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ArticleJune 14, 2016

Molecular Level Understanding of the Factors Affecting the Stability of Dimethoxy Benzene Catholyte Candidates from First-Principles Investigations
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Rajeev S. Assary^*^†^‡
Lu Zhang^†^§
Jinhua Huang^†^§
Larry A. Curtiss^†^‡

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The Journal of Physical Chemistry C

Cite this: J. Phys. Chem. C 2016, 120, 27, 14531–14538

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https://doi.org/10.1021/acs.jpcc.6b04263

Published June 14, 2016

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Abstract

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First-principles simulations are performed to gain molecular level insights into the factors affecting the stability of seven 1,4-dimethoxybenzene (DMB) derivatives. These molecules are potential catholyte candidates for nonaqueous redox flow battery systems. Computations are performed to predict oxidation potentials in various dielectric mediums, intrinsic-reorganization energies, and structural changes of these representative catholyte molecules during the redox process. In order to understand the stability of the DMB-based radical cations, the thermodynamic feasibility of the following reactions is computed using density functional theory: (a) deprotonation, (b) dimerization, (c) hydrolysis, and (d) demethylation. The computations indicate that radical cations of the 2,3-dimethyl and 2,5-dimethyl derivatives are the most stable among the DMB derivatives considered in this study. In the presence of solvents with high-proton solvating ability (water, DMSO, acetonitrile), degradation of cation radical occurring via deprotonation is the most likely mechanism. In the presence of solvents such as propylene carbonate (PC), demethylation was found to be the most likely reaction that causes degradation of radical cations. From the computed enthalpy of activation (ΔH^‡) for a demethylation reaction in PC, the 2,5-dimethyl DMB cation radical would exhibit better kinetic stability in comparison to the other candidates. This investigation suggests that computational studies of structural properties such as redox potentials, reorganization energies, and the computed reaction energetics (deprotonation and demethylation) of charged species can be used to predict the relative stability of a large set of molecules required for the discovery of novel redox active materials for flow battery applications.

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1 Introduction

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Effective energy storage in organic molecules has a role to play in the area of the grid electricity storage due to the increased production of intermittent renewable electricity. (1-3) Redox couples for such applications need to be chemically and electrochemically (oxidation and reduction) stable to perform thousands of cycles with no significant degradation. In a redox couple, molecules that undergo oxidation and reduction are classified into “catholyte” and “anolyte”, respectively. Prerequisites of ideal candidates for flow battery applications are their adequate electrochemical window, solubility, fast redox kinetics, stability, and cost. Detailed molecular level understandings of all the above properties are crucial to the development of new and improved candidates for grid storage applications.

Recently, the use of redox active organic molecules for energy storage has made exceptional progress in aqueous systems, (4-7) while finding desirable redox flow molecules (catholytes and anolytes) in nonaqueous media was found to be exceptionally challenging. (8-11) Among, many possible catholyte candidates as redox active materials in nonaqueous media, 2,5-di-tert-butyl-1,4-dimethoxybenzene was found to be promising material in the battery operating conditions by Dahn et al. (12, 13) These materials have a reasonable oxidation window (>3.5 V Li/Li+) and stability as radical cations compared to many materials investigated. (14) Similarly, further studies suggest that derivatives of 1,4-dimethoxybenzene (DMB) can be used as stable material as redox shuttle in lithium ion batteries. (14-16) In general, due to the electrochemical window, any DMB derivatives with solubility and stability can be used as catholyte materials in flow batteries. In terms of assessing the feasibility of redox materials, there have been many studies of their redox properties. (17-23) However, the reasons for the differences in stability of organic molecules for flow batteries are poorly understood due to the complexity of the reactivity patterns (24, 25) of the radical cations (RC) in solution. However, information regarding the reactivity of radical cations is critical toward understanding the factors that control stability. Therefore, in this investigation we utilize the predictive ability of density functional theory (DFT) to gain insights into factors controlling the stability of seven catholyte candidates based on DMB derivatives. The structures of the seven molecules are shown in Figure 1.

Figure 1. Schematic of dimethoxybenzene (DMB) derivatives considered in this study.

A critical bottleneck for understanding the stability is the complexity of the radical cations due to their many intrinsic and extrinsic properties. Intrinsic properties of radical cations include the following: (i) delocalization of spin and charge, (ii) release of strain, (iii) stereo electronic factors, and (iv) steric hindrance. Extrinsic properties include the following: (v) solvent medium, (vi) anode materials, and (vii) electrolytes. A detailed understanding of these intrinsic and extrinsic effects requires comprehensive experimental and theoretical studies. In this paper we present a theoretical investigation focused on understanding key features that affect the stability of radical cations (RC) formed during the oxidation of the catholytes. These features and associated quantum chemical descriptors that can be computed are schematically shown in Figure 2. We present results for the electrochemical windows, structural distortion, and reactivity of radical cations of the seven DMB derivatives. The oxidation potential and reorganization energy are hypothesized as the descriptors for electrochemical window and structural distortion, respectively. Additionally, computed free energies of reactions such as deprotonation, dimerization, hydrolysis, and demethylation of cation radicals are hypothesized as the descriptors for their reactivity.

Figure 2. Key properties controlling the stability of redox active species and the quantum chemical descriptors that can assess the properties are schematically shown. Note: G, E, and λ represent Gibbs free energy, redox potential, and reorganization energy, respectively.

Details of the computational approach employed in this investigation are presented in the next section. In Results and Discussion, computational studies are presented that rank the stability of selected catholyte molecules using structural and energetic parameters.

2 Computational Details

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All computations were performed using the Gaussian 09 software. (26) The B3LYP/6-31G(2df,p) level of theory was used to compute the structure, electronic energy, vibrational frequencies, and free energy corrections of all species. Solvation free energies were computed with the SMD solvation model by performing single point energy evaluations with a specific dielectric medium: diethyl ether (ε = 4.24), acetone (ε = 20.49), dimethyl sulfoxide (ε = 46.83), or water (ε = 78.36). Reorganization energies were computed (section 3.1.2) from the structures of the molecules optimized in the diethyl ether solvent medium. We note that explicit interactions of solvent molecules may be essential to represent charged states of the molecules/ions are not included in this investigation due to the computational complexity, which is beyond the scope of this manuscript.

The oxidation potentials (E^ox w.r.t. Li/Li⁺) were computed from the computation of Gibbs free energy change (ΔG^ox, eV) at 298 K in the solution (dielectric) for the removal of an electron from the species of interest, using the following equation:

where F is the Faraday constant (in eV) and n is the number of electrons involved in the oxidation process. The addition of the constant “–1.24 V” is required to convert the free energy changes to oxidation potential (Li/Li⁺ reference electrode), a commonly used convention to compute the redox potentials in solution. (27, 28) The change in energy of electrons when going from vacuum to nonaqueous solution is treated as zero, similar to what has been used by others. (29) Further details regarding the computation of redox potential can be found elsewhere. (29-34)

Solvation energy contributions of a species are added to the gas phase enthalpies and free energies to approximate enthalpies (H_soln = H_gas phase + ΔG_solv) and free energies (G_soln = G_gas phase + ΔG_solv) in solution. The demethylation (C–O bond breaking) reaction barriers presented in section 3.2.2 are apparent enthalpy barriers (see SI for activation free energy barriers), computed as the difference between the enthalpy of the transition state structure (H^†) and the sum of enthalpies of reactants in solution at 298 K. Here we used the solution phase enthalpy approximation because the main free energy contributions (G_CDS: cavitation, dispersion, and solvent structure terms) cancel out (or are negligible) when computing the apparent barriers (H(TS)–H(reactants)). Therefore, the dominant solvation contributions are electronic, nuclear, and polarization terms (G_ENP), which are included in the computation of solution enthalpies. A similar approach was successfully employed elsewhere. (35) The B3LYP/6-31G(2df,p) level of theory is used to compute the transition state structures in the gas phase and, subsequently, a single point solvation energy calculation is performed using the SMD solvation model (diethyl ether and water) at the same level of theory. Thus, the enthalpy barriers (ΔH^†) presented in section 3.2.2 are the sum of the gas phase enthalpy barrier and the solvation energy (E^SMD):

3 Results and Discussion

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In this section, computation of various structural and reactivity related properties (Figure 2) of the seven DMB-based molecules are described. The structure related properties, electrochemical windows and structural deviation upon oxidation, are discussed in section 3.1. The reactivity related properties such as thermochemical data for deprotonation, dimerization, hydrolysis, and demethylation reactions of radical cations and their relevance are discussed in section 3.2.

3.1 Structure

Three properties related to structural stability of the dimethoxybenzene (DMB) derivatives were computed at the B3LYP/6-31G(2df,p) level of theory. These properties are (i) oxidation potential, (ii) reorganization energy (λ), and (iii) the Kabsch root-mean-square deviation (RMSD) between the optimized coordinates of the oxidized and neutral state of the molecules.

3.1.1 Simulated Oxidation Potentials (E^ox)

The electrochemical windows of the seven DMB derivatives in various dielectric media including diethyl ether (DEE; ε = 4.24), acetone (ε = 20.49), dimethyl sulfoxide (DMSO; ε = 46.83), and water (ε = 78.36) were computed and are presented in Table 1. In a low dielectric medium such as in diethyl ether, the oxidation potentials are in the range of 3.9–4.5 V for the DMB candidate molecules. Among the molecules in diethyl ether medium, the 25-DMB and 23-DMB exhibit lower oxidation potentials (3.9 V each), while 235-DMB exhibits a high oxidation potential (4.5 V). At high dielectrics such as DMSO (ε = 46.83) and water (ε = 78.36), the computed oxidation potentials are on the order of 3.5–4.1 V. This slight reduction in oxidation potential in a high dielectric medium compared to low dielectric conditions is expected due to the increased stabilization of the charged state by the high dielectric medium compared to the neutral state.

Table 1. Computed Oxidation Potentials (V), Reorganization Energies (λ_total, λ_ox), and Root Mean Squared Deviation (RMSD) between the Optimized Cartesian Coordinates of Neutral and Oxidized Species of DMB Derivatives (see Figure 1)^a

	computed oxidation potential (V, Li/Li⁺)				reorganization energy (kcal/mol)
species	E_DEE	E_acetone	E_DMSO	E_H₂O	λ_ox	λ_total	Kabsch RMSD
DMB^a	4.2	3.8	3.7	3.8	5.6	11.3	0.04
2-DMB	4.1	3.7	3.7	3.7	5.5	11.2	0.04
23-DMB	3.9	3.6	3.5	3.6	15.6	19.7	0.63
25-DMB	3.9	3.6	3.5	3.6	5.4	11.1	0.04
26-DMB	4.1	3.8	3.7	3.7	10.9	21.3	0.59
235-DMB	4.5	4.1	4.1	4.1	9.1	20.4	0.44
2356-DMB^b	4.3	3.9	3.9	3.9	14.6	31.9	0.52

Note: The total reorganization energy (λ_total) is defined as the sum of the reorganization energy during the oxidation (λ_ox) and reduction (λ_red). Abbreviations: DEE, diethyl ether; DMSO, dimethyl sulfoxide; H₂O, water. Notes: (a) experimentally (12) measured oxidation potentials in propylene carbonate are 3.9 and 4.1 V for DMB (a) and 2356-DMB (b) molecules, respectively.

3.1.2 Reorganization Energy (λ)

The reorganization energy (λ) of a molecule during a redox process is computed using the energies and geometries of the neutral and oxidized states. The connection between the nuclear configuration and ground state energies of a given molecule and its cation radical is schematically shown in Figure 3. The total reorganization energy (λ_total) is defined as the sum of the reorganization energy during the oxidation (λ_ox) and reduction (λ_red). We note that zero-point energy correction is not considered in the reorganization energy computations. The λ_ox is the energy difference between the vertical detachment energy (VDE) and adiabatic detachment energy (ADE). The (λ_red) is the energy difference between ADE and vertical electron affinity (VEA). The absolute values of reorganization energy are used here and are tabulated in Table 1. Lower reorganization energy suggests that the molecular system achieves energy closer to the minimum energy upon electron addition or removal. For oxidation processes, based on the computed λ_ox presented in Table 1, the 25-DMB has the lowest and the 23-DMB has the highest requirement of reorganization energy. We note that, the 2356-DMB molecule requires highest total reorganization energy as expected due to six substituent groups in the benzene ring. In general, from Table 1, the unsubstituted DMB, 2-DMB, and the 25-DMB have lower values for λ_ox and λ_total.

Figure 3. Schematic of the computation of reorganization energy during oxidation (λ_ox) and reduction (λ_red) process from the computed energies of a molecule in its neutral and singly positive charge state. Total reorganization energy ((λ_total) is the sum of reorganization energy required during the oxidation (λ_ox) and reduction (λ_red) process.

3.1.3 Root Mean Squared Deviation (RMSD) between the Geometries

Another measure of the structural deviation during the redox processes is the RMSD between the optimized Cartesian coordinates of neutral and oxidized species of the DMB derivatives. From the neutral and unipositive DFT optimized (xyz) structures of molecules (all optimized structures are given in the Supporting Information, Figure S1 and Table S1); the root-mean-square deviation (RMSD) was computed using the Kabsch algorthm. (36) The computed RMSD is also shown in Table 1. Based on the computations, the 25-DMB exhibits a lowest RMSD for geometry change (0.04) and also the lowest reorganization energy, suggesting stabilization over the rest of the candidate molecules. Two other derivatives (DMB and 2-DMB) have similarly low RMSDs during oxidation consistent with the prediction of reorganization energy.

To compare the oxidation potentials (E^ox), reorganization energies (λ^ox), and RMSD during oxidation of the candidate molecules, these three properties are plotted in Figure 4. From the figure it is evident that the 25-DMB exhibits low values for all three properties. Having lower oxidation potentials (E^ox) in a given series of molecules is an indicator of relative stability of cation radicals. This is based on the assumption that these cation radicals are less reactive toward typical deprotonation reactions. Methylated DMB derivatives have similar C–H bond dissociation energies; therefore, the acidity of the corresponding cation radicals increase with the oxidation potential of neutral species. (37) In this aspect, the 25-DMB and 23-DMB are less acidic (lower computed oxidation potential, see Table 1) compared to the rest of the candidates. In addition by incorporating other structural features such as reorganization energy (λ) or RMSD during the oxidation, it is evident that 25-DMB can be considered to be the most stable catholyte material from the seven DMB derivatives considered. In general, plots similar to that of Figure 4 can be obtained for a series of molecules to assess the relative stability potential redox active materials. Further understanding of the fate of the radical cation is also essential to understand the relative stability of radical cations and the overall stability of materials. We consider this in section 3.2).

Figure 4. Comparison of computed oxidation potentials (E^oxn V unit w.r.t. Li/Li⁺ ref electrode), reorganization energy during oxidation (λ^ox), and root-mean-square deviation (RMSD) between the optimized *xyz* coordinates of neutral and oxidized states of various DMB derivatives (x-axis). Note: The data associated with the plot is given in Table 1.

3.2 Reactivity

To assess the reactivity of cation radicals, it is essential to understand the thermodynamic driving forces and kinetic feasibilities of likely reactions. In subsection 3.2.1, the computed thermochemistry of selected probe reactions of radical cations (deprotonation, dimerization, hydrolysis, and demethylation) is discussed. In subsection 3.2.2, the kinetic barriers of demethylation reactions are evaluated and discussed.

3.2.1 Thermodynamic Feasibility of Chemical Reactions

The thermodynamic feasibility (ΔG) of deprotonation (rxn1), demethylation (rxn2), hydrolysis (rxn3), and dimerization (rxn4) reactions were computed for the radical cations of (RCs) the candidates. These four reactions are schematically shown for DMB in Figure 5. The deprotonation (rxn 1) of the cation radical in water is chosen as the probe reaction, where deprotonation results into the formation of neutral radical and a solvated proton. This reaction is considered due to the available experimental data of free energy of solvation of the proton in aqueous medium ΔG^{proton solvation} = −265.9 kcal/mol, (38) which is needed to understand the thermochemistry of protonation and deprotonation. (39) Deprotonation of DMB derivatives could occur from the methoxy (−OCH₃) or from the methyl (−CH₃) group. Note that the deprotonation from the benzene ring is unlikely due to the largely endothermic nature (∼+40 kcal/mol) of the reaction. Similar to deprotonation, demethylation (rxn 2) is another way of eliminating the positive charge of the radical cations. To address rxn 2, we have computed the thermochemistry for methyl transfer between the radical cation (RC) and propylene carbonate (PC), a probe molecule, as shown in Figure 5. In addition to the deprotonation and demethylation reactions, a hydrolysis reaction (rxn 3), a probe reaction for solvolysis, is considered. In this reaction the thermochemistry of etheric cleavage by a water molecule to form CH₃OH and hydroxyl radical cations (see Figure 5, rxn3) was studied. Finally, dimerization (rxn4) of the cation radical was also considered as a probe reaction. The computed Gibbs free energies of rxns1–4 are presented in Table 2. Using the Gibbs free energy of deprotonation (ΔG(H⁺)_aq) and Gibbs free energy of hydrolysis (ΔG_HOH), the pK_a and K_HOH equilibrium constants, respectively, were also computed and presented in Table 2.

Figure 5. Schematic of selected reactions (rxn1–4) of 1,4-dimethoxybenzene (DMB) radical cation. Abbreviation: PC, propylene carbonate molecule. All reactions are modeled in aqueous dielectric medium. In rxn1, the cation radical reacts with water to form solvated proton and neutral radical. In rxn4, two cation radicals interact with water to form a dimer and two solvated protons.

Table 2. Computed Thermochemical Data (in kcal/mol) of DMB Cation Radicals at the B3LYP/6-31G(2df,p) Level of Theory^a

	deprotonation from “–OCH₃” group			deprotonation from “–CH₃” group
radical cation (RC)	ΔG(H⁺)_aq^b	pK_a^c	ΔG^dimer_aq^d	ΔG(H⁺)_aq^e	pK_a^c	ΔG^dimer_aq^d	^gΔG(CH₃⁺)_PC	ΔG^HOH^h	K_HOHⁱ
DMB	28.3	7.3	–12.0	NA	NA	NA	14.0	0.6	1.0 × 10^–01
2-DMB	29.9	7.7	–8.3	21.1	5.4	–1.1	13.7	0.1	6.5 × 10^–01
23-DMB	31.3	8.1	–5.0	22.7	5.8	4.2	14.9	–0.2	2.3 × 10⁰⁰
25-DMB	32.4	8.3	–3.3	23.4	6.0	3.6	15.4	0.2	5.0 × 10^–01
26-DMB	27.2	7.0	–9.3	20.8	5.4	–3.6	10.0	–4.5	3.6 × 10⁰⁷
235-DMB	19.3	5.0	–27.0	^f10.8	2.8	–19.9	9.4	–6.9	4.2 × 10¹¹
2356-DMB	22.7	5.9	–20.4	14.9	3.8	–10.7	5.9	–7.1	8.6 × 10¹¹

Computed data includes deprotonation free energies (ΔG(H⁺)_aq), dimerization energies (ΔG^dimer_aq), and demethylation energies (ΔG(CH₃⁺)) of DMB cation radicals.

Free energy change during the deprotonation reaction (rxn1) from the methoxy group of the DMB derivative using the experimental Gibbs free energy of proton in aqueous solution (−265.9 kcal/mol).

pK_a = ΔG/2.303RT, where R is the gas constant and T is the absolute temperature.

ΔG^dimer is the free energy change during the dimerization (rxn4) of two cation radicals to form neutral dimer, where two protons are solvated by an aqueous medium.

The computed free energy change for deprotonation (rxn1) from the methyl group to the aqueous medium using the Gibbs free energy of a proton in aqueous solution: −265.9 kcal/mol. (38)

For 235-DMB, the deprotonation from the 3-methyl position (ΔG(H⁺)_aq = 10.8 kcal/mol) is thermodynamically more preferred than from the 2-methyl (ΔG(H⁺)_aq = 12.3 kcal/mol) and from the 5-methyl (ΔG(H⁺)_aq = 11.7 kcal/mol) position.

ΔG = G(RC) + G(PC) → G(PC–CH₃⁺) + G(RC-demethylated), see rxn2 in Figure 5.

ΔG = G(RC) + G(H₂O) → G(CH₃OH) + G(RC-H) radical, barrier is not computed.

ⁱ

The equilibrium constant for hydrolysis, K_HOH = exp(−ΔG^HOH/2.303RT).

Based on the computed data presented in Table 2, the following important points can be drawn:

1.	Deprotonation of DMB radical cations are endothermic processes in aqueous solution (10–32 kcal/mol). Deprotonation from a methyl group (10–23 kcal/mol) is thermodynamically more likely than from the methoxy groups (19–32 kcal/mol) for all DMB derivatives.
2.	Dimerization of cation radicals are generally thermodynamically downhill, and it is likely that the rate-determining deprotonation is the first step. Dimerization of neutral radical formed at the methoxy group (−OCH₂−) is thermodynamically more favorable than that formed at the methyl (−CH₂– group). See Figure S3 of the Supporting Information for details.
3.	Demethylation reactions of RCs by a PC molecule in aqueous medium are endoergic reactions (5–16 kcal/mol). This is the most likely reaction based on the free energy of the reaction.
4.	Similar to demethylation, hydrolysis can also cleave the etheric C–O bond. Based on the computations in Table 2, hydrolysis of DMB analogues with steric constraints (due to methyl groups) are thermodynamically favorable.

Additionally, deprotonation energetics are likely to be more thermodynamically uphill in solvents such as methanol (ΔG^{proton solvation} = −263.5 kcal/mol (38)) or acetonitrile (ΔG^{proton solvation} = −260.2 kcal/mol (38)) than in water medium, due to the reduced proton solvation energy of methanol and acetonitrile compared to the aqueous medium (ΔG^{proton solvation} = −265.9 kcal/mol). In terms of the reactivity of cation radicals, deprotonation and demethylation are the most likely reactions since both of them are ideal for the radical cations to lose its charge to the solvent medium. The dimerization reaction is a consequence of the deprotonation and hydrolysis reaction (or solvolysis) is one of the routes for the demethylation reaction. In general, based on the computed reaction free energies of demethylation reaction, most likely based on the free energies, the 25-DMB radical cations would exhibit the least reactivity in the solution, indicating better relative stability as catholyte materials.

3.2.2 Demethylation: Computed Activation Barriers

Table 3. Computed Enthalpy of Activation (kcal/mol) for Demethylation of DMB Radical Cations by a Propylene Carbonate Molecule at the B3LYP/6-31G(2df,p) Level of Theory in Water and Diethyl Ether Solvent Dielectric^a

	water dielectric			diethyl ether dielectric
radical cation (RC)	ΔH^‡(CH₃⁺)_PC	K^b (s^–1)	T_1/2^c (h)	ΔH^‡(CH₃⁺)_PC^b	K^c (s^–1)	T_1/2^d (h)
DMB	26.7	1.6 × 10^–7	1.2 × 10³	23.6	3.0 × 10^–5	6.4
2-DMB	26.5	2.5 × 10^–7	7.9 × 10²	23.8	2.6 × 10^–5	8.5
23-DMB	27.2	6.8 × 10^–8	2.8 × 10³	24.7	4.9 × 10^–6	3.9 × 10¹
25-DMB	27.9	2.1 × 10^–8	9.1 × 10³	25.6	9.9 × 10^–7	1.9 × 10²
26-DMB	23.8	2.3 × 10^–5	8.5	21.0	2.5 × 10^–3	7.7 × 10^–2
256-DMB	16.5	4.8	4.0 × 10^–5	13.9	4.4 × 10²	4.4 × 10^–7
2356-DMB	23.7	2.0 × 10^–5	9.6	21.5	1.0 × 10^–3	1.9 × 10^–1

Using the enthalpy of activation in solution, the rate constant (K) and half-life assuming first order kinetics are shown.

ΔG^‡(CH₃⁺)_PC is gven in the Table S2 of the Supporting Information;

K = (K_bT/hc)exp(−ΔG^‡/RT).

T_1/2 = ln(2)/K (h^–1).

In addition to the relative stability predictions based on the computed free energy changes associated with the deprotonation/demethylation reactions, the activation barriers for demethylation of DMB analogues were also computed. In general, calculation of activation barriers using the demethylation model can provide qualitative trends for describing the initial decomposition reactions of cation radicals. Demethylation of DMB cation analogues by a propylene carbonate molecule in water dielectric medium is used as a model to compute the activation barriers. In Figure 6, a computed enthalpy profile for the demethylation reaction of 25-DMB is shown, where the computed apparent activation enthalpy (ΔH^‡) is ∼28 kcal/mol. In the energy profile (Figure 6), first a weakly bound complex between 25-DMB cation radical and a PC molecule is formed. Subsequently, the complex undergoes C–O_methyl bond cleavage via the transition state shown in the Figure 6. Complete transfer of methyl cation to the PC results in the formation of a 25-DMB demethylated radical. Note that the demethylation reactions are endothermic in nature, as explained in the previous section (see Table 2). In Table 3, the computed apparent activation barriers (ΔH^‡) for all radical cations in aqueous and diethyl ether solvent model are presented. A comparison of computed activation enthalpies and the free energy changes associated with the demethylation of DMB cation radicals by propylene carbonate is schematically shown in Figure 7. Using this activation enthalpy in solution, we have approximated the rate constant (K, s^–1) using the Eyring equation (see Table 3). Additionally, using first order kinetics approximation, the half-life of the reactions (T_1/2, h) is also computed, also shown in Table 3. Based on the computations, the activation barriers for the demethylation are smaller in diethyl ether compared to water. Most importantly, the computed activation barriers are in the order: 25-DMB > 23-DMB > 2-DMB > DMB > 2356-DMB > 26-DMB > 256-DMB in ether dielectric medium. The computed half-life of the demethylation reaction suggest that both 25-DMB and 23-DMB would require approximately hundreds of hours to decompose via demethylation reaction, while the rest of the cation radicals require tens of hours or less. This is consistent with the conclusion above based on the computed thermochemistry that these two would be the most stable.

Figure 6. Computed solution phase enthalpy profile (ΔH_soln) of demethylation of the 25-DMB cation radical by a propylene carbonate molecule. Computed apparent enthalpy of activation (ΔH^‡) of all DMB candidates are given in Table 3.

Figure 7. Comparison of the computed free energies changes (ΔG(CH₃⁺)_PC) and activation enthalpies (ΔH^‡(CH₃⁺)_PC) required for the demethylation of DMB cation radicals by propylene carbonate. The data associated with (ΔG(CH₃⁺)_PC) and (ΔH^‡(CH₃⁺)_PC) are given in Tables 2 and 3, respectively.

4 Summary

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Stable, electrochemically active and soluble organic materials are required for redox flow energy storage applications. Understanding the stability of organic materials in the battery operating conditions is a complex problem, but essential to provide guidelines for materials discovery. In this study, first-principles simulations are performed to gain molecular level insights into the factors affecting the stability of seven 1,4-dimethoxybenzene (DMB) derivatives. These molecules are potential catholyte candidates for nonaqueous redox flow battery systems. Computations are performed to predict oxidation potentials in various dielectric mediums, intrinsic-reorganization energies, and structural changes of these representative catholytes during the redox process. In order to understand the stability of the DMB-based radical cations, the thermodynamic feasibility of the following reactions is computed using density functional theory: (a) deprotonation, (b) dimerization, (c) hydrolysis, and (d) demethylation.

The computations indicate that radical cations of the 2,3-dimethyl and 2,5-dimethyl derivatives are the most stable among the DMB derivatives considered in this study. In the presence of solvents with high-proton solvating ability (water, DMSO, acetonitrile), degradation of the cation radical likely occurs via deprotonation. In the presence of solvents such as propylene carbonate (PC), demethylation was found to be the most likely reaction that causes degradation of radical cations. From the computed enthalpy of activation (ΔH^‡) for a demethylation reaction in PC, the 2,5-dimethyl DMB cation radical would exhibit better kinetic stability in comparison with the other candidates.

This investigation suggests that computational studies of structural properties such as redox potentials, reorganization energies, and the computed reaction energetics (deprotonation and demethylation) of charged species could be used to predict the relative stabilities of large sets of molecules required for the discovery of novel redox active materials for flow battery applications. First, computationally less demanding, reorganization energies or RMSD between structures can be used as a descriptor for relative stability. Second, a reactivity indicator from computed free energy change of the most likely reaction can be used to screen a large set of molecules to rank the stability trends. Finally, as part of the deep dive study, for a small subset of radical cations kinetic feasibility of the most likely reaction can be computed to assess the reactivity and lifetime. We also anticipate that these first-principles-based computations can be used in the future to derive quantitative relationships to predict the stability of materials for redox flow applications.

Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b04263.

Optimized structures of DMB derivatives are shown in Figure S1. Figure 2 suggests most important structural parameters and these parameters are tabulated in Table S1. Figure S3 presents the computed deprotonation energies (rxn 1 of Figure 5) and dimerization energies (rxn 4 of the Figure 5) of radical cations at the B3LYP/6-31G(2df,p) level of theory, and Table S2 shows computed enthalpies and free energies of activation required for the demethylation reactions in water and diethyl ether dielectric media (PDF).

jp6b04263_si_001.pdf (476.11 kb)

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Author Information

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Corresponding Author
- Rajeev S. Assary - †Joint Center for Energy Storage Research, ‡Materials Science Division, and §Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States; Email: [email protected]
Authors
- Lu Zhang - †Joint Center for Energy Storage Research, ‡Materials Science Division, and §Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States
- Jinhua Huang - †Joint Center for Energy Storage Research, ‡Materials Science Division, and §Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States
- Larry A. Curtiss - †Joint Center for Energy Storage Research, ‡Materials Science Division, and §Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States
Notes
The authors declare no competing financial interest.

Acknowledgment

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This work was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. We gratefully acknowledge the computing resources provided on “Blues”, a 320-node computing cluster operated by the Laboratory Computing Resource Center at Argonne National Laboratory.

References

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We introduce a novel Org. Redox Flow Battery (ORBAT), for meeting the demanding requirements of cost, eco-friendliness, and durability for large-scale energy storage. ORBAT employs two different water-sol. org. redox couples on the pos. and neg. side of a flow battery. Redox couples such as quinones are particularly attractive for this application. No precious metal catalyst is needed because of the fast proton-coupled electron transfer processes. Furthermore, in acid media, the quinones exhibit good chem. stability. These properties render quinone-based redox couples very attractive for high-efficiency metal-free rechargeable batteries. We demonstrate the rechargeability of ORBAT with anthraquinone-2-sulfonic acid or anthraquinone-2,6-disulfonic acid on the neg. side, and 1,2-dihydrobenzoquinone-3,5-disulfonic acid on the pos. side. The ORBAT cell uses a membrane-electrode assembly configuration similar to that used in polymer electrolyte fuel cells. Such a battery can be charged and discharged multiple times at high faradaic efficiency without any noticeable degrdn. of performance. We show that soly. and mass transport properties of the reactants and products are paramount to achieving high current densities and high efficiency. The ORBAT configuration presents a unique opportunity for developing an inexpensive and sustainable metal-free rechargeable battery for large-scale elec. energy storage.
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Nonaq. redox flow batteries are emerging flow-based energy storage technologies that have the potential for higher energy densities than their aq. counterparts because of their wider voltage windows. However, their performance has lagged far behind their inherent capability due to one major limitation of low soly. of the redox species. Here, a mol. structure engineering strategy towards high performance nonaq. electrolyte is reported with significantly increased soly. Its performance outweighs that of the state-of-the-art nonaq. redox flow batteries. In particular, an ionic-derivatized ferrocene compd. is designed and synthesized that has more than 20 times increased soly. in the supporting electrolyte. The solvation chem. of the modified ferrocene compd. Electrochem. cycling testing in a hybrid lithium-org. redox flow battery using the as-synthesized ionic-derivatized ferrocene as the catholyte active material demonstrates that the incorporation of the ionic-charged pendant significantly improves the system energy d. When coupled with a lithium-graphite hybrid anode, the hybrid flow battery exhibits a cell voltage of 3.49 V, energy d. about 50 Wh L-1, and energy efficiency over 75%. These results reveal a generic design route towards high performance nonaq. electrolyte by rational functionalization of the org. redox species with selective ligand.
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The integration of renewable energy sources into the elec. grid requires low-cost energy storage systems that mediate the variable and intermittent flux of energy assocd. with most renewables. Nonaq. redox-flow batteries have emerged as a promising technol. for grid-scale energy storage applications. Because the cost of the system scales with mass, the electroactive materials must have a low equiv. wt. (ideally 150 g/(mol·e-) or less), and must function with low mol. wt. supporting electrolytes such as LiBF4. However, sol. anolyte materials that undergo reversible redox processes in the presence of Li-ion supports are rare. We report the evolutionary design of a series of pyridine-based anolyte materials that exhibit up to two reversible redox couples at low potentials in the presence of Li-ion supporting electrolytes. A combination of cyclic voltammetry of anolyte candidates and independent synthesis of their corresponding charged-states was performed to rapidly screen for the most promising candidates. Results of this workflow provided evidence for possible decompn. pathways of first-generation materials and guided synthetic modifications to improve the stability of anolyte materials under the targeted conditions. This iterative process led to the identification of a promising anolyte material, N-Me 4-acetylpyridinium tetrafluoroborate. This compd. is sol. in nonaq. solvents, is prepd. in a single synthetic step, has a low equiv. wt. of 111 g/(mol·e-), and undergoes two reversible 1e- redns. in the presence of LiBF4 to form reduced products that are stable over days in soln.
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Journal of the Electrochemical Society (2005), 152 (12), A2390-A2399CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
Fifty eight arom. org. mols. were screened as chem. shuttles to provide overcharge protection for LiFePO4/graphite and LiFePO4/Li4/3Ti5/3O4 Li-ion cells. The majority of the mols. were based on methoxybenzene and on dimethoxybenzene with a variety of ligands added to explore their effect. The added ligands affect the redox potential of the mols. through their electron-withdrawing effect and affect the stability of the radical cation. Of all the mols. tested, only 2,5-di-tert-butyl-1,4-dimethoxybenzene shows an appropriate redox potential of 3.9 V vs. Li/Li+ and long-term stability during extended abusive overcharge totaling over 300 cycles of 100% overcharge per cycle. The reasons for the success of this mol. are explored.
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Lithium-ion batteries suitable for the mass consumer market require robust safety and tolerance to repeated overdischarge and overcharge to avoid costly charge control circuitry and to allow simple replacement of individual cells by consumers. A chem. redox shuttle electrolyte additive is shown to provide this protection. Using 2,5-di-tert-butyl-1,4-dimethoxybenzene as electrolyte additive provides overcharge and overdischarge protection for hundreds of charge-discharge cycles in both single cells and series-connected batteries.
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Zhang, Z.; Zhang, L.; Schlueter, J. A.; Redfern, P. C.; Curtiss, L.; Amine, K. Understanding the Redox shuttle stability of 3,5-di-tert-butyl-1,2-dimethoxybenzene for Overcharge Protection of Lithium-ion Batteries J. Power Sources 2010, 195, 4957– 4962 DOI: 10.1016/j.jpowsour.2010.02.075
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Journal of Power Sources (2010), 195 (15), 4957-4962CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
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A simple computational approach for predicting ground-state redn. potentials based upon gas phase geometry optimizations at a moderate level of d. functional theory followed by single-point energy calcns. at higher levels of theory in the gas phase or with polarizable continuum solvent models is described. Energies of the gas phase optimized geometries of the S and one-electron-reduced D states of 35 planar arom. org. mols. spanning three distinct families of org. photooxidants are computed in the gas phase as well as well in implicit solvent with IPCM and CPCM solvent models. Correlation of the D - S energy difference (essentially an electron affinity) with exptl. redn. potentials from the literature (in acetonitrile vs SCE) within a single family, or across families when solvent models are used, yield correlations with r2 values in excess of 0.97 and residuals of about 100 mV or less, without resorting to computationally expensive vibrational calcns. or thermodn. cycles.
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Chemical Science (2015), 6 (2), 885-893CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
Inspired by the electron transfer properties of quinones in biol. systems, we recently showed that quinones are also very promising electroactive materials for stationary energy storage applications. Due to the practically infinite chem. space of org. mols., the discovery of addnl. quinones or other redox-active org. mols. for energy storage applications is an open field of inquiry. Here, we introduce a high-throughput computational screening approach that we applied to an accelerated study of a total of 1710 quinone (Q) and hydroquinone (QH2) (i.e., two-electron two-proton) redox couples. We identified the promising candidates for both the neg. and pos. sides of org.-based aq. flow batteries, thus enabling an all-quinone battery. To further aid the development of addnl. interesting electroactive small mols. we also provide emerging quant. structure-property relationships.
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Assary, R. S.; Brushett, F. R.; Curtiss, L. A. Reduction Potential Predictions of some Aromatic Nitrogen-Containing Molecules RSC Adv. 2014, 4, 57442– 57451 DOI: 10.1039/C4RA08563A
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Reduction potential predictions of some aromatic nitrogen-containing molecules
Assary, Rajeev S.; Brushett, Fikile R.; Curtiss, Larry A.
RSC Advances (2014), 4 (101), 57442-57451CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
Accurate quantum chem. methods offer a reliable alternative to time-consuming exptl. evaluations for obtaining a priori electrochem. knowledge of a large no. of redox active mols. In this contribution, quantum chem. calcns. are performed to investigate the redox behavior of quinoxalines, a promising family of active materials for non-aq. flow batteries, as a function of substituent group. The redn. potentials of 40 quinoxaline derivs. with a range of electron-donating and electron-withdrawing groups are computed. Calcns. indicate the addn. of electron-donating groups, particularly alkyl groups, can significantly lower the redn. potential albeit with a concomitant decrease in oxidative stability. A simple descriptor is derived for computing redn. potentials of quinoxaline derivs. from the LUMO energies of the neutral mols. without time-consuming free energy calcns. The relationship was validated for a broader set of arom. nitrogen-contg. mols. including pyrazine, phenazine, bipyridine, pyridine, pyrimidine, pyridazine, and quinoline, suggesting that it is a good starting point for large high-throughput computations to screen redn. windows of novel mols.
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Bachman, J. E.; Curtiss, L. A.; Assary, R. S. Investigation of the Redox Chemistry of Anthraquinone Derivatives Using Density Functional Theory J. Phys. Chem. A 2014, 118, 8852– 8860 DOI: 10.1021/jp5060777
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Investigation of the Redox Chemistry of Anthraquinone Derivatives Using Density Functional Theory
Bachman, Jonathan E.; Curtiss, Larry A.; Assary, Rajeev S.
Journal of Physical Chemistry A (2014), 118 (38), 8852-8860CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
Application of d. functional calcns. to compute electrochem. properties such as redox windows, effect of substitution by electron donating and electron withdrawing groups on redox windows, and solvation free energies for ∼50 anthraquinone (AQ) derivs. are presented because of their potential as anolytes in all-org. redox flow batteries. Computations suggest that lithium ions can increase (by ∼0.4 V) the redn. potential of anthraquinone due to the lithium ion pairing by forming a Lewis base-Lewis acid complex. To design new redox active species, the substitution by electron donating groups is essential to improve the redn. window of AQ with adequate oxidative stability. For instance, a complete methylation of AQ can improve its redn. window by ∼0.4 V. The quantum chem. studies of the ∼50 AQ derivs. are used to derive a relationship that connects the computed LUMO energy and the redn. potential that can be applied as a descriptor for screening thousands of AQ derivs. Our computations also suggest that incorporating oxy-Me dioxolane substituents in the AQ framework can increase its interaction with nonaq. solvent and improve its soly. Thermochem. calcns. for likely bond breaking decompn. reactions of unsubstituted AQ anions suggest that the dianions are relatively stable in the soln. These studies provide an ideal platform to perform further combined exptl. and theor. studies to understand the electrochem. reversibility and soly. of new quinone mols. as energy storage materials.
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Hernández-Burgos, K.; Burkhardt, S. E.; Rodríguez-Calero, G. G.; Hennig, R. G.; Abruña, H. D. Theoretical Studies of Carbonyl-Based Organic Molecules for Energy Storage Applications: The Heteroatom and Substituent Effect J. Phys. Chem. C 2014, 118, 6046– 6051 DOI: 10.1021/jp4117613
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Theoretical Studies of Carbonyl-Based Organic Molecules for Energy Storage Applications: The Heteroatom and Substituent Effect
Hernandez-Burgos, Kenneth; Burkhardt, Stephen E.; Rodriguez-Calero, Gabriel G.; Hennig, Richard G.; Abruna, Hector D.
Journal of Physical Chemistry C (2014), 118 (12), 6046-6051CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
Org. compds. represent an attractive choice for cathode materials in rechargeable Li batteries. Among all the org. functionalities, carbonyl-based org. mols. (C-bOMs) exhibit rapid and generally chem. reversible electrochem. behavior, and their reduced forms (enolates) can have strong ionic interactions with small radii cations (such as Li+). Also, a wide range of chem. variations/modifications can be performed on C-bOM structures via synthesis. The authors have systematically studied how to modify their electrochem. behavior by shifting the formal potential, maximizing the interaction of the various redox forms with Li ions, and maximizing the no. of electrons transferred while minimizing the mol. wt. of the compd., thus maximizing their gravimetric energy d. The authors have performed d.-functional calcns. to predict the formal potentials of the C-bOMs materials (E = 2.0-4.0 V) and identify the most promising candidates. The addn. of electron-withdrawing and -donating groups can be used to tune the formal potentials and Li ion binding energies. Also, by using the LUMO energy levels and the aromaticity, which was calcd. with nuclear independent chem. shift (NICS), it was possible to study the stability of these systems. Also, the authors were able to design and computationally characterize new C-bOMs mols., which represent new potentially high gravimetric energy d. cathode materials for elec. energy storage applications.
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Pineda Flores, S. D.; Martin-Noble, G. C.; Phillips, R. L.; Schrier, J. Bio-Inspired Electroactive Organic Molecules for Aqueous Redox Flow Batteries. 1. Thiophenoquinones J. Phys. Chem. C 2015, 119, 21800– 21809 DOI: 10.1021/acs.jpcc.5b05346
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Bio-Inspired Electroactive Organic Molecules for Aqueous Redox Flow Batteries. 1. Thiophenoquinones
Pineda Flores, Sergio D.; Martin-Noble, Geoffrey C.; Phillips, Richard L.; Schrier, Joshua
Journal of Physical Chemistry C (2015), 119 (38), 21800-21809CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
Redox flow batteries (RFB) utilizing water-sol. org. redox couples are a new strategy for low-cost, eco-friendly, and durable stationary elec. energy storage. Previous studies have focused on benzoquinones, naphthoquinones, and anthraquinones as the electroactive species. Here, we explore a new class of mols.-thiophenoquinones-specifically focusing on the caldariellaquinone-, sulfolobusquinone-, and benzodithiophenoquinone-like frameworks that are used for metabolic processes in thermophilic aerobic Sulfolobus archaebacteria. We demonstrated that B3LYP/6-311+G(d,p) thermochem. calcns. (using the SMD solvation model) reproduce exptl. redn. potentials to within ±0.04 V. We then studied the effect of amine, hydroxyl, Me, fluoro, phosphonic acid, sulfonic acid, carboxylic acid, and nitro functional group modifications on the redn. potential and Gibbs energy of solvation in water (using d. functional theory) and aq. soly. (using cheminformatics). Next we enumerated all of the 10 611 possible combinations of functional group substitutions on these frameworks and identified 1056 potential mols. with solubilities exceeding 2 mol/L; of these, 36 mols. have redn. potentials below 0.25 V and 15 mols. above 0.95 V (vs. the std. hydrogen electrode (SHE)). The combination of high soly. and wide voltage range makes these mols. promising candidates for high performance aq. RFB applications. Finally, using our data set of ab initio redn. potentials, we developed a cheminformatics model that predicts ab initio redn. potentials to within ±0.09 V based solely on mol. connectivity. We found that a model trained with as few as 200 examples generates rank-ordered predictions allowed us to identify the highest performance candidates with half the no. of ab initio calcns. This offers a strategy for improving the tractability of future computational searches for high performance RFB mols.
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Karlsson, C.; Jämstorp, E.; Strømme, M.; Sjödin, M. Computational electrochemistry study of 16 isoindole-4, 7-diones as candidates for organic cathode materials J. Phys. Chem. C 2012, 116, 3793– 3801 DOI: 10.1021/jp211851f
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Coote, M. L.; Lin, C. Y.; Beckwith, A. L. J.; Zavitsas, A. A. A comparison of methods for measuring relative radical stabilities of carbon-centred radicals Phys. Chem. Chem. Phys. 2010, 12, 9597– 9610 DOI: 10.1039/c003880f
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A comparison of methods for measuring relative radical stabilities of carbon-centered radicals
Coote, Michelle L.; Lin, Ching Yeh; Beckwith, Athelstan L. J.; Zavitsas, Andreas A.
Physical Chemistry Chemical Physics (2010), 12 (33), 9597-9610CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
This article discusses and compares various methods for defining and measuring radical stability, including the familiar radical stabilization energy (RSE), along with some lesser-known alternatives based on cor. carbon-carbon bond energies, and more direct measures of the extent of radical delocalization. As part of this work, a large set of R-H, R-CH3, R-Cl and R-R BDEs (R√ = √CH2X, √CH(CH3)X, √C(CH3)2X and X = H, BH2, CH3, NH2, OH, F, SiH3, PH2, SH, Cl, Br, N(CH3)2, NHCH3, NHCHO, NHCOCH3, NO2, OCF3, OCH2CH3, OCH3, OCHO, OCOCH3, Si(CH3)3, P(CH3)2, SC(CH3)2CN, SCH2COOCH3, SCH2COOCH3, SCH2Ph, SCH3, SO2CH3, S(O)CH3, Ph, C6H4-pCN, C6H4-pNO2, C6H4-pOCH3, C6H4-pOH, CF2CF3, CF2H, CF3, CCl2H, CCl3, CH2Cl, CH2F, CH2OH, CH2Ph, cyclo-CH(CH2)2, CH2CH:CH2, CH2CH3, CH(CH3)2, C(CH3)3, C≡CH, CH:CH2, CH:CHCH3, CHO, CN, COCH3, CON(CH2CH3)2, CONH2, CONHCH3, COOC(CH3)3, COOCH2CH3, COOCH3, COOH, COPh), and assocd. radical stability values are calcd. using the high-level ab initio MO theory method G3(MP2)-RAD. These are used to compare the alternative radical stability schemes and illustrate principal structure-reactivity trends.
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Cheng, L.; Assary, R. S.; Qu, X.; Jain, A.; Ong, S. P.; Rajput, N. N.; Persson, K.; Curtiss, L. A. Accelerating Electrolyte Discovery for Energy Storage with High-Throughput Screening J. Phys. Chem. Lett. 2015, 6, 283– 291 DOI: 10.1021/jz502319n
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Accelerating Electrolyte Discovery for Energy Storage with High-Throughput Screening
Cheng, Lei; Assary, Rajeev S.; Qu, Xiaohui; Jain, Anubhav; Ong, Shyue Ping; Rajput, Nav Nidhi; Persson, Kristin; Curtiss, Larry A.
Journal of Physical Chemistry Letters (2015), 6 (2), 283-291CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
A review. Computational screening techniques are an effective alternative to the trial and error of experimentation for discovery of new materials. With increased interest in development of advanced elec. energy storage systems, it is essential to find new electrolytes that function effectively. This Perspective reviews various methods for screening electrolytes and then describes a hierarchical computational scheme to screen multiple properties of advanced elec. energy storage electrolytes using high-throughput quantum chem. calcns. The approach effectively down-selects a large pool of candidates based on successive property evaluation. As an example, results of screening are presented for redox potentials, solvation energies, and structural changes of ∼1400 org. mols. for nonaq. redox flow batteries. Importantly, from high-throughput screening, in silico design of suitable candidate mols. for synthesis and electrochem. testing can be achieved. The authors anticipate that the computational approach described in this Perspective coupled with experimentation will have a significant role to play in the discovery of materials for future energy needs.
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Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Gaussian 09; Gaussian, Inc.: Wallingford, CT, U.S.A., 2009.
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Kelly, C. P.; Cramer, C. J.; Truhlar, D. G. Aqueous Solvation Free Energies of Ions and Ion-Water Clusters Based on an Accurate Value for the Absolute Aqueous Solvation Free Energy of the Proton J. Phys. Chem. B 2006, 110, 16066– 16081 DOI: 10.1021/jp063552y
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Aqueous Solvation Free Energies of Ions and Ion-Water Clusters Based on an Accurate Value for the Absolute Aqueous Solvation Free Energy of the Proton
Kelly, Casey P.; Cramer, Christopher J.; Truhlar, Donald G.
Journal of Physical Chemistry B (2006), 110 (32), 16066-16081CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
Thermochem. cycles that involve pKa, gas-phase acidities, aq. solvation free energies of neutral species, and gas-phase clustering free energies have been used with the cluster pair approxn. to det. the abs. aq. solvation free energy of the proton. The best value obtained in this work is in good agreement with the value reported by Tissandier et al. (Tissandier, M. D.; Cowen, K. A.; Feng, W. Y.; Gundlach, E.; Cohen, M. J.; Earhart, A. D.; Coe, J. V., J. Phys. Chem. A 1998, 102, 7787), who applied the cluster pair approxn. to a less diverse and smaller data set of ions. We agree with previous workers who advocated the value of -265.9 kcal/mol for the abs. aq. solvation free energy of the proton. Considering the uncertainties assocd. with the exptl. gas-phase free energies of ions that are required to use the cluster pair approxn. as well as analyses of various subsets of data, we est. an uncertainty for the abs. aq. solvation free energy of the proton of no less than 2 kcal/mol. Using a value of -265.9 kcal/mol for the abs. aq. solvation free energy of the proton, we expand and update our previous compilation of abs. aq. solvation free energies; this new data set contains conventional and abs. aq. solvation free energies for 121 unclustered ions (not including the proton) and 147 conventional and abs. aq. solvation free energies for 51 clustered ions contg. from 1 to 6 water mols. When tested against the same set of ions that was recently used to develop the SM6 continuum solvation model, SM6 retains its previously detd. high accuracy; indeed, in most cases the mean unsigned error improves when it is tested against the more accurate ref. data.
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Bhattacharyya, S.; Stankovich, M. T.; Truhlar, D. G.; Gao, J. Combined Quantum Mechanical and Molecular Mechanical Simulations of One- and Two-Electron Reduction Potentials of Flavin Cofactor in Water, Medium-Chain Acyl-CoA Dehydrogenase, and Cholesterol Oxidase J. Phys. Chem. A 2007, 111, 5729– 5742 DOI: 10.1021/jp071526+
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Combined Quantum Mechanical and Molecular Mechanical Simulations of One- and Two-Electron Reduction Potentials of Flavin Cofactor in Water, Medium-Chain Acyl-CoA Dehydrogenase, and Cholesterol Oxidase
Bhattacharyya, Sudeep; Stankovich, Marian T.; Truhlar, Donald G.; Gao, Jiali
Journal of Physical Chemistry A (2007), 111 (26), 5729-5742CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
FAD (FAD) is a common cofactor in redox proteins, and its redn. potentials are controlled by the protein environment. This regulation is mainly responsible for the versatile catalytic functions of flavoenzymes. In this article, we report computations of the redn. potentials of FAD in medium-chain acyl-CoA dehydrogenase (MCAD) and cholesterol oxidase (CHOX). In addn., the redn. potentials of lumiflavin in aq. soln. have also been computed. Using mol. dynamics and free-energy perturbation techniques, we obtained the free-energy changes for two-electron/two-proton as well as one-electron/one-proton addn. steps. We employed a combined quantum mech. and mol. mech. (QM/MM) potential, in which the flavin ring was represented by the self-consistent-charge d. functional tight-binding (SCC-DFTB) method, while the rest of the enzyme-solvent system was treated by classical force fields. The computed two-electron/two-proton redn. potentials for lumiflavin and the two enzyme-bound FADs are in reasonable agreement with exptl. data. The calcns. also yielded the pKa values for the one-electron reduced semiquinone (FH•) and the fully reduced hydroquinone (FH2) forms. The pKa of the FAD semiquinone in CHOX was found to be around 4, which is 4 units lower than that in the enzyme-free state and 2 units lower than that in MCAD; this supports the notion that oxidases have a greater ability than dehydrogenases to stabilize anionic semiquinones. In MCAD, the flavin ring interacts with four hydrophobic residues and has a significantly bent structure, even in the oxidized state. The present study shows that this bending of the flavin imparts a significant destabilization (∼5 kcal/mol) to the oxidized state. The redn. potential of lumiflavin was also computed using DFT (M06-L and B3LYP functionals with 6-31+G(d,p) basis set) with the SM6 continuum solvation model, and the results are in good agreement with results from explicit free-energy simulations, which supports the conclusion that the SCC-DFTB/MM computation is reasonably accurate for both 1e-/1H+ and 2e-/2H+ redn. processes. These results suggest that the first coupled electron-proton addn. is stepwise for both the free and the two enzyme-bound flavins. In contrast, the second coupled electron-proton addn. is also stepwise for the free flavin but is likely to be concerted when the flavin is bound to either the dehydrogenase or the oxidase enzyme.
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Guerard, J. J.; Arey, J. S. Critical Evaluation of Implicit Solvent Models for Predicting Aqueous Oxidation Potentials of Neutral Organic Compounds J. Chem. Theory Comput. 2013, 9, 5046– 5058 DOI: 10.1021/ct4004433
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Critical Evaluation of Implicit Solvent Models for Predicting Aqueous Oxidation Potentials of Neutral Organic Compounds
Guerard, Jennifer J.; Arey, J. Samuel
Journal of Chemical Theory and Computation (2013), 9 (11), 5046-5058CODEN: JCTCCE; ISSN:1549-9618. (American Chemical Society)
Quantum chem. implicit solvent models are used widely to est. aq. redox potentials. We compared the accuracy of several popular implicit solvent models (SM8, SMD, C-PCM, IEF-PCM, and COSMO-RS) for the prediction of aq. single electron oxidn. potentials of a diverse test set of neutral org. compds. for which accurate exptl. oxidn. potential and gas-phase ionization energy data are available. Using a thermodn. cycle, we decompd. the free energy of oxidn. into contributions arising from the gas-phase adiabatic ionization energy, the solvation free energy of the closed-shell neutral species, and the solvation free energy of the radical cation species. For aq. oxidn. potentials, implicit solvent models exhibited mean unsigned errors (MUEs) ranging from 0.27 to 0.50 V, depending on the model. The principal source of error was attributed to the computed solvation free energy of the oxidized radical cation. Based on these results, a recommended implicit solvation approach is the SMD model for the solvation free energy combined with CBS-QB3 for the gas-phase ionization energy. With this approach, the MUE in computed oxidn. potentials was 0.27 V, and the MUE in solvation free energy of the charged open-shell species was 0.32 eV. This baseline assessment provides a compiled benchmark test set of vetted exptl. data that may be used to judge newly developed solvation models for their ability to produce improved predictions for aq. oxidn. potentials and related properties.
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Moens, J.; Geerlings, P.; Roos, G. A Conceptual DFT Approach for the Evaluation and Interpretation of Redox Potentials Chem. - Eur. J. 2007, 13, 8174– 8184 DOI: 10.1002/chem.200601896
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A conceptual DFT approach for the evaluation and interpretation of redox potentials
Moens, Jan; Geerlings, Paul; Roos, Goedele
Chemistry - A European Journal (2007), 13 (29), 8174-8184CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
Conceptual DFT aims at describing the properties of mols. in interactions by using chem. reactivity descriptors. Herein, the redox behavior of a given species, as quantified by the redox potential, is linked to DFT-based descriptors. We made use of a hierarchical decompn. of the corresponding half-reactions into one-electron redn., protonation, dissocn. and water-forming or dissocn. reactions. Most of these reactions can be readily described through reactivity descriptors, such as the electrophilicity, nucleofugality and electrofugality, as defined in conceptual DFT. The final expression linking the corresponding free energy changes to the redox potential seems to give correct predictions for the redox potentials of bromo, chloro and nitro oxo acids in the gas phase, as in a polarized continuum model.
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Borodin, O.; Behl, W.; Jow, T. R. Oxidative Stability and Initial Decomposition Reactions of Carbonate, Sulfone, and Alkyl Phosphate-Based Electrolytes J. Phys. Chem. C 2013, 117, 8661– 8682 DOI: 10.1021/jp400527c
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Oxidative Stability and Initial Decomposition Reactions of Carbonate, Sulfone, and Alkyl Phosphate-Based Electrolytes
Borodin, Oleg; Behl, Wishvender; Jow, T. Richard
Journal of Physical Chemistry C (2013), 117 (17), 8661-8682CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
The oxidative stability and initial oxidn.-induced decompn. reactions of common electrolyte solvents for batteries and elec. double layer capacitors were studied using quantum chem. (QC) calcns. The studied electrolytes consisted of linear (DMC, EMC) and cyclic carbonate (EC, PC, VC), sulfone (TMS), sulfonate, and alkyl phosphate solvents paired with BF4-, PF6-, bis(fluorosulfonyl)imide (FSI-), difluoro-(oxalato)borate (DFOB-), dicyanotriazolate (DCTA-), and B(CN)4- anions. Most QC calcns. were performed using the M05-2X, LC-ωPBE d. functional and compared with the G4MP2 results where feasible. The calcd. oxidn. potentials were compared with previous and new exptl. data. The intrinsic oxidn. potential of most solvent mols. is higher than exptl. values for electrolytes even after the solvation contribution was included in the QC calcns. via a polarized continuum model. The presence of BF4-, PF6-, B(CN)4-, and FSI- anions near the solvents significantly decreases the oxidative stability of many solvents due to the spontaneous or low barrier (for FSI-) H- and F-abstraction reaction that followed the initial electron removal step. Such spontaneous H-abstraction reactions were not obsd. for the solvent complexes with DCTA- or DFOB- anions or for VC/anion, TMP/PF6- complexes. Spontaneous H-transfer reactions were also found for dimers of the oxidized carbonates (EC, DMC), alkyl phosphates (TMP), while low barrier H-transfer was found for dimers of sulfones (TMS and EMS). These reactions resulted in a significant decrease of the dimer oxidn. potential compared to the oxidn. potential of the isolated solvent mols. The presence of anions or an explicitly included solvent mol. next to the oxidized solvent mols. also reduced the barriers for the oxidn.-induced decompn. reaction and often changed the decompn. products. When a Li+ cation polarized the solvent in the ECn/LiBF4 and ECn/LiPF6 complexes, the complex oxidn. potential was 0.3-0.6 eV higher than the oxidn. potential of ECn/BF4- and ECn/PF6-.
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Vollmer, J. M.; Curtiss, L. A.; Vissers, D. R.; Amine, K. Reduction Mechanisms of Ethylene, Propylene, and Vinylethylene Carbonates: A Quantum Chemical Study J. Electrochem. Soc. 2004, 151, A178– A183 DOI: 10.1149/1.1633765
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Reduction Mechanisms of Ethylene, Propylene, and Vinylethylene Carbonates
Vollmer, James M.; Curtiss, Larry A.; Vissers, Donald R.; Amine, Khalil
Journal of the Electrochemical Society (2004), 151 (1), A178-A183CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
Quantum chem. methods were used to study redn. mechanisms of ethylene carbonate (EC), propylene carbonate (PC), and vinylethylene carbonate (VEC), in electrolyte solns. The feasibility of direct 2-electron redn. of these species was assessed, and for VEC no barriers to the reactions were found for the formation of Li2CO3 and 1,4-butadiene. In contrast EC and PC have barriers to reactions of ∼0.5 eV. The ready formation of Li2CO3 when VEC is reduced may explain why it acts as a good passivating agent in Li-ion batteries.
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Kelly, C. P.; Cramer, C. J.; Truhlar, D. G. Single-Ion Solvation Free Energies and the Normal Hydrogen Electrode Potential in Methanol, Acetonitrile, and Dimethyl Sulfoxide J. Phys. Chem. B 2007, 111, 408– 422 DOI: 10.1021/jp065403l
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Single-Ion Solvation Free Energies and the Normal Hydrogen Electrode Potential in Methanol, Acetonitrile, and Dimethyl Sulfoxide
Kelly, Casey P.; Cramer, Christopher J.; Truhlar, Donald G.
Journal of Physical Chemistry B (2007), 111 (2), 408-422CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
The division of thermodn. solvation free energies of electrolytes into contributions from individual ionic constituents is conventionally accomplished by using the single-ion solvation free energy of one ref. ion, conventionally the proton, to set the single-ion scales. Thus, the detn. of the free energy of solvation of the proton in various solvents is a fundamental issue of central importance in soln. chem. In the present article, relative solvation free energies of ions and ion-solvent clusters in methanol, acetonitrile, and DMSO have been detd. using a combination of exptl. and theor. gas-phase free energies of formation, soln.-phase redn. potentials and acid dissocn. consts., and gas-phase clustering free energies. Applying the cluster pair approxn. to differences between these relative solvation free energies leads to values of -263.5, -260.2, and -273.3 kcal/mol for the abs. solvation free energy of the proton in methanol, acetonitrile, and DMSO, resp. The final abs. proton solvation free energies are used to assign abs. values for the normal hydrogen electrode potential and the solvation free energies of other single ions in the solvents mentioned above.
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The Li-S battery (secondary cell or redox flow) technol. is a promising future alternative to the present Li intercalation-based energy storage, and, therefore, a mol. level understanding of the chem. processes and properties such as stability of intermediates, reactivity of polysulfides, and reactivity toward the nonaq. electrolytes in the Li-S batteries is of great interest. Quantum chem. methods (G4MP2, MP2, and B3LYP) were used to compute redn. potentials of Li polysulfides and polysulfide mol. clusters, energetics of disproportionation and assocn. reactions of likely intermediates, and their reactions with ether-based electrolytes. Based on the computed reaction energetics in soln., a probable mechanism during the discharge process for polysulfide anions and Li polysulfides in soln. is proposed and likely intermediates such as S42-, S32-, S22-, and S31- radical were identified. Addnl., the stability and reactivity of propylene carbonate and tetraglyme solvent mols. were assessed against the above-mentioned intermediates and other reactive species by computing the reaction energetics required to initiate the solvent decompn. reactions in soln. Calcns. suggest that the propylene carbonate mol. is unstable against the polysulfide anions such as S22-, S32-, and S42- (ΔH† < 0.8 eV) and highly reactive toward Li2S2 and Li2S3. Even though the tetraglyme solvent mol. exhibits increased stability toward polysulfide anions compared to propylene carbonate, this mol. too is vulnerable to nucleophilic attack from Li2S2 and Li2S3 species in solns. Hence, long-term stability of the ether mols. is unlikely if a high concn. of these reactive intermediates is present in the Li-S energy storage systems.
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Kelly, Casey P.; Cramer, Christopher J.; Truhlar, Donald G.
Journal of Physical Chemistry B (2007), 111 (2), 408-422CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
The division of thermodn. solvation free energies of electrolytes into contributions from individual ionic constituents is conventionally accomplished by using the single-ion solvation free energy of one ref. ion, conventionally the proton, to set the single-ion scales. Thus, the detn. of the free energy of solvation of the proton in various solvents is a fundamental issue of central importance in soln. chem. In the present article, relative solvation free energies of ions and ion-solvent clusters in methanol, acetonitrile, and DMSO have been detd. using a combination of exptl. and theor. gas-phase free energies of formation, soln.-phase redn. potentials and acid dissocn. consts., and gas-phase clustering free energies. Applying the cluster pair approxn. to differences between these relative solvation free energies leads to values of -263.5, -260.2, and -273.3 kcal/mol for the abs. solvation free energy of the proton in methanol, acetonitrile, and DMSO, resp. The final abs. proton solvation free energies are used to assign abs. values for the normal hydrogen electrode potential and the solvation free energies of other single ions in the solvents mentioned above.
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Physical Chemistry Chemical Physics (2012), 14 (48), 16603-16611CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
Mol. level understanding of acid-catalyzed conversion of sugar mols. to platform chems. such as hydroxy-Me furfural (HMF), furfuryl alc. (FAL), and levulinic acid (LA) is essential for efficient biomass conversion. In this paper, the high-level G4MP2 method along with the SMD solvation model is employed to understand detailed reaction energetics of the acid-catalyzed decompn. of glucose and fructose to HMF. Based on protonation free energies of various hydroxyl groups of the sugar mol., the relative reactivity of gluco-pyranose, fructo-pyranose and fructo-furanose are predicted. Calcns. suggest that, in addn. to the protonated intermediates, a solvent assisted dehydration of one of the fructo-furanosyl intermediates is a competing mechanism, indicating the possibility of multiple reaction pathways for fructose to HMF conversion in aq. acidic medium. Two reaction pathways were explored to understand the thermodn. of glucose to HMF; the first one is initiated by the protonation of a C2-OH group and the second one through an enolate intermediate involving acyclic intermediates. Addnl., a pathway is proposed for the formation of furfuryl alc. from glucose initiated by the protonation of a C2-OH position, which includes a C-C bond cleavage, and the formation of formic acid. The detailed free energy landscapes predicted in this study can be used as benchmarks for further exploring the sugar decompn. reactions, prediction of possible intermediates, and finally designing improved catalysts for biomass conversion chem. in the future.
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Abstract
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Figure 1
Figure 1. Schematic of dimethoxybenzene (DMB) derivatives considered in this study.
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Figure 2
Figure 2. Key properties controlling the stability of redox active species and the quantum chemical descriptors that can assess the properties are schematically shown. Note: G, E, and λ represent Gibbs free energy, redox potential, and reorganization energy, respectively.
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Figure 3
Figure 3. Schematic of the computation of reorganization energy during oxidation (λ_ox) and reduction (λ_red) process from the computed energies of a molecule in its neutral and singly positive charge state. Total reorganization energy ((λ_total) is the sum of reorganization energy required during the oxidation (λ_ox) and reduction (λ_red) process.
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Figure 4
Figure 4. Comparison of computed oxidation potentials (E^oxn V unit w.r.t. Li/Li⁺ ref electrode), reorganization energy during oxidation (λ^ox), and root-mean-square deviation (RMSD) between the optimized xyz coordinates of neutral and oxidized states of various DMB derivatives (x-axis). Note: The data associated with the plot is given in Table 1.
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Figure 5
Figure 5. Schematic of selected reactions (rxn1–4) of 1,4-dimethoxybenzene (DMB) radical cation. Abbreviation: PC, propylene carbonate molecule. All reactions are modeled in aqueous dielectric medium. In rxn1, the cation radical reacts with water to form solvated proton and neutral radical. In rxn4, two cation radicals interact with water to form a dimer and two solvated protons.
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Figure 6
Figure 6. Computed solution phase enthalpy profile (ΔH_soln) of demethylation of the 25-DMB cation radical by a propylene carbonate molecule. Computed apparent enthalpy of activation (ΔH^‡) of all DMB candidates are given in Table 3.
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Figure 7
Figure 7. Comparison of the computed free energies changes (ΔG(CH₃⁺)_PC) and activation enthalpies (ΔH^‡(CH₃⁺)_PC) required for the demethylation of DMB cation radicals by propylene carbonate. The data associated with (ΔG(CH₃⁺)_PC) and (ΔH^‡(CH₃⁺)_PC) are given in Tables 2 and 3, respectively.
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  Huskinson, Brian; Marshak, Michael P.; Suh, Changwon; Er, Sueleyman; Gerhardt, Michael R.; Galvin, Cooper J.; Chen, Xudong; Aspuru-Guzik, Alan; Gordon, Roy G.; Aziz, Michael J.
  Nature (London, United Kingdom) (2014), 505 (7482), 195-198CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
  A class of energy storage materials that exploits the favorable chem. and electrochem. properties of a family of mols. known as quinones is described. The metal-free flow battery based on the redox chem. of 9,10-anthraquinone-2,7-disulfonic acid (AQDS) is demonstrated. AQDS undergoes extremely rapid and reversible 2-electron 2-proton redn. on a glassy C electrode in H2SO4. An aq. flow battery with inexpensive C electrodes, combining the quinone/hydroquinone couple with the Br2/Br- redox couple, yields a peak galvanic power d. exceeding 0.6 W cm-2 at 1.3 A cm-2. Cycling of this quinone-bromide flow battery showed >99% storage capacity retention per cycle. The org. anthraquinone species can be synthesized from inexpensive commodity chems. This org. approach permits tuning of important properties such as the redn. potential and soly. by adding functional groups: for example, the addn. of 2 hydroxy groups to AQDS increases the open circuit potential of the cell by 11% and a pathway for further increases in cell voltage is described. The use of π-arom. redox-active org. mols. instead of redox-active metals represents a new and promising direction for realizing massive elec. energy storage at greatly reduced cost.
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5. 5
  Lin, K.; Chen, Q.; Gerhardt, M. R.; Tong, L.; Kim, S. B.; Eisenach, L.; Valle, A. W.; Hardee, D.; Gordon, R. G.; Aziz, M. J.; Marshak, M. P. Alkaline Quinone Flow Battery Science 2015, 349, 1529– 1532 DOI: 10.1126/science.aab3033
  5
  Alkaline quinone flow battery
  Lin, Kaixiang; Chen, Qing; Gerhardt, Michael R.; Tong, Liuchuan; Kim, Sang Bok; Eisenach, Louise; Valle, Alvaro W.; Hardee, David; Gordon, Roy G.; Aziz, Michael J.; Marshak, Michael P.
  Science (Washington, DC, United States) (2015), 349 (6255), 1529-1532CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
  Storage of photovoltaic and wind electricity in batteries could solve the mismatch problem between the intermittent supply of these renewable resources and variable demand. Flow batteries permit more economical long-duration discharge than solid-electrode batteries by using liq. electrolytes stored outside of the battery. We report an alk. flow battery based on redox-active org. mols. that are composed entirely of Earth-abundant elements and are nontoxic, nonflammable, and safe for use in residential and com. environments. The battery operates efficiently with high power d. near room temp. These results demonstrate the stability and performance of redox-active org. mols. in alk. flow batteries, potentially enabling cost-effective stationary storage of renewable energy.
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6. 6
  Yang, B.; Hoober-Burkhardt, L.; Wang, F.; Surya Prakash, G. K.; Narayanan, S. R. An Inexpensive Aqueous Flow Battery for Large-Scale Electrical Energy Storage Based on Water-Soluble Organic Redox Couples J. Electrochem. Soc. 2014, 161, A1371– A1380 DOI: 10.1149/2.1001409jes
  6
  An Inexpensive Aqueous Flow Battery for Large-Scale Electrical Energy Storage Based on Water-Soluble Organic Redox Couples
  Yang, Bo; Hoober-Burkhardt, Lena; Wang, Fang; Surya Prakash, G. K.; Narayanan, S. R.
  Journal of the Electrochemical Society (2014), 161 (9), A1371-A1380CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
  We introduce a novel Org. Redox Flow Battery (ORBAT), for meeting the demanding requirements of cost, eco-friendliness, and durability for large-scale energy storage. ORBAT employs two different water-sol. org. redox couples on the pos. and neg. side of a flow battery. Redox couples such as quinones are particularly attractive for this application. No precious metal catalyst is needed because of the fast proton-coupled electron transfer processes. Furthermore, in acid media, the quinones exhibit good chem. stability. These properties render quinone-based redox couples very attractive for high-efficiency metal-free rechargeable batteries. We demonstrate the rechargeability of ORBAT with anthraquinone-2-sulfonic acid or anthraquinone-2,6-disulfonic acid on the neg. side, and 1,2-dihydrobenzoquinone-3,5-disulfonic acid on the pos. side. The ORBAT cell uses a membrane-electrode assembly configuration similar to that used in polymer electrolyte fuel cells. Such a battery can be charged and discharged multiple times at high faradaic efficiency without any noticeable degrdn. of performance. We show that soly. and mass transport properties of the reactants and products are paramount to achieving high current densities and high efficiency. The ORBAT configuration presents a unique opportunity for developing an inexpensive and sustainable metal-free rechargeable battery for large-scale elec. energy storage.
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7. 7
  Liu, T.; Wei, X.; Nie, Z.; Sprenkle, V.; Wang, W. A Total Organic Aqueous Redox Flow Battery Employing a Low Cost and Sustainable Methyl Viologen Anolyte and 4-HO-TEMPO Catholyte Adv. Energy Mater. 2016, 6, 1501449– 1501456 DOI: 10.1002/aenm.201501449
  There is no corresponding record for this reference.
8. 8
  Wei, X.; Cosimbescu, L.; Xu, W.; Hu, J. Z.; Vijayakumar, M.; Feng, J.; Hu, M. Y.; Deng, X.; Xiao, J.; Liu, J.; Sprenkle, V.; Wang, W. Towards High-Performance Nonaqueous Redox Flow Electrolyte Via Ionic Modification of Active Species Adv. Energy Mater. 2015, 5, 1400678– 1400685 DOI: 10.1002/aenm.201400678
  8
  Towards High-Performance Nonaqueous Redox Flow Electrolyte Via Ionic Modification of Active Species
  Wei, Xiaoliang; Cosimbescu, Lelia; Xu, Wu; Hu, Jian Zhi; Vijayakumar, M.; Feng, Ju; Hu, Mary Y.; Deng, Xuchu; Xiao, Jie; Liu, Jun; Sprenkle, Vincent; Wang, Wei
  Advanced Energy Materials (2015), 5 (1), 1400678/1-1400678/7CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)
  Nonaq. redox flow batteries are emerging flow-based energy storage technologies that have the potential for higher energy densities than their aq. counterparts because of their wider voltage windows. However, their performance has lagged far behind their inherent capability due to one major limitation of low soly. of the redox species. Here, a mol. structure engineering strategy towards high performance nonaq. electrolyte is reported with significantly increased soly. Its performance outweighs that of the state-of-the-art nonaq. redox flow batteries. In particular, an ionic-derivatized ferrocene compd. is designed and synthesized that has more than 20 times increased soly. in the supporting electrolyte. The solvation chem. of the modified ferrocene compd. Electrochem. cycling testing in a hybrid lithium-org. redox flow battery using the as-synthesized ionic-derivatized ferrocene as the catholyte active material demonstrates that the incorporation of the ionic-charged pendant significantly improves the system energy d. When coupled with a lithium-graphite hybrid anode, the hybrid flow battery exhibits a cell voltage of 3.49 V, energy d. about 50 Wh L-1, and energy efficiency over 75%. These results reveal a generic design route towards high performance nonaq. electrolyte by rational functionalization of the org. redox species with selective ligand.
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9. 9
  Brushett, F. R.; Vaughey, J. T.; Jansen, A. N. An All-Organic Non-aqueous Lithium-Ion Redox Flow Battery Adv. Energy Mater. 2012, 2, 1390– 1396 DOI: 10.1002/aenm.201200322
  9
  An all-organic non-aqueous lithium-ion redox flow battery
  Brushett, Fikile R.; Vaughey, John T.; Jansen, Andrew N.
  Advanced Energy Materials (2012), 2 (11), 1390-1396CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)
  A non-aq. lithium-ion redox flow battery employing org. mols. is proposed and investigated. 2,5-Di-tert-butyl-1,4-bis(2-methoxyethoxy)benzene and a variety of mols. derived from quinoxaline are employed as initial high-potential and low-potential active materials, resp. Electrochem. measurements highlight that the choice of electrolyte and of substituent groups can have a significant impact on redox species performance. The charge-discharge characteristics are investigated in a modified coin-cell configuration. After an initial break-in period, coulombic and energy efficiencies for this unoptimized system are ∼70% and ∼37%, resp., with major charge and discharge plateaus between 1.8-2.4 V and 1.7-1.3 V, resp., for 30 cycles. Performance enhancements are expected with improvements in cell design and materials processing.
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10. 10
  Gong, K.; Fang, Q.; Gu, S.; Li, S. F. Y.; Yan, Y. Nonaqueous Redox-flow Batteries: Organic Solvents, Supporting electrolytes, and Redox pairs Energy Environ. Sci. 2015, 8, 3515– 3530 DOI: 10.1039/C5EE02341F
  There is no corresponding record for this reference.
11. 11
  Sevov, C. S.; Brooner, R. E. M.; Chénard, E.; Assary, R. S.; Moore, J. S.; Rodríguez-López, J.; Sanford, M. S. Evolutionary Design of Low Molecular Weight Organic Anolyte Materials for Applications in Nonaqueous Redox Flow Batteries J. Am. Chem. Soc. 2015, 137, 14465– 14472 DOI: 10.1021/jacs.5b09572
  11
  Evolutionary Design of Low Molecular Weight Organic Anolyte Materials for Applications in Nonaqueous Redox Flow Batteries
  Sevov, Christo S.; Brooner, Rachel E. M.; Chenard, Etienne; Assary, Rajeev S.; Moore, Jeffrey S.; Rodriguez-Lopez, Joaquin; Sanford, Melanie S.
  Journal of the American Chemical Society (2015), 137 (45), 14465-14472CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  The integration of renewable energy sources into the elec. grid requires low-cost energy storage systems that mediate the variable and intermittent flux of energy assocd. with most renewables. Nonaq. redox-flow batteries have emerged as a promising technol. for grid-scale energy storage applications. Because the cost of the system scales with mass, the electroactive materials must have a low equiv. wt. (ideally 150 g/(mol·e-) or less), and must function with low mol. wt. supporting electrolytes such as LiBF4. However, sol. anolyte materials that undergo reversible redox processes in the presence of Li-ion supports are rare. We report the evolutionary design of a series of pyridine-based anolyte materials that exhibit up to two reversible redox couples at low potentials in the presence of Li-ion supporting electrolytes. A combination of cyclic voltammetry of anolyte candidates and independent synthesis of their corresponding charged-states was performed to rapidly screen for the most promising candidates. Results of this workflow provided evidence for possible decompn. pathways of first-generation materials and guided synthetic modifications to improve the stability of anolyte materials under the targeted conditions. This iterative process led to the identification of a promising anolyte material, N-Me 4-acetylpyridinium tetrafluoroborate. This compd. is sol. in nonaq. solvents, is prepd. in a single synthetic step, has a low equiv. wt. of 111 g/(mol·e-), and undergoes two reversible 1e- redns. in the presence of LiBF4 to form reduced products that are stable over days in soln.
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12. 12
  Buhrmester, C.; Chen, J.; Moshurchak, L.; Jiang, J.; Wang, R. L.; Dahn, J. R. Studies of Aromatic Redox Shuttle Additives for LiFePO4-Based Li-Ion Cells J. Electrochem. Soc. 2005, 152, A2390– A2399 DOI: 10.1149/1.2098265
  12
  Studies of Aromatic Redox Shuttle Additives for LiFePO4-Based Li-Ion Cells
  Buhrmester, Claudia; Chen, Jun; Moshurchak, Lee; Jiang, Junwei; Wang, Richard Liangchen; Dahn, J. R.
  Journal of the Electrochemical Society (2005), 152 (12), A2390-A2399CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
  Fifty eight arom. org. mols. were screened as chem. shuttles to provide overcharge protection for LiFePO4/graphite and LiFePO4/Li4/3Ti5/3O4 Li-ion cells. The majority of the mols. were based on methoxybenzene and on dimethoxybenzene with a variety of ligands added to explore their effect. The added ligands affect the redox potential of the mols. through their electron-withdrawing effect and affect the stability of the radical cation. Of all the mols. tested, only 2,5-di-tert-butyl-1,4-dimethoxybenzene shows an appropriate redox potential of 3.9 V vs. Li/Li+ and long-term stability during extended abusive overcharge totaling over 300 cycles of 100% overcharge per cycle. The reasons for the success of this mol. are explored.
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13. 13
  Chen, J.; Buhrmester, C.; Dahn, J. R. Chemical Overcharge and Overdischarge Protection for Lithium-Ion Batteries Electrochem. Solid-State Lett. 2005, 8, A59– A62 DOI: 10.1149/1.1836119
  13
  Chemical Overcharge and Overdischarge Protection for Lithium-Ion Batteries
  Chen, Jun; Buhrmester, Claudia; Dahn, J. R.
  Electrochemical and Solid-State Letters (2005), 8 (1), A59-A62CODEN: ESLEF6; ISSN:1099-0062. (Electrochemical Society)
  Lithium-ion batteries suitable for the mass consumer market require robust safety and tolerance to repeated overdischarge and overcharge to avoid costly charge control circuitry and to allow simple replacement of individual cells by consumers. A chem. redox shuttle electrolyte additive is shown to provide this protection. Using 2,5-di-tert-butyl-1,4-dimethoxybenzene as electrolyte additive provides overcharge and overdischarge protection for hundreds of charge-discharge cycles in both single cells and series-connected batteries.
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14. 14
  Zhang, L.; Zhang, Z.; Redfern, P. C.; Curtiss, L. A.; Amine, K. Molecular Engineering towards Safer Lithium-ion Batteries: A highly Stable and Compatible Redox shuttle for Overcharge protection Energy Environ. Sci. 2012, 5, 8204– 8207 DOI: 10.1039/c2ee21977h
  14
  Molecular engineering towards safer lithium-ion batteries: a highly stable and compatible redox shuttle for overcharge protection
  Zhang, Lu; Zhang, Zhengcheng; Redfern, Paul C.; Curtiss, Larry A.; Amine, Khalil
  Energy & Environmental Science (2012), 5 (8), 8204-8207CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
  Overcharge abuse is one of the most common and dangerous safety issues with state-of-the-art lithium-ion batteries. Thus, incorporation of overcharge prevention into the lithium-ion battery pack is key to its practical application. Redox shuttle mols. that can be reversibly oxidized and reduced at specific potentials (redox potential) provide an effective and economic method to prevent overcharge abuse for lithium-ion batteries. We have developed a novel oligo(ethylene glycol)-functionalized redox shuttle, 2,5-di-tert-butyl-1,4-bis(2-methoxyethoxy)benzene (DBBB), that is not only capable of providing efficient and long-lasting overcharge protection to lithium-ion batteries (capable of withstanding over 180 cycles of 100% overcharge at the C/2 rate), but is also compatible with the state-of-the-art lithium-ion cell system. D. functional theory calcns. provided an understanding of the stability properties of this new redox shuttle.
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15. 15
  Zhang, L.; Zhang, Z.; Wu, H.; Amine, K. Novel redox shuttle Additive for High-Voltage Cathode Materials Energy Environ. Sci. 2011, 4, 2858– 2862 DOI: 10.1039/c0ee00733a
  15
  Novel redox shuttle additive for high-voltage cathode materials
  Zhang, Lu; Zhang, Zhengcheng; Wu, Huiming; Amine, Khalil
  Energy & Environmental Science (2011), 4 (8), 2858-2862CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
  A high voltage redox shuttle additive, tetraethyl-2,5-di-tert-butyl-1,4-phenylene diphosphate (TEDBPDP), has been synthesized and explored as an overcharge protection additive for lithium-ion cells. Cyclic voltammetry results indicate that the new shuttle mol. exhibits an oxidn. potential at 4.8 V vs. Li/Li+, the highest one of any redox shuttles ever synthesized and reported in the literature. The charge-discharge tests for lithium ion cells with LiMn2O4 and Li1.2Ni0.15Co0.1Mn0.55O2 as cathode materials indicated that the TEDBPDP additive can provide successful overcharge protection at 4.75 V vs. Li/Li+, which is a suitable redox shuttle additive for high voltage cathode materials. In addn., the incorporated organophosphate groups in the mol. structure can provide an addnl. safety feature as a flame retardant additive, making this high-voltage redox shuttle even more attractive.
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16. 16
  Zhang, Z.; Zhang, L.; Schlueter, J. A.; Redfern, P. C.; Curtiss, L.; Amine, K. Understanding the Redox shuttle stability of 3,5-di-tert-butyl-1,2-dimethoxybenzene for Overcharge Protection of Lithium-ion Batteries J. Power Sources 2010, 195, 4957– 4962 DOI: 10.1016/j.jpowsour.2010.02.075
  16
  Understanding the redox shuttle stability of 3,5-di-tert-butyl-1,2-dimethoxybenzene for overcharge protection of lithium-ion batteries
  Zhang, Zhengcheng; Zhang, Lu; Schlueter, John A.; Redfern, Paul C.; Curtiss, Larry; Amine, Khalil
  Journal of Power Sources (2010), 195 (15), 4957-4962CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
  3,5-Di-tert-butyl-1,2-dimethoxybenzene (DBDB) has been synthesized as a new redox shuttle additive for overcharge protection of lithium-ion batteries. DBDB can easily dissolve in carbonate-based electrolytes, which facilitates its practical use in lithium-ion batteries; however, it has poor electrochem. stability compared to 2,5-di-tert-butyl-1,4-dimethoxybenzene (DDB). The structures of DBDB and DDB were investigated using X-ray crystallog. and d. functional calcns. The structures differ in the conformations of the alkoxy bonds probably due to the formation of an intramol. hydrogen bond in the case of DBDB. We investigated reaction energies for decompn. pathways of neutral DBDB and DDB and their radical cations and found little difference in the reaction energies, although it is clear that kinetically, decompn. of DBDB is more favorable.
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17. 17
  Speelman, A. L.; Gillmore, J. G. Efficient Computational Methods for Accurately Predicting Reduction Potentials of Organic Molecules J. Phys. Chem. A 2008, 112, 5684– 5690 DOI: 10.1021/jp800782e
  17
  Efficient Computational Methods for Accurately Predicting Reduction Potentials of Organic Molecules
  Speelman, Amy L.; Gillmore, Jason G.
  Journal of Physical Chemistry A (2008), 112 (25), 5684-5690CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
  A simple computational approach for predicting ground-state redn. potentials based upon gas phase geometry optimizations at a moderate level of d. functional theory followed by single-point energy calcns. at higher levels of theory in the gas phase or with polarizable continuum solvent models is described. Energies of the gas phase optimized geometries of the S and one-electron-reduced D states of 35 planar arom. org. mols. spanning three distinct families of org. photooxidants are computed in the gas phase as well as well in implicit solvent with IPCM and CPCM solvent models. Correlation of the D - S energy difference (essentially an electron affinity) with exptl. redn. potentials from the literature (in acetonitrile vs SCE) within a single family, or across families when solvent models are used, yield correlations with r2 values in excess of 0.97 and residuals of about 100 mV or less, without resorting to computationally expensive vibrational calcns. or thermodn. cycles.
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18. 18
  Er, S.; Suh, C.; Marshak, M. P.; Aspuru-Guzik, A. Computational Design of Molecules for an all-Quinone Redox Flow Battery Chem. Sci. 2015, 6, 885– 893 DOI: 10.1039/C4SC03030C
  18
  Computational design of molecules for an all-quinone redox flow battery
  Er, Suleyman; Suh, Changwon; Marshak, Michael P.; Aspuru-Guzik, Alan
  Chemical Science (2015), 6 (2), 885-893CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
  Inspired by the electron transfer properties of quinones in biol. systems, we recently showed that quinones are also very promising electroactive materials for stationary energy storage applications. Due to the practically infinite chem. space of org. mols., the discovery of addnl. quinones or other redox-active org. mols. for energy storage applications is an open field of inquiry. Here, we introduce a high-throughput computational screening approach that we applied to an accelerated study of a total of 1710 quinone (Q) and hydroquinone (QH2) (i.e., two-electron two-proton) redox couples. We identified the promising candidates for both the neg. and pos. sides of org.-based aq. flow batteries, thus enabling an all-quinone battery. To further aid the development of addnl. interesting electroactive small mols. we also provide emerging quant. structure-property relationships.
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19. 19
  Assary, R. S.; Brushett, F. R.; Curtiss, L. A. Reduction Potential Predictions of some Aromatic Nitrogen-Containing Molecules RSC Adv. 2014, 4, 57442– 57451 DOI: 10.1039/C4RA08563A
  19
  Reduction potential predictions of some aromatic nitrogen-containing molecules
  Assary, Rajeev S.; Brushett, Fikile R.; Curtiss, Larry A.
  RSC Advances (2014), 4 (101), 57442-57451CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
  Accurate quantum chem. methods offer a reliable alternative to time-consuming exptl. evaluations for obtaining a priori electrochem. knowledge of a large no. of redox active mols. In this contribution, quantum chem. calcns. are performed to investigate the redox behavior of quinoxalines, a promising family of active materials for non-aq. flow batteries, as a function of substituent group. The redn. potentials of 40 quinoxaline derivs. with a range of electron-donating and electron-withdrawing groups are computed. Calcns. indicate the addn. of electron-donating groups, particularly alkyl groups, can significantly lower the redn. potential albeit with a concomitant decrease in oxidative stability. A simple descriptor is derived for computing redn. potentials of quinoxaline derivs. from the LUMO energies of the neutral mols. without time-consuming free energy calcns. The relationship was validated for a broader set of arom. nitrogen-contg. mols. including pyrazine, phenazine, bipyridine, pyridine, pyrimidine, pyridazine, and quinoline, suggesting that it is a good starting point for large high-throughput computations to screen redn. windows of novel mols.
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20. 20
  Bachman, J. E.; Curtiss, L. A.; Assary, R. S. Investigation of the Redox Chemistry of Anthraquinone Derivatives Using Density Functional Theory J. Phys. Chem. A 2014, 118, 8852– 8860 DOI: 10.1021/jp5060777
  20
  Investigation of the Redox Chemistry of Anthraquinone Derivatives Using Density Functional Theory
  Bachman, Jonathan E.; Curtiss, Larry A.; Assary, Rajeev S.
  Journal of Physical Chemistry A (2014), 118 (38), 8852-8860CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
  Application of d. functional calcns. to compute electrochem. properties such as redox windows, effect of substitution by electron donating and electron withdrawing groups on redox windows, and solvation free energies for ∼50 anthraquinone (AQ) derivs. are presented because of their potential as anolytes in all-org. redox flow batteries. Computations suggest that lithium ions can increase (by ∼0.4 V) the redn. potential of anthraquinone due to the lithium ion pairing by forming a Lewis base-Lewis acid complex. To design new redox active species, the substitution by electron donating groups is essential to improve the redn. window of AQ with adequate oxidative stability. For instance, a complete methylation of AQ can improve its redn. window by ∼0.4 V. The quantum chem. studies of the ∼50 AQ derivs. are used to derive a relationship that connects the computed LUMO energy and the redn. potential that can be applied as a descriptor for screening thousands of AQ derivs. Our computations also suggest that incorporating oxy-Me dioxolane substituents in the AQ framework can increase its interaction with nonaq. solvent and improve its soly. Thermochem. calcns. for likely bond breaking decompn. reactions of unsubstituted AQ anions suggest that the dianions are relatively stable in the soln. These studies provide an ideal platform to perform further combined exptl. and theor. studies to understand the electrochem. reversibility and soly. of new quinone mols. as energy storage materials.
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21. 21
  Hernández-Burgos, K.; Burkhardt, S. E.; Rodríguez-Calero, G. G.; Hennig, R. G.; Abruña, H. D. Theoretical Studies of Carbonyl-Based Organic Molecules for Energy Storage Applications: The Heteroatom and Substituent Effect J. Phys. Chem. C 2014, 118, 6046– 6051 DOI: 10.1021/jp4117613
  21
  Theoretical Studies of Carbonyl-Based Organic Molecules for Energy Storage Applications: The Heteroatom and Substituent Effect
  Hernandez-Burgos, Kenneth; Burkhardt, Stephen E.; Rodriguez-Calero, Gabriel G.; Hennig, Richard G.; Abruna, Hector D.
  Journal of Physical Chemistry C (2014), 118 (12), 6046-6051CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
  Org. compds. represent an attractive choice for cathode materials in rechargeable Li batteries. Among all the org. functionalities, carbonyl-based org. mols. (C-bOMs) exhibit rapid and generally chem. reversible electrochem. behavior, and their reduced forms (enolates) can have strong ionic interactions with small radii cations (such as Li+). Also, a wide range of chem. variations/modifications can be performed on C-bOM structures via synthesis. The authors have systematically studied how to modify their electrochem. behavior by shifting the formal potential, maximizing the interaction of the various redox forms with Li ions, and maximizing the no. of electrons transferred while minimizing the mol. wt. of the compd., thus maximizing their gravimetric energy d. The authors have performed d.-functional calcns. to predict the formal potentials of the C-bOMs materials (E = 2.0-4.0 V) and identify the most promising candidates. The addn. of electron-withdrawing and -donating groups can be used to tune the formal potentials and Li ion binding energies. Also, by using the LUMO energy levels and the aromaticity, which was calcd. with nuclear independent chem. shift (NICS), it was possible to study the stability of these systems. Also, the authors were able to design and computationally characterize new C-bOMs mols., which represent new potentially high gravimetric energy d. cathode materials for elec. energy storage applications.
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22. 22
  Pineda Flores, S. D.; Martin-Noble, G. C.; Phillips, R. L.; Schrier, J. Bio-Inspired Electroactive Organic Molecules for Aqueous Redox Flow Batteries. 1. Thiophenoquinones J. Phys. Chem. C 2015, 119, 21800– 21809 DOI: 10.1021/acs.jpcc.5b05346
  22
  Bio-Inspired Electroactive Organic Molecules for Aqueous Redox Flow Batteries. 1. Thiophenoquinones
  Pineda Flores, Sergio D.; Martin-Noble, Geoffrey C.; Phillips, Richard L.; Schrier, Joshua
  Journal of Physical Chemistry C (2015), 119 (38), 21800-21809CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
  Redox flow batteries (RFB) utilizing water-sol. org. redox couples are a new strategy for low-cost, eco-friendly, and durable stationary elec. energy storage. Previous studies have focused on benzoquinones, naphthoquinones, and anthraquinones as the electroactive species. Here, we explore a new class of mols.-thiophenoquinones-specifically focusing on the caldariellaquinone-, sulfolobusquinone-, and benzodithiophenoquinone-like frameworks that are used for metabolic processes in thermophilic aerobic Sulfolobus archaebacteria. We demonstrated that B3LYP/6-311+G(d,p) thermochem. calcns. (using the SMD solvation model) reproduce exptl. redn. potentials to within ±0.04 V. We then studied the effect of amine, hydroxyl, Me, fluoro, phosphonic acid, sulfonic acid, carboxylic acid, and nitro functional group modifications on the redn. potential and Gibbs energy of solvation in water (using d. functional theory) and aq. soly. (using cheminformatics). Next we enumerated all of the 10 611 possible combinations of functional group substitutions on these frameworks and identified 1056 potential mols. with solubilities exceeding 2 mol/L; of these, 36 mols. have redn. potentials below 0.25 V and 15 mols. above 0.95 V (vs. the std. hydrogen electrode (SHE)). The combination of high soly. and wide voltage range makes these mols. promising candidates for high performance aq. RFB applications. Finally, using our data set of ab initio redn. potentials, we developed a cheminformatics model that predicts ab initio redn. potentials to within ±0.09 V based solely on mol. connectivity. We found that a model trained with as few as 200 examples generates rank-ordered predictions allowed us to identify the highest performance candidates with half the no. of ab initio calcns. This offers a strategy for improving the tractability of future computational searches for high performance RFB mols.
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23. 23
  Karlsson, C.; Jämstorp, E.; Strømme, M.; Sjödin, M. Computational electrochemistry study of 16 isoindole-4, 7-diones as candidates for organic cathode materials J. Phys. Chem. C 2012, 116, 3793– 3801 DOI: 10.1021/jp211851f
  There is no corresponding record for this reference.
24. 24
  Coote, M. L.; Lin, C. Y.; Beckwith, A. L. J.; Zavitsas, A. A. A comparison of methods for measuring relative radical stabilities of carbon-centred radicals Phys. Chem. Chem. Phys. 2010, 12, 9597– 9610 DOI: 10.1039/c003880f
  24
  A comparison of methods for measuring relative radical stabilities of carbon-centered radicals
  Coote, Michelle L.; Lin, Ching Yeh; Beckwith, Athelstan L. J.; Zavitsas, Andreas A.
  Physical Chemistry Chemical Physics (2010), 12 (33), 9597-9610CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
  This article discusses and compares various methods for defining and measuring radical stability, including the familiar radical stabilization energy (RSE), along with some lesser-known alternatives based on cor. carbon-carbon bond energies, and more direct measures of the extent of radical delocalization. As part of this work, a large set of R-H, R-CH3, R-Cl and R-R BDEs (R√ = √CH2X, √CH(CH3)X, √C(CH3)2X and X = H, BH2, CH3, NH2, OH, F, SiH3, PH2, SH, Cl, Br, N(CH3)2, NHCH3, NHCHO, NHCOCH3, NO2, OCF3, OCH2CH3, OCH3, OCHO, OCOCH3, Si(CH3)3, P(CH3)2, SC(CH3)2CN, SCH2COOCH3, SCH2COOCH3, SCH2Ph, SCH3, SO2CH3, S(O)CH3, Ph, C6H4-pCN, C6H4-pNO2, C6H4-pOCH3, C6H4-pOH, CF2CF3, CF2H, CF3, CCl2H, CCl3, CH2Cl, CH2F, CH2OH, CH2Ph, cyclo-CH(CH2)2, CH2CH:CH2, CH2CH3, CH(CH3)2, C(CH3)3, C≡CH, CH:CH2, CH:CHCH3, CHO, CN, COCH3, CON(CH2CH3)2, CONH2, CONHCH3, COOC(CH3)3, COOCH2CH3, COOCH3, COOH, COPh), and assocd. radical stability values are calcd. using the high-level ab initio MO theory method G3(MP2)-RAD. These are used to compare the alternative radical stability schemes and illustrate principal structure-reactivity trends.
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25. 25
  Cheng, L.; Assary, R. S.; Qu, X.; Jain, A.; Ong, S. P.; Rajput, N. N.; Persson, K.; Curtiss, L. A. Accelerating Electrolyte Discovery for Energy Storage with High-Throughput Screening J. Phys. Chem. Lett. 2015, 6, 283– 291 DOI: 10.1021/jz502319n
  25
  Accelerating Electrolyte Discovery for Energy Storage with High-Throughput Screening
  Cheng, Lei; Assary, Rajeev S.; Qu, Xiaohui; Jain, Anubhav; Ong, Shyue Ping; Rajput, Nav Nidhi; Persson, Kristin; Curtiss, Larry A.
  Journal of Physical Chemistry Letters (2015), 6 (2), 283-291CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
  A review. Computational screening techniques are an effective alternative to the trial and error of experimentation for discovery of new materials. With increased interest in development of advanced elec. energy storage systems, it is essential to find new electrolytes that function effectively. This Perspective reviews various methods for screening electrolytes and then describes a hierarchical computational scheme to screen multiple properties of advanced elec. energy storage electrolytes using high-throughput quantum chem. calcns. The approach effectively down-selects a large pool of candidates based on successive property evaluation. As an example, results of screening are presented for redox potentials, solvation energies, and structural changes of ∼1400 org. mols. for nonaq. redox flow batteries. Importantly, from high-throughput screening, in silico design of suitable candidate mols. for synthesis and electrochem. testing can be achieved. The authors anticipate that the computational approach described in this Perspective coupled with experimentation will have a significant role to play in the discovery of materials for future energy needs.
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26. 26
  Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Gaussian 09; Gaussian, Inc.: Wallingford, CT, U.S.A., 2009.
  There is no corresponding record for this reference.
27. 27
  Kelly, C. P.; Cramer, C. J.; Truhlar, D. G. Aqueous Solvation Free Energies of Ions and Ion-Water Clusters Based on an Accurate Value for the Absolute Aqueous Solvation Free Energy of the Proton J. Phys. Chem. B 2006, 110, 16066– 16081 DOI: 10.1021/jp063552y
  27
  Aqueous Solvation Free Energies of Ions and Ion-Water Clusters Based on an Accurate Value for the Absolute Aqueous Solvation Free Energy of the Proton
  Kelly, Casey P.; Cramer, Christopher J.; Truhlar, Donald G.
  Journal of Physical Chemistry B (2006), 110 (32), 16066-16081CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
  Thermochem. cycles that involve pKa, gas-phase acidities, aq. solvation free energies of neutral species, and gas-phase clustering free energies have been used with the cluster pair approxn. to det. the abs. aq. solvation free energy of the proton. The best value obtained in this work is in good agreement with the value reported by Tissandier et al. (Tissandier, M. D.; Cowen, K. A.; Feng, W. Y.; Gundlach, E.; Cohen, M. J.; Earhart, A. D.; Coe, J. V., J. Phys. Chem. A 1998, 102, 7787), who applied the cluster pair approxn. to a less diverse and smaller data set of ions. We agree with previous workers who advocated the value of -265.9 kcal/mol for the abs. aq. solvation free energy of the proton. Considering the uncertainties assocd. with the exptl. gas-phase free energies of ions that are required to use the cluster pair approxn. as well as analyses of various subsets of data, we est. an uncertainty for the abs. aq. solvation free energy of the proton of no less than 2 kcal/mol. Using a value of -265.9 kcal/mol for the abs. aq. solvation free energy of the proton, we expand and update our previous compilation of abs. aq. solvation free energies; this new data set contains conventional and abs. aq. solvation free energies for 121 unclustered ions (not including the proton) and 147 conventional and abs. aq. solvation free energies for 51 clustered ions contg. from 1 to 6 water mols. When tested against the same set of ions that was recently used to develop the SM6 continuum solvation model, SM6 retains its previously detd. high accuracy; indeed, in most cases the mean unsigned error improves when it is tested against the more accurate ref. data.
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28. 28
  Bhattacharyya, S.; Stankovich, M. T.; Truhlar, D. G.; Gao, J. Combined Quantum Mechanical and Molecular Mechanical Simulations of One- and Two-Electron Reduction Potentials of Flavin Cofactor in Water, Medium-Chain Acyl-CoA Dehydrogenase, and Cholesterol Oxidase J. Phys. Chem. A 2007, 111, 5729– 5742 DOI: 10.1021/jp071526+
  28
  Combined Quantum Mechanical and Molecular Mechanical Simulations of One- and Two-Electron Reduction Potentials of Flavin Cofactor in Water, Medium-Chain Acyl-CoA Dehydrogenase, and Cholesterol Oxidase
  Bhattacharyya, Sudeep; Stankovich, Marian T.; Truhlar, Donald G.; Gao, Jiali
  Journal of Physical Chemistry A (2007), 111 (26), 5729-5742CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
  FAD (FAD) is a common cofactor in redox proteins, and its redn. potentials are controlled by the protein environment. This regulation is mainly responsible for the versatile catalytic functions of flavoenzymes. In this article, we report computations of the redn. potentials of FAD in medium-chain acyl-CoA dehydrogenase (MCAD) and cholesterol oxidase (CHOX). In addn., the redn. potentials of lumiflavin in aq. soln. have also been computed. Using mol. dynamics and free-energy perturbation techniques, we obtained the free-energy changes for two-electron/two-proton as well as one-electron/one-proton addn. steps. We employed a combined quantum mech. and mol. mech. (QM/MM) potential, in which the flavin ring was represented by the self-consistent-charge d. functional tight-binding (SCC-DFTB) method, while the rest of the enzyme-solvent system was treated by classical force fields. The computed two-electron/two-proton redn. potentials for lumiflavin and the two enzyme-bound FADs are in reasonable agreement with exptl. data. The calcns. also yielded the pKa values for the one-electron reduced semiquinone (FH•) and the fully reduced hydroquinone (FH2) forms. The pKa of the FAD semiquinone in CHOX was found to be around 4, which is 4 units lower than that in the enzyme-free state and 2 units lower than that in MCAD; this supports the notion that oxidases have a greater ability than dehydrogenases to stabilize anionic semiquinones. In MCAD, the flavin ring interacts with four hydrophobic residues and has a significantly bent structure, even in the oxidized state. The present study shows that this bending of the flavin imparts a significant destabilization (∼5 kcal/mol) to the oxidized state. The redn. potential of lumiflavin was also computed using DFT (M06-L and B3LYP functionals with 6-31+G(d,p) basis set) with the SM6 continuum solvation model, and the results are in good agreement with results from explicit free-energy simulations, which supports the conclusion that the SCC-DFTB/MM computation is reasonably accurate for both 1e-/1H+ and 2e-/2H+ redn. processes. These results suggest that the first coupled electron-proton addn. is stepwise for both the free and the two enzyme-bound flavins. In contrast, the second coupled electron-proton addn. is also stepwise for the free flavin but is likely to be concerted when the flavin is bound to either the dehydrogenase or the oxidase enzyme.
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29. 29
  Guerard, J. J.; Arey, J. S. Critical Evaluation of Implicit Solvent Models for Predicting Aqueous Oxidation Potentials of Neutral Organic Compounds J. Chem. Theory Comput. 2013, 9, 5046– 5058 DOI: 10.1021/ct4004433
  29
  Critical Evaluation of Implicit Solvent Models for Predicting Aqueous Oxidation Potentials of Neutral Organic Compounds
  Guerard, Jennifer J.; Arey, J. Samuel
  Journal of Chemical Theory and Computation (2013), 9 (11), 5046-5058CODEN: JCTCCE; ISSN:1549-9618. (American Chemical Society)
  Quantum chem. implicit solvent models are used widely to est. aq. redox potentials. We compared the accuracy of several popular implicit solvent models (SM8, SMD, C-PCM, IEF-PCM, and COSMO-RS) for the prediction of aq. single electron oxidn. potentials of a diverse test set of neutral org. compds. for which accurate exptl. oxidn. potential and gas-phase ionization energy data are available. Using a thermodn. cycle, we decompd. the free energy of oxidn. into contributions arising from the gas-phase adiabatic ionization energy, the solvation free energy of the closed-shell neutral species, and the solvation free energy of the radical cation species. For aq. oxidn. potentials, implicit solvent models exhibited mean unsigned errors (MUEs) ranging from 0.27 to 0.50 V, depending on the model. The principal source of error was attributed to the computed solvation free energy of the oxidized radical cation. Based on these results, a recommended implicit solvation approach is the SMD model for the solvation free energy combined with CBS-QB3 for the gas-phase ionization energy. With this approach, the MUE in computed oxidn. potentials was 0.27 V, and the MUE in solvation free energy of the charged open-shell species was 0.32 eV. This baseline assessment provides a compiled benchmark test set of vetted exptl. data that may be used to judge newly developed solvation models for their ability to produce improved predictions for aq. oxidn. potentials and related properties.
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30. 30
  Moens, J.; Geerlings, P.; Roos, G. A Conceptual DFT Approach for the Evaluation and Interpretation of Redox Potentials Chem. - Eur. J. 2007, 13, 8174– 8184 DOI: 10.1002/chem.200601896
  30
  A conceptual DFT approach for the evaluation and interpretation of redox potentials
  Moens, Jan; Geerlings, Paul; Roos, Goedele
  Chemistry - A European Journal (2007), 13 (29), 8174-8184CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Conceptual DFT aims at describing the properties of mols. in interactions by using chem. reactivity descriptors. Herein, the redox behavior of a given species, as quantified by the redox potential, is linked to DFT-based descriptors. We made use of a hierarchical decompn. of the corresponding half-reactions into one-electron redn., protonation, dissocn. and water-forming or dissocn. reactions. Most of these reactions can be readily described through reactivity descriptors, such as the electrophilicity, nucleofugality and electrofugality, as defined in conceptual DFT. The final expression linking the corresponding free energy changes to the redox potential seems to give correct predictions for the redox potentials of bromo, chloro and nitro oxo acids in the gas phase, as in a polarized continuum model.
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31. 31
  Borodin, O.; Behl, W.; Jow, T. R. Oxidative Stability and Initial Decomposition Reactions of Carbonate, Sulfone, and Alkyl Phosphate-Based Electrolytes J. Phys. Chem. C 2013, 117, 8661– 8682 DOI: 10.1021/jp400527c
  31
  Oxidative Stability and Initial Decomposition Reactions of Carbonate, Sulfone, and Alkyl Phosphate-Based Electrolytes
  Borodin, Oleg; Behl, Wishvender; Jow, T. Richard
  Journal of Physical Chemistry C (2013), 117 (17), 8661-8682CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
  The oxidative stability and initial oxidn.-induced decompn. reactions of common electrolyte solvents for batteries and elec. double layer capacitors were studied using quantum chem. (QC) calcns. The studied electrolytes consisted of linear (DMC, EMC) and cyclic carbonate (EC, PC, VC), sulfone (TMS), sulfonate, and alkyl phosphate solvents paired with BF4-, PF6-, bis(fluorosulfonyl)imide (FSI-), difluoro-(oxalato)borate (DFOB-), dicyanotriazolate (DCTA-), and B(CN)4- anions. Most QC calcns. were performed using the M05-2X, LC-ωPBE d. functional and compared with the G4MP2 results where feasible. The calcd. oxidn. potentials were compared with previous and new exptl. data. The intrinsic oxidn. potential of most solvent mols. is higher than exptl. values for electrolytes even after the solvation contribution was included in the QC calcns. via a polarized continuum model. The presence of BF4-, PF6-, B(CN)4-, and FSI- anions near the solvents significantly decreases the oxidative stability of many solvents due to the spontaneous or low barrier (for FSI-) H- and F-abstraction reaction that followed the initial electron removal step. Such spontaneous H-abstraction reactions were not obsd. for the solvent complexes with DCTA- or DFOB- anions or for VC/anion, TMP/PF6- complexes. Spontaneous H-transfer reactions were also found for dimers of the oxidized carbonates (EC, DMC), alkyl phosphates (TMP), while low barrier H-transfer was found for dimers of sulfones (TMS and EMS). These reactions resulted in a significant decrease of the dimer oxidn. potential compared to the oxidn. potential of the isolated solvent mols. The presence of anions or an explicitly included solvent mol. next to the oxidized solvent mols. also reduced the barriers for the oxidn.-induced decompn. reaction and often changed the decompn. products. When a Li+ cation polarized the solvent in the ECn/LiBF4 and ECn/LiPF6 complexes, the complex oxidn. potential was 0.3-0.6 eV higher than the oxidn. potential of ECn/BF4- and ECn/PF6-.
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32. 32
  Vollmer, J. M.; Curtiss, L. A.; Vissers, D. R.; Amine, K. Reduction Mechanisms of Ethylene, Propylene, and Vinylethylene Carbonates: A Quantum Chemical Study J. Electrochem. Soc. 2004, 151, A178– A183 DOI: 10.1149/1.1633765
  32
  Reduction Mechanisms of Ethylene, Propylene, and Vinylethylene Carbonates
  Vollmer, James M.; Curtiss, Larry A.; Vissers, Donald R.; Amine, Khalil
  Journal of the Electrochemical Society (2004), 151 (1), A178-A183CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
  Quantum chem. methods were used to study redn. mechanisms of ethylene carbonate (EC), propylene carbonate (PC), and vinylethylene carbonate (VEC), in electrolyte solns. The feasibility of direct 2-electron redn. of these species was assessed, and for VEC no barriers to the reactions were found for the formation of Li2CO3 and 1,4-butadiene. In contrast EC and PC have barriers to reactions of ∼0.5 eV. The ready formation of Li2CO3 when VEC is reduced may explain why it acts as a good passivating agent in Li-ion batteries.
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33. 33
  Kelly, C. P.; Cramer, C. J.; Truhlar, D. G. Single-Ion Solvation Free Energies and the Normal Hydrogen Electrode Potential in Methanol, Acetonitrile, and Dimethyl Sulfoxide J. Phys. Chem. B 2007, 111, 408– 422 DOI: 10.1021/jp065403l
  33
  Single-Ion Solvation Free Energies and the Normal Hydrogen Electrode Potential in Methanol, Acetonitrile, and Dimethyl Sulfoxide
  Kelly, Casey P.; Cramer, Christopher J.; Truhlar, Donald G.
  Journal of Physical Chemistry B (2007), 111 (2), 408-422CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
  The division of thermodn. solvation free energies of electrolytes into contributions from individual ionic constituents is conventionally accomplished by using the single-ion solvation free energy of one ref. ion, conventionally the proton, to set the single-ion scales. Thus, the detn. of the free energy of solvation of the proton in various solvents is a fundamental issue of central importance in soln. chem. In the present article, relative solvation free energies of ions and ion-solvent clusters in methanol, acetonitrile, and DMSO have been detd. using a combination of exptl. and theor. gas-phase free energies of formation, soln.-phase redn. potentials and acid dissocn. consts., and gas-phase clustering free energies. Applying the cluster pair approxn. to differences between these relative solvation free energies leads to values of -263.5, -260.2, and -273.3 kcal/mol for the abs. solvation free energy of the proton in methanol, acetonitrile, and DMSO, resp. The final abs. proton solvation free energies are used to assign abs. values for the normal hydrogen electrode potential and the solvation free energies of other single ions in the solvents mentioned above.
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34. 34
  Ho, J.; Coote, M. L. First-principles prediction of acidities in the gas and solution phase Wiley Interdisc. Rev.: Comput. Mol. Sci. 2011, 1, 649– 660 DOI: 10.1002/wcms.43
  34
  First-principles prediction of acidities in the gas and solution phase
  Ho, Junming; Coote, Michelle L.
  Wiley Interdisciplinary Reviews: Computational Molecular Science (2011), 1 (5), 649-660CODEN: WIRCAH; ISSN:1759-0884. (Wiley-Blackwell)
  A review. This paper provides an overview of contemporary computational protocols toward accurate prediction of acidities in the gas and aq. phase. The performance of various d. functional theory (DFT) methods and ab initio composite procedures, such as the G3MP2(+) method, for the prediction of gas-phase acidities of a range of neutral and cationic acids is presented. Various methods for soln. pKa predictions are also reviewed where the emphasis is on thermodn. cycle-based methods that combine ab initio or exptl. gas-phase energies with solvation free energies from continuum solvent models. The prediction of accurate solvation free energies, esp. for ionic species, represents the bottleneck for accurate pKa prediction via the direct or abs. method. The success and limitations of alternative thermodn. cycles are discussed and some of the difficulties and future challenges assocd. with the applications of these methods on more complicated mols. are also highlighted.
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35. 35
  Assary, R. S.; Curtiss, L. A.; Moore, J. S. Toward a Molecular Understanding of Energetics in Li–S Batteries Using Nonaqueous Electrolytes: A High-Level Quantum Chemical Study J. Phys. Chem. C 2014, 118, 11545– 11558 DOI: 10.1021/jp5015466
  35
  Toward a Molecular Understanding of Energetics in Li-S Batteries Using Nonaqueous Electrolytes: A High-Level Quantum Chemical Study
  Assary, Rajeev S.; Curtiss, Larry A.; Moore, Jeffrey S.
  Journal of Physical Chemistry C (2014), 118 (22), 11545-11558CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
  The Li-S battery (secondary cell or redox flow) technol. is a promising future alternative to the present Li intercalation-based energy storage, and, therefore, a mol. level understanding of the chem. processes and properties such as stability of intermediates, reactivity of polysulfides, and reactivity toward the nonaq. electrolytes in the Li-S batteries is of great interest. Quantum chem. methods (G4MP2, MP2, and B3LYP) were used to compute redn. potentials of Li polysulfides and polysulfide mol. clusters, energetics of disproportionation and assocn. reactions of likely intermediates, and their reactions with ether-based electrolytes. Based on the computed reaction energetics in soln., a probable mechanism during the discharge process for polysulfide anions and Li polysulfides in soln. is proposed and likely intermediates such as S42-, S32-, S22-, and S31- radical were identified. Addnl., the stability and reactivity of propylene carbonate and tetraglyme solvent mols. were assessed against the above-mentioned intermediates and other reactive species by computing the reaction energetics required to initiate the solvent decompn. reactions in soln. Calcns. suggest that the propylene carbonate mol. is unstable against the polysulfide anions such as S22-, S32-, and S42- (ΔH† < 0.8 eV) and highly reactive toward Li2S2 and Li2S3. Even though the tetraglyme solvent mol. exhibits increased stability toward polysulfide anions compared to propylene carbonate, this mol. too is vulnerable to nucleophilic attack from Li2S2 and Li2S3 species in solns. Hence, long-term stability of the ether mols. is unlikely if a high concn. of these reactive intermediates is present in the Li-S energy storage systems.
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36. 36
  Kabsch, W. A solution for the Best Rotation to Relate Two Sets of Vectors Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, 32, 922– 923 DOI: 10.1107/S0567739476001873
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  Schmittel, M.; Burghart, A. Understanding Reactivity Patterns of Radical Cations Angew. Chem., Int. Ed. Engl. 1997, 36, 2550– 2589 DOI: 10.1002/anie.199725501
  There is no corresponding record for this reference.
38. 38
  Kelly, C. P.; Cramer, C. J.; Truhlar, D. G. Single-Ion Solvation Free Energies and the Normal Hydrogen Electrode Potential in Methanol, Acetonitrile, and Dimethyl Sulfoxide J. Phys. Chem. B 2007, 111, 408– 422 DOI: 10.1021/jp065403l
  38
  Single-Ion Solvation Free Energies and the Normal Hydrogen Electrode Potential in Methanol, Acetonitrile, and Dimethyl Sulfoxide
  Kelly, Casey P.; Cramer, Christopher J.; Truhlar, Donald G.
  Journal of Physical Chemistry B (2007), 111 (2), 408-422CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
  The division of thermodn. solvation free energies of electrolytes into contributions from individual ionic constituents is conventionally accomplished by using the single-ion solvation free energy of one ref. ion, conventionally the proton, to set the single-ion scales. Thus, the detn. of the free energy of solvation of the proton in various solvents is a fundamental issue of central importance in soln. chem. In the present article, relative solvation free energies of ions and ion-solvent clusters in methanol, acetonitrile, and DMSO have been detd. using a combination of exptl. and theor. gas-phase free energies of formation, soln.-phase redn. potentials and acid dissocn. consts., and gas-phase clustering free energies. Applying the cluster pair approxn. to differences between these relative solvation free energies leads to values of -263.5, -260.2, and -273.3 kcal/mol for the abs. solvation free energy of the proton in methanol, acetonitrile, and DMSO, resp. The final abs. proton solvation free energies are used to assign abs. values for the normal hydrogen electrode potential and the solvation free energies of other single ions in the solvents mentioned above.
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39. 39
  Assary, R. S.; Kim, T.; Low, J. J.; Greeley, J.; Curtiss, L. A. Glucose and Fructose to Platform Chemicals: Understanding the Thermodynamic Landscapes of Acid-Catalysed Reactions Using High-Level Ab Initio Methods Phys. Chem. Chem. Phys. 2012, 14, 16603– 16611 DOI: 10.1039/c2cp41842h
  39
  Glucose and fructose to platform chemicals: understanding the thermodynamic landscapes of acid-catalysed reactions using high-level ab initio methods
  Assary, Rajeev S.; Kim, Taejin; Low, John J.; Greeley, Jeff; Curtiss, Larry A.
  Physical Chemistry Chemical Physics (2012), 14 (48), 16603-16611CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
  Mol. level understanding of acid-catalyzed conversion of sugar mols. to platform chems. such as hydroxy-Me furfural (HMF), furfuryl alc. (FAL), and levulinic acid (LA) is essential for efficient biomass conversion. In this paper, the high-level G4MP2 method along with the SMD solvation model is employed to understand detailed reaction energetics of the acid-catalyzed decompn. of glucose and fructose to HMF. Based on protonation free energies of various hydroxyl groups of the sugar mol., the relative reactivity of gluco-pyranose, fructo-pyranose and fructo-furanose are predicted. Calcns. suggest that, in addn. to the protonated intermediates, a solvent assisted dehydration of one of the fructo-furanosyl intermediates is a competing mechanism, indicating the possibility of multiple reaction pathways for fructose to HMF conversion in aq. acidic medium. Two reaction pathways were explored to understand the thermodn. of glucose to HMF; the first one is initiated by the protonation of a C2-OH group and the second one through an enolate intermediate involving acyclic intermediates. Addnl., a pathway is proposed for the formation of furfuryl alc. from glucose initiated by the protonation of a C2-OH position, which includes a C-C bond cleavage, and the formation of formic acid. The detailed free energy landscapes predicted in this study can be used as benchmarks for further exploring the sugar decompn. reactions, prediction of possible intermediates, and finally designing improved catalysts for biomass conversion chem. in the future.
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Supporting Information
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b04263.
- Optimized structures of DMB derivatives are shown in Figure S1. Figure 2 suggests most important structural parameters and these parameters are tabulated in Table S1. Figure S3 presents the computed deprotonation energies (rxn 1 of Figure 5) and dimerization energies (rxn 4 of the Figure 5) of radical cations at the B3LYP/6-31G(2df,p) level of theory, and Table S2 shows computed enthalpies and free energies of activation required for the demethylation reactions in water and diethyl ether dielectric media (PDF).
- jp6b04263_si_001.pdf (476.11 kb)
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Molecular Level Understanding of the Factors Affecting the Stability of Dimethoxy Benzene Catholyte Candidates from First-Principles InvestigationsClick to copy article linkArticle link copied!

The Journal of Physical Chemistry C

Abstract

1 Introduction

Figure 1

Figure 2

2 Computational Details

3 Results and Discussion

3.1 Structure

3.1.1 Simulated Oxidation Potentials (Eox)

3.1.2 Reorganization Energy (λ)

Figure 3

3.1.3 Root Mean Squared Deviation (RMSD) between the Geometries

Figure 4

3.2 Reactivity

3.2.1 Thermodynamic Feasibility of Chemical Reactions

Figure 5

3.2.2 Demethylation: Computed Activation Barriers

Figure 6

Figure 7

4 Summary

Supporting Information

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Acknowledgment

References

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The Journal of Physical Chemistry C

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Abstract

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

References

Supporting Information

Supporting Information

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Molecular Level Understanding of the Factors Affecting the Stability of Dimethoxy Benzene Catholyte Candidates from First-Principles Investigations
Click to copy article linkArticle link copied!

3.1.1 Simulated Oxidation Potentials (E^ox)