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ArticleAugust 8, 2016

DFT Modeling of the Adsorption of Trimethylphosphine Oxide at the Internal and External Surfaces of Zeolite MFI
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The Journal of Physical Chemistry C

Cite this: J. Phys. Chem. C 2016, 120, 34, 19097–19106

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https://doi.org/10.1021/acs.jpcc.6b03448

Published August 8, 2016

Abstract

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The characterization of the acidity of zeolites allows a direct correlation with their catalytic activity. To this end, probe molecules are utilized to obtain a ranking of acid strengths. Trimethylphosphine oxide (TMPO) is a widely used probe molecule, which allows the sensing of solid acids by using ³¹P NMR. We have performed calculations based on the density functional theory to investigate the Brønsted acid (BA) sites in zeolite MFI by adsorbing TMPO as a probe molecule. We have considered the substitution of silicon at the T2 site by aluminum, both at the internal cavity and at the external surface. The different acid strengths observed in the zeolite MFI when probed by TMPO (very strong, strong, and weak) may depend on the basicity of the centers sharing the acid proton. If the proton lies between the TMPO and one of the framework oxygen atoms binding the Al, the acidity is strong. When the framework oxygen atom is not directly binding the Al, it is less basic and a shortening of the TMPO–H distance is observed, causing an acid response of very strong. Finally, if two TMPO molecules share the proton, the TMPO–H distance elongates, rendering a weak acid character.

Subjects

1 Introduction

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Zeolites are well-known microporous materials whose frameworks are formed by corner-sharing SiO₄ tetrahedra. A negative charge is created when Al³⁺ substitutes Si⁴⁺ at the center of a tetrahedron, which is compensated by extraframework cations within the pore system of the zeolite. Brønsted acid (BA) sites are generated when the counterbalancing cation is a proton that covalently binds the O atom bridging the Al and the Si. (1) These BA sites, together with the size selectivity of the pore system, are the driving forces behind the wide range of catalytic applications of zeolites. (2-5)

The characterization of the zeolite’s acidity by number, density, and strength may allow a direct correlation to its catalytic activity. However, in contrast with an acid in an aqueous medium, there is no unique way to rank the acidity of solid materials. (6) In zeolites, the proficiency of each BA as a proton donor will depend on its location within the pore system and its accessibility by the adsorbed reactant, (6) which hinders the analysis of the zeolite’s acidity.

The strength and number of the BA sites are usually measured through the adsorption of probe molecules, which act as bases and are protonated upon interaction with the BA sites. There are several analytical methods to evaluate the extent of the protonation of the probe molecule, (1, 6) for example, the nuclear magnetic resonance (NMR), which provides high accuracy and specificity to examine the consequences of the proton transfer. (7) The movement of the acid proton from the BA site to the probe molecule may be sensed directly when recording the ¹H NMR spectrum, as in the case of the pyridine/pyridiniun system. (8) In addition, other nuclei different from ¹H may also be analyzed if their magnetic response to the applied field is affected by the protonation of the probe molecule, e.g., ¹³C in acetone (9) and ³¹P in trimethylphosphine oxide. (10)

Trimethylphosphine oxide (TMPO, see Figure 1 for a representation of the molecule’s structure) has shown to be a suitable molecule for sensing BA sites of different strength, owing to the high sensibility of the TMPO’s ³¹P nucleus to the intensity of the phosphine–BA interaction. (7) The ³¹P chemical shift moves from 39 ppm (crystalline TMPO) toward a range of 50–105 ppm upon protonation of the TMPO’s oxygen atom; the stronger the proton transfer, the larger the chemical shift. (7) Therefore, the strength of BA sites in zeolites can be classified according to the value of the ³¹P chemical shift as very strong (90–80 ppm), strong (80–70 ppm), and weak (70–60 ppm), where 86 ppm is the calculated threshold of superacidity for TMPO. (7, 10-12)

Figure 1. Representation of the BA sites at the internal (bottom, left panel) and at the external (bottom, right panel) surfaces. One of the two pentasil layers that form the slab is identified by a black-line rectangle (top, right panel). The Al-substituted T2-site (light blue ball) with the proton (white ball) at the O1 position (red ball) are shown. The rest of the O atoms and silanol OH groups were deleted for a better view; Si atoms are represented by orange sticks. A molecule of trimethylphosphine oxide is shown in the bottom-right corner; H is represented in white, C in gray, P in brown, and O in red.

The present work proposes a density functional theory (DFT) study of the adsorption of TMPO in zeolite MFI. Zeolite Socony Mobil-5 (ZSM-5) has the framework type of MFI, which is a material with a wide array of industrial applications and high versatility. (13-16) We have described in detail the TMPO–BA interaction at the internal and external surfaces of zeolite MFI, providing atomic level information to complement previous experimental reports. The MFI’s internal and external surfaces behave differently: three types of BA sites of variable strength have been detected at the internal surface (very strong, strong, and weak), whereas the strong BA sites tend to disappear at the external surface. (10, 12) Thus, to distinguish between these sites, alternative experimental methods must be used. For instance, silica chemical vapor deposition (Si-CVD) deactivates the external surface while keeping the internal sites undamaged, (10) whereas the inclusion of tributylphosphine oxide (TBPO), which is too big to diffuse into the zeolite’s pore system, allows the exclusive sensing of the external surface. (10, 12)

We have also explored how a variable number of TMPO molecules affects the acid response of the BA site. Zheng et al. have previously reported the enhancement of the Brønsted acidity in mordenite by the intermolecular solvent effect when several TMPO molecules are confined in the micropore. (17) The authors relate this effect to van der Waals interactions among the TMPO molecules, which provoke simultaneous decrease and increase of the O–H distance of the TMPO molecules, generating in consequence superacidity and weak acidity, respectively. (6, 7, 17)

2 Computational Methods

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The results presented in this work were obtained using the DFT approximation as implemented in the Vienna Ab-initio Simulation Package (VASP). (18-21) The generalized gradient approximation (GGA) under the scheme proposed by Perdew, Burke, and Ernzerhof (PBE) was used in all calculations. (22) Grimme’s method of the sum of pairs was added to the GGA energy to account for the long-range-interactions (DFT-D2). (23) The inclusion of dispersion corrections improves the predicted cell parameters and elastic properties of the zeolite MFI. (24) The valence electrons were treated explicitly using a basis set of plane waves, although their nodal features and interactions with the ion were included within the projected-augmented-wave method (PAW). (25, 26) The number of plane waves considered during the calculations was determined by a maximum kinetic energy of 500 eV. The electronic convergence was improved using the Gaussian smearing method with a bandwidth of 0.1 eV for the zeolite and zeolite–TMPO systems and 0.01 eV for the isolated TMPO molecule. (27, 28) Only the Gamma point was taken into account during the numerical integration over the Brillouin zone due to the large dimensions of the MFI unit cell. Finally, the electronic threshold was set to 10^–5 eV.

The orthorhombic unit cell of the MFI framework has 12 nonequivalent T-sites to be substituted by Al (the center of each tetrahedron is referred as T-site and numbered according to the symmetry of the zeolite’s unit cell). In conjunction, the acid proton can bind four O atoms for each T-site, which produces a high number of possible combinations, making practically impossible a detailed analysis of each arrangement. We have therefore opted for Al substitution in the T2-site, binding the proton to O1 (see Figure 1), which allowed us to focus on the acid response of that BA site to the number and orientation of TMPO molecules, as well as the consequences of the BA site location at the internal or the external surface.

The T2-site was chosen for the Al substitution, and the charge-compensating proton was bound to the O1 atom, leading to the formation of the BA site (see Figure 1). This specific position was chosen because T2 is at the interception of straight and sinusoidal channels and is easily accessible by the probe molecule, allowing a fair study of the TMPO agglomeration around the acid site. We have studied the proton transfer to TMPO, accommodating up to three molecules simultaneously. The optimized cell parameters of the zeolite MFI were 20.317, 19.979, and 13.413 Å along the a, b, and c directions, respectively, (24) within 1% of the experimental measurements. (29) Using periodic boundary conditions, the external surface was described by a slab formed of two pentasil layers, (30) as shown in Figure 1, with a surface area per unit cell of 272.512 Å² and a perpendicular vacuum gap of 20 Å between the slabs. In this termination, each dangling Si–O bond was properly saturated by hydroxyl, thus forming silanol groups at the bottom and top surfaces of the slab; in the case of pentasil layers, there is only one dangling Si–O bond per Si atom. (24) The isolated TMPO molecule was first optimized in a 20 × 21 × 22 Å³ cell, before it was loaded in the zeolite, varying its numbers and orientations, ahead of a full geometry optimization.

The initial geometries set for optimization were constructed by loading the TMPO molecules in close proximity to the acid proton of the zeolite. These geometries were locally optimized using a conjugate gradient algorithm until all forces acting on the ions were smaller than 0.03 eV/Å. The local relaxations were followed by short quantum molecular dynamics (MD) simulations over a simulation time of 10 ps, with a time step of 0.5 fs. A microcanonical ensemble was used for each geometry during the first 2.5 ps of MD while keeping the temperature at 300 K. A canonical ensemble centered at 300 K was simulated during the next 7.5 ps of MD, controlling the temperature fluctuation with a Nosé thermostat. (31-33) We have used the last 2.5 ps to obtain the average structural parameters of interest.

All the images related to structural and charge density visualizations were obtained with the code Visualization for Electronic and Structural Analysis (VESTA 3). (34)

2.1 Classification of the Acid Strength

Brønsted acid sites of different strength are detected in zeolite MFI when probed with TMPO. According to Seo et al., the acid strength can be classified as very strong (86 ppm), strong (76 ppm), and weak (68 and 66 ppm), (12) in agreement with previous experimental reports. (10) Zheng et al. reproduced the range of ³¹P chemical shifts by modeling the adsorption of TMPO on zeolite MFI, which was represented by a cluster of eight T-sites. (11) The authors tuned the acid strength by varying the terminal Si–H distances of the cluster, thus controlling the extent of the proton transfer from the BA site to the TMPO’s oxygen atom, hereafter referred to as O_(P). Zheng et al. reported a wide spectrum of O_(P)–H distances after optimization, calculating the ³¹P chemical shift for each geometry; those distances ranged from 1.368 Å (45.5 ppm) to 1.014 Å (88.9 ppm). (11)

We noted that our calculated O_(P)–H distances changed within the limits proposed by Zheng et al., and hence we assumed that the theoretical work in ref 11 matches the expected ³¹P chemical shift for our own data. Figure 2 shows the correlation between the O_(P)–H distance and the ³¹P chemical shift as calculated in ref 11. From that curve, we extrapolated the O_(P)–H distances corresponding to the experimental measurement of the ³¹P chemical shift that classifies the BA sites by strength (see Figure 2). (12) Taking the middle points between the extrapolated O_(P)–H distances, we obtained approximate ranges of O_(P)–H distances to classify the effective acid strength of our model (see Figure 2). The ranges are as follows: very strong acid (from 1.001 to 1.041 Å), strong acid (from 1.041 to 1.097 Å), and weak acid (from 1.097 to 1.188 Å). In comparison, we calculated a O_(P)–H distance of 0.978 Å for a fully protonated TMPO molecule loaded in the zeolite, for which the position of the P atom was fixed at the center of the pore interception of zeolite MFI to avoid any H-bonding with nearby framework O atoms. In addition, on the basis of Car–Parrinello MD of a hydronium ion in aqueous medium, Tuckerman et al. have reported an O–H distance of 1.3 Å for the shared proton between two H₂O molecules in the complex (H₅O₂)⁺. (35)

Figure 2. Correlation of the O_(P)–H distances and the ³¹P chemical shifts using Zheng et al. data (black squares linked with black lines). (11) The experimental classification of the acid sites of the zeolite MFI according to the TMPO ³¹P chemical shift is indicated by vertical dashed blue lines. (10, 12) The experimental classification is used to extrapolate the expected O_(P)–H distances from its interception with the theoretical curve (indicated by red circles); the corresponding distance values are written above the horizontal dashed black lines. The spectrum of O_(P)–H distances is divided into three zones taking the middle points between the extrapolated O_(P)–H distances. These zones are shaded alternately in light gray and white, corresponding to (top) weak acids, (center) strong acids, and (bottom) very strong acids; the limits of each range are indicated by the red numbers at the left-hand side of the graph.

3 Results and Discussion

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3.1 Adsorption of One TMPO Molecule

In this section we have presented the adsorption of a single TMPO molecule in close proximity to the BA proton. The TMPO was adsorbed on the internal and external surfaces of the zeolite by placing the O_(P) atom at 2.0 Å from the acidic proton. Two different orientations of the TMPO molecule with respect to the BA proton were tested, varying the values of the P–O_(P)–H and O_(P)–H–O1_(Al) angles (the label O1_(Al) refers to the framework O atom binding simultaneously the Al atom and the acidic proton; in the present case, position O1).

Table 1. Relevant Interatomic Distances (Å) of the 1TMPO/1BA Configurations Shown in Figure 3a–d after Local Optimization

	internal BA		external BA
	Figure 3a^a	Figure 3b	Figure 3c	Figure 3d
O_(P)–H	1.060 (2.000)	1.045 (2.000)	1.066 (2.000)	1.060 (2.000)
O_(Al)–H	1.459 (0.975)	1.483 (0.975)	1.429 (0.975)	1.465 (0.975)
O_(P)–O_(Al)	2.518 (2.975)	2.526 (2.225)	2.492 (2.975)	2.522 (2.225)
P–O_(P)^b	1.556	1.561	1.556	1.556

The values before the structural optimization are presented within parentheses.

The optimized P−O_(P) distance in gas phase is 1.494 Å.

Figure 3. Representation of the interaction of a single TMPO molecule with (a, b) the internal and (c, d) the external BA sites after local optimization; H in white, C in gray, P in brown, O in red, Al in light blue, and Si represented by orange sticks. All the framework O atoms (except the protonated one) and silanol OH groups were deleted for an enhanced view. Related structural values are presented in Table 1.

During the local relaxations, the proton was transferred from the BA site to the TMPO molecule, at both the internal and external surfaces, regardless of the initial configuration. The Bader analysis of atomic charges (36-38) confirmed the movement of the proton; the total charge obtained for protonated phosphine oxides ranged from +0.8 to 0.9 e^–, and more than 80% of the corresponding negative charge left in the zeolite framework was associated with the AlO₄ tetrahedron. Figure 3 shows the final equilibrium structures with the corresponding structural parameters listed in Table 1. The O_(P)–H bond length ranged from 1.045 to 1.066 Å, i.e., larger than in the MFI external silanol groups (<0.970 Å) and MFI acidic protons with intraframework H-bonds (<1.014 Å). (24) These longer bonds were due to strong H-bonds established between the protonated TMPO molecules and the O_(Al) atoms of the framework, with O_(P)–O_(Al) distances below 2.6 Å and H–O_(Al)–O_(P) angles smaller than 3°. (39) According to the division of the spectrum of possible O_(P)–H distances, shown in Figure 2, the acid strength of the BA site may be classified as strong for the four different configurations after local optimization. No evidence of very strong or weak acidity was obtained at this stage. The variation of the adsorption energy could not be correlated with corresponding changes in the O_(P)–H distance. Nonetheless, the average TMPO interaction energy was larger for the internal BA when compared with the external BA: −190 and −167 kJ/mol, respectively. The contribution of dispersion forces to the total adsorption energy was 47% and 38% for the internal and external BA site, respectively. However, these percentages should be viewed with reservation because the DFT-D2 method tends to overestimate attractive interactions owing to the omission of terms above the two-body contribution; the error can increase beyond 10% for supramolecular complexes. (40)

The MD simulation did not result in significant changes when compared to the local optimization; Table 2 shows the average values and the standard deviations of the last 2.5 ps. The average values of the O_(P)–H bond length remained within the range 1.05–1.08 Å, with fluctuations between 0.05 and 0.08 Å, which confirms that the strength of the modeled BA site can only be classified as strong when a single TMPO is adsorbed. Furthermore, we observed the replacement of O1_(Al) by O2_(Al) as the acceptor of the H-bond for the configuration in Figure 3b (see Figure 4 for a representation of the structure after 10 ps of MD). This movement occurred during the first 2.5 ps of MD simulation, although the acid response continued to be strong.

Table 2. Relevant Interatomic Distances (Å) after 10 ps of MD Simulation Taking the 1TMPO/1BA Configurations Shown in Figure 3a–d as Input Geometries

	internal BA		external BA
	Figure 3a^a	Figure 3b	Figure 3c	Figure 3d
O_(P)–H	1.06 ± 0.05	1.05 ± 0.05	1.08 ± 0.08	1.07 ± 0.06
O_(Al)–H	1.5 ± 0.1	1.6 ± 0.2	1.5 ± 0.1	1.5 ± 0.1
O_(P)–O_(Al)	2.51 ± 0.09	2.6 ± 0.1	2.53 ± 0.09	2.53 ± 0.09
P–O_(P)	1.56 ± 0.03	1.57 ± 0.03	1.57 ± 0.04	1.57 ± 0.03

Average and standard deviation over the last 2.5 ps of MD simulation.

Figure 4. Configuration after 10 ps of MD simulation of the locally optimized structure in Figure 3b.

3.2 Adsorption of Two TMPO Molecules

We next adsorbed two TMPO molecules at a short distance from the BA site to analyze their effect on the proton transfer. Figure 5 shows the most stable structures after local optimization, out of several tested configurations at the internal and external BA sites; related structural parameters are compiled in Table 3. The O_(P)–H bond length of the configurations in Figure 5 decreased to 1.016 and 1.033 Å for the internal and external surfaces, respectively, suggesting an apparent stronger acidity triggered by the presence of the second TMPO molecule. In consequence, the acid strength of the internal and external acid sites can be classified as very strong according to the scheme in Figure 2.

Figure 5. Representation of the adsorption of two TMPO molecules on (a) the internal and (b) the external BA sites after local optimization. H in white, C in gray, P in brown, O in red, Al in light blue, and Si represented by orange sticks. All the framework O atoms (except the protonated one) and silanol OH groups were deleted for an enhanced view. Related structural values are presented in Table 3.

Table 3. Relevant Interatomic Distances (Å) of the 2TMPO/1BA Configurations Shown in Figure 5a,b after Local Optimization

	internal BA	external BA
	Figure 5a^a	Figure 5b
O_(P1)–H	1.016 (2.000)	1.033 (2.000)
O_(P2)–H	2.406 (2.000)	2.746 (2.000)
O_(P1)–O_(P2)	2.868 (2.462)	2.908 (2.462)
O_(Al)–H	1.635 (0.975)	1.541 (0.975)
O_(P1)–O_(Al)	2.547 (2.372)	2.544 (2.372)
P–O_(P1)	1.561	1.555
P–O_(P2)	1.504	1.499

The values before the structural optimization are presented within parentheses.

We have calculated average adsorption energies per TMPO of −125 kJ/mol for the internal surface and −112 kJ/mol for the external surface, with the dispersion forces representing 84% and 54% of the total adsorption energy, respectively, indicating that the role of van der Waals interactions increases with the number of probe molecules.

The second TMPO is a source of steric hindrance that affects the interaction through H-bonding between the first TMPO and the BA site. Because the protonated TMPO cannot form an ideal orientation in front of the bridging O atom, the H-bond is not strong enough, which produces a further shift of the proton position toward O_(P1). We did not observe any evidence that the second TMPO interfered through direct competition for the proton: the O_(P2)–H distance increased above 2.5 Å during local optimization, which diminishes the possibility of H-bonding.

In contrast to the adsorption of a single TMPO molecule, where only strong acids are observed after both local optimization and MD, the systems composed of two TMPO molecules continued evolving during the MD simulation, modifying the acid strength classification given by the local optimization. At the internal surface, the movement of the TMPO molecules is capped owing to the reduced space inside the pore system. Therefore, we noticed a small shift of the protonated TMPO from O1_(Al) to O6_(Al), which was still enough to increase the separation from the second TMPO and establish a strong H-bond with O6_(Al) (see Figure 6a and Table 4). As a consequence, the average O_(P1)–H bond length increased to 1.05 Å after 10 ps of MD, from an initial value of 1.016 Å. The absence of confinement effects at the external surface allowed greater movement of the second TMPO, moving away from the first TMPO, to finally establish two strong H-bonds with nearby silanol groups (see Figure 6b). The withdrawal of the second TMPO from the vicinity of the protonated phosphine reduced the steric effects and hence enabled the reorientation of the first TMPO. Without steric interference, the acid proton could be better shared between the bridging O1_(Al) and O_(P1), which increased the O_(P1)–H bond length from 1.033 Å after local optimization to an average value of 1.07 Å after 10 ps of MD. Therefore, the very strong acidity predicted by the local relaxation was not conserved during the MD simulation; the BA site behaved as a strong acid instead.

Figure 6. Representation of two TMPO molecules on (a) the internal BA site and (b) the external BA site after 10 ps of MD simulation of the locally optimized structure in Figure 5.

Table 4. Relevant Interatomic Distances (Å) after 10 ps of MD Simulation Taking the 2TMPO/1BA Configurations Shown in Figure 5a,b as Input Geometries

	internal BA	external BA
	Figure 5a^a	Figure 5b
O_(P1)–H	1.05 ± 0.04	1.07 ± 0.06
O_(P2)–H	5.0 ± 0.3	8.7 ± 0.2
O_(P1)–O_(P2)	4.7 ± 0.4	8.7 ± 0.2
O_(Al)–H	1.5 ± 0.1	1.5 ± 0.1
O_(P1)–O_(Al)	2.52 ± 0.09	2.5 ± 0.1
P–O_(P1)	1.57 ± 0.03	1.56 ± 0.02
P–O_(P2)	1.51 ± 0.02	1.52 ± 0.01

Average and standard deviation over the last 2.5 ps of simulation.

3.3 Adsorption of Three TMPO Molecules

Proton transfer can be classified as weak for O_(P)–H distances that range from 1.097 to 1.188 Å (see Figure 2). We observed this weak acidity when a direct O_(P)–Al interaction was induced after adding a third phosphine oxide into the 2TMPO/1BA systems shown in Figure 5.

Figure 7 shows two equivalent structures after local optimization, out of several tested during our work, where O_(P1)–H bond lengths of 1.131 and 1.154 Å were obtained after adsorption on the internal and external surfaces, respectively; structural parameters of interest are compiled in Table 5. The proton was almost evenly shared between O_(Al) and O_(P1), rendering the acid response of the BA site weak. On this occasion, the average interaction energies for the internal and external BA sites differed by only 2 kJ/mol: −99 kJ/mol (internal) versus −97 kJ/mol (external). The adsorption at the internal surface, without considering the correction for dispersion forces, was repulsive, by 12 kJ/mol on average. Once the van der Waals interactions were taken into consideration, the overall adsorption energy became attractive, with a value of −99 kJ/mol. At the external surface, the dispersion contribution represented 77% of an average adsorption energy of −97 kJ/mol.

Figure 7. Representation of the adsorption of three TMPO molecules on (a) the internal and (b) the external BA sites after local optimization. H in white, C in gray, P in brown, O in red, Al in light blue, and Si represented by orange sticks. All the framework O atoms (except the protonated one) and silanol OH groups were deleted for an enhanced view. The interaction between one of the nonprotonated TMPO and the Al atom is represented by a stick connecting O_(P2) to the Al atom. Related structural values are presented in Table 5.

Table 5. Relevant Interatomic Distances (Å) of the 3TMPO/1BA Configurations Shown in Figure 7a,b after Local Optimization

	internal BA	external BA
	Figure 7a^a	Figure 7b
O_(P1)–H	1.131 (2.000)	1.154 (2.000)
O_(P2)–H	2.878 (2.000)	3.134 (2.000)
O_(P3)–H	3.125 (2.000)	3.599 (2.000)
O_(P1)–O_(P2)	3.548 (3.570)	4.034 (3.572)
O_(P1)–O_(P3)	3.473 (3.230)	4.144 (3.227)
O_(P2)–Al	1.993 (3.080)	2.033 (3.073)
O_(Al)–H	1.250 (0.975)	1.232 (0.975)
O_(P1)–O_(Al)	2.381 (2.627)	2.379 (2.627)
P–O_(P1)	1.533	1.531
P–O_(P2)	1.511	1.511
P–O_(P3)	1.501	1.496

The values before the structural optimization are presented within parentheses.

Our interpretation of the weakening of the acid as proton donor after the adsorption of three TMPO molecules was based on the strengthening of the O_(Al)–H bond. The O_(Al)–Al bond was weakened by the interaction between the Al atom and one of the nonprotonated TMPO molecules, which was reflected in O_(P)–Al distances that remained around 2.0 Å for both surfaces, as shown in Figure 7 and Table 5. There was also a large displacement of the Al atom during the optimization toward the nonprotonated phosphine. Under conditions of significant agglomeration of TMPO molecules around the BA site, the Al center may show residual Lewis acidity, with subsequent repercussions on the acid strength of the BA site. We did not observe direct competition for the acid proton by the nonprotonated phosphines, emphasized by O_(P2)–H and O_(P3)–H distances above 2.8 Å after local optimization.

In order to observe the perturbation and reordering of the electronic charge density derived from the TMPO-Al interaction, we have calculated the charge density difference for the configurations shown in Figure 7 according to the equation

(1)The ρ_{zeolite+3TMPO} term corresponds to the electronic density of the whole system with three TMPO molecules, while the density of the zeolite without the TMPO molecule closest to the Al atom is represented by the ρ_{zeolite+2TMPO} term. The electronic density of the isolated phosphine oxide with exactly the same structure as when it was interacting with Al atom is denoted by the ρ_1TMPO term. We have used the same grid of points to represent the charge density and the same box size for the three calculations. The resulting distribution shows an increase of the charge density between O_(P2) and Al atoms (see Figure 8), which highlights the formation of bonding interactions between both centers.

Figure 8. Charge difference isosurfaces with values of 0.005 bohr^–3 calculated from eq 1. (a) Internal BA site; (b) external BA site. The structures correspond to those shown in Figure 7. H in white, C in gray, P in brown, O in red, Al in light blue, and Si represented by orange sticks. All the framework O atoms (except the protonated one) and silanol OH groups were deleted for an enhanced view.

Figure 9 shows the projected density of states (PDOS) onto O_(P2)(2p) of the TMPO interacting with the Al atom, as represented in Figure 7a, corresponding to the interior of the zeolite. The O_(P2)(2p) PDOS presents intense peaks at the upper occupied edge of the energy spectra, between −2 and 0.0 eV (see Figure 9a). These O_(P)(2p) states are complemented by small peaks in the Al(3s,3p) PDOS (Figure 9b,c) which disappear once the TMPO–Al interaction is deleted by removing the phosphine molecule (Figure 9d,e). In addition, the Al(3s,3p) PDOS reports the appearance of empty states above the Fermi energy as a result of the phosphine removal (Figure 9d,e). These transformations emphasize electron donation from the TMPO into empty electronic states associated with the Al atom.

Figure 9. Projected density of states (PDOS) of (a) O_(P2)(2p), (b) Al(3s), and (c) Al(3p) for the 3TMPO/1BA configuration observed in the cavity according to Figure 7a. PDOS of (d) Al(3s) and (e) Al(3p) for the structure without the nonprotonated TMPO that interacts with the Al center.

After local optimization, there were differences in the behavior of the internal and external BA sites in the presence of three TMPO molecules. The internal ones almost always performed as weak acids, whereas the external BA sites varied in strength between strong and weak, depending on the configuration; Table 6 summarizes these tendencies. When the O_(P2) atom remained at less than 2.15 Å from the Al, the proton transfer was weak. Once this distance increased and the nonprotonated TMPO moved away from the Al atom, which was the case in four out of six configurations considered for the external BA site, the acidity was classified as strong. These findings underline the importance of the TMPO–Al interaction for the presence of the weak acidity.

Table 6. Relevant Interatomic Distances (Å) for Six Different Configurations 3TMPO/1BA after Local Optimization

internal BA configurations^a
	O_(P2)–Al	O_(Al)–Al	O_(P1)–H	acid class^c
a	2.099	1.853	1.146	weak
b^b	1.993	1.846	1.131	weak
c	2.149	1.839	1.107	weak
d	1.988	1.852	1.137	weak
e	2.245	1.822	1.093	strong
f	2.002	1.844	1.116	weak

external BA configurations^a
	O_(P2)–Al	O_(Al)–Al	O_(P1)–H	acid class^c
a	3.050	1.793	1.050	strong
b^b	2.033	1.842	1.154	weak
c	5.554	1.812	1.053	strong
d	3.637	1.785	1.059	strong
e	3.059	1.789	1.047	strong
f	2.059	1.829	1.100	weak

Second column: distance between O_(P2) and Al in Figure 7. Third column: distance between O_(Al) and Al. Fourth column: O_(P1)–H distance for the protonated TMPO in Figure 7.

Configurations shown in Figure 7

Acid classification according to Figure 2.

The locally optimized arrangements of three TMPO molecules evolved differently during the MD simulation, depending on whether the adsorption took place at the internal or external surface. The MD calculations were performed on the configurations shown in Figure 7, with the average of the main structural parameters compiled in Table 7. The internal BA site continued to behave as a weak acid after 10 ps of dynamics, mostly due to the conservation of the initial configuration given as an input, with O_(P2) atom binding the Al center. However, at the external surface the agglomeration of three TMPO molecules at the acid site was not preserved; both nonprotonated TMPO molecules moved away toward nearby silanol groups to establish H-bonds. Once the O_(P2)–Al link was broken, the acid strength of the external BA was again strong, illustrated by an average O_(P1)–H bond length of 1.05 Å. This exemplifies how equivalent BA sites are driven to behave differently as a consequence of the confinement produced by the pore system. The same confinement may be mimicked at the external surface if a large enough number of TMPO molecules is adsorbed in the vicinity of the BA site, limiting the movement of those phosphines that are in direct contact with the acid.

Table 7. Relevant Interatomic Distances (Å) after 10 ps of MD Simulation Taking the 3TMPO/1BA Configurations Shown in Figure 7a,b as Input Geometries

	internal BA	external BA
	Figure 7a^a	Figure 7b
O_(P1)–H	1.2 ± 0.1	1.05 ± 0.05
O_(P2)–H	3.7 ± 0.2	6.5 ± 0.3
O_(P3)–H	5.6 ± 0.5	8.8 ± 0.5
O_(P1)–O_(P2)	4.9 ± 0.2	6.6 ± 0.3
O_(P1)–O_(P3)	6.3 ± 0.5	9.5 ± 0.5
O_(P2)–Al	2.0 ± 0.1	6.3 ± 0.2
O_(Al)–H	1.3 ± 0.2	1.5 ± 0.1
O_(P1)–O_(Al)	2.46 ± 0.09	2.52 ± 0.09
P–O_(P1)	1.55 ± 0.03	1.56 ± 0.02
P–O_(P2)	1.52 ± 0.02	1.53 ± 0.02
P–O_(P3)	1.50 ± 0.02	1.51 ± 0.03

Average and standard deviation over the last 2.5 ps of simulation.

3.4 Full Deprotonation of the Brønsted Acid Site

We have considered the migration of a protonated TMPO from the vicinity of the BA site by moving the phosphine to the second pore interception that is present within the MFI unit cell. The distance between the acid proton and any of the four O atoms binding the Al center was larger than 5 Å before relaxation, rendering the acid site fully deprotonated. After local optimization, the O_(P)–H bond length decreased to 0.979 and 1.005 Å for adsorption at the internal and the external surfaces, respectively; Figure 10 shows the optimized geometries, with the O–H distances compiled in Table 8. Those bond lengths correspond to ³¹P chemical shifts of around 86 ppm, which translates into an acid classification of very strong (see Figure 2). The framework O atoms that do not bind the Al center are not basic enough to lengthen the O_(P)–H bond, even if a H-bond is established. The MD simulation did not result in further modifications, with average O_(P)–H bond lengths remaining around 1.00 Å (see Table 8).

Figure 10. Representation of (a, c) TMPOH⁺ and (b, d) (TMPO)₂H⁺ at more than 5 Å away from the BA site after local optimization. (a, b) Internal surface; (c, d) external surface. H in white, C in gray, P in brown, O in red, and Si represented by orange sticks. All the framework O atoms (except the protonated one) and silanol OH groups were deleted for an enhanced view. Related structural values are presented in Table 8.

Table 8. Relevant Interatomic Distances (Å) of the Fully Protonated TMPO Molecules at More than 5 Å from the BA Site

	internal BA		external BA
	TMPOH⁺ (Figure 10a)	(TMPO)₂H⁺ (Figure 10b)	TMPOH⁺ (Figure 10c)	(TMPO)₂H⁺ (Figure 10d)
local optimization^a
O_(P1)–H	0.979	0.996	1.005	1.072
O_(P2)–H	−	2.167	−	1.431
P–O_(P1)	1.594	1.600	1.576	1.570
P–O_(P2)	−	1.515	−	1.522
O_(Si)–H	2.802	−	1.708	−
MD simulation^b
O_(P1)–H	1.00 ± 0.04	1.11 ± 0.08	1.00 ± 0.02	1.2 ± 0.1
O_(P2)–H	−	1.4 ± 0.1	−	1.3 ± 0.1
P–O_(P1)	1.59 ± 0.04	1.57 ± 0.03	1.60 ± 0.05	1.55 ± 0.03
P–O_(P2)	−	1.53 ± 0.02	−	1.54 ± 0.03
O_(Si)–H	1.9 ± 0.4	−	2.3 ± 0.5	−

Values after local optimization. The optimized structures are shown in Figure 10a–d.

Average values over the last 2.5 ps out of 10 ps of MD simulation taking the configurations shown in Figure 10a–d as input geometries.

We also placed a second TMPO molecule in close proximity to the protonated phosphine in order to form the species (TMPO)₂H⁺ and thus to induce the sharing of the acid proton between both molecules. After local optimization, the O_(P1)–H bond length at the interior of the zeolite remained 0.996 Å, corresponding to a very strong protonation. The equivalent value at the external surface increased to 1.072 Å, but still within the range of O_(P)–H distances sensed as a strong protonation by ³¹P NMR. The second TMPO molecule had O_(P2)–H distances of 2.167 and 1.431 Å for the internal and external surfaces, respectively.

During the MD simulation the proton became more evenly shared between both TMPO molecules, increasing the average O_(P1)–H bond length to 1.1 or 1.2 Å and decreasing the O_(P2)–H distance to 1.4 or 1.3 Å depending on the surface (see Table 8); those values correspond to ³¹P chemical shifts that classify the acid strength as weak. Therefore, full proton transfer may be probed experimentally as either very strong or weak, depending on whether the proton is binding a single TMPO or is shared between two phosphine molecules, respectively. During the adsorption of TMPO on Keggin-type 12-tungstophosphoric acid (HPW), (41) TMPOH⁺ species appear within the region 95–85 ppm, while the formation of (TMPO)₂H⁺ moves the ³¹P chemical shift into the range 75–53 ppm, in agreement with our calculated O_(P)–H distances. However, the signals associated with (TMPO)₂H⁺ disappear after thermal pretreatment of the sample, to emerge again when the sample is oversaturated with TMPO molecules. (41) Extrapolating to the zeolite MFI, it is more probable that the formation of (TMPO)₂H⁺ occurs at the external surface, where the acid sites are exposed to higher loads of TMPO molecules than at the internal surface.

The implications of the (TMPO)₂H⁺ formation depend on its coexistence with the other arrangements analyzed above. The acid proton may be shared between O_(P) and O_(Al), or between O_(P1) and O_(P2); the former alternative produces a strong proton transfer, as it was observed above after the MD simulation, while the latter corresponds to (TMPO)₂H⁺, interpreted as weak acidity in the ³¹P NMR spectrum. The relative occurrence of the first or the second alternative should depend on the number and mobility of TMPO molecules throughout the framework. For instance, a second TMPO can get close to the acid site where a protonated phosphine is already interacting through an H-bond with one of the O_(Al) atoms. The second O_(P2) may replace O_(Al) within the H-bond to form (TMPO)₂H⁺ (our MD simulation was too short to observe this transformation when more than one TMPO was adsorbed). Although O_(P2) is more basic than O_(Al), both TMPO molecules must have a specific orientation to properly share the proton. Owing to the presence of the (CH₃)₃P segment, steric effects may play against the formation of (TMPO)₂H⁺, which are accentuated by the confinement along the pore system. At the external surface, the TMPO’s mobility and number increase as well as the access to the acid sites. This favors the formation of (TMPO)₂H⁺ over the adsorption of a single TMPO at the acid site and may be the reason behind the disappearance of the ³¹P signal for strong acids at the external surface of zeolite MFI. (10, 12) Once the (TMPO)₂H⁺ is formed, an additional equilibrium between (TMPO)₂H⁺ and (TMPOH⁺ + TMPO) can be established. This suggestion may explain the presence of very strong proton transfer, considering that we only observed an average O_(P)–H bond length below 1.01 Å after MD simulation for the TMPOH⁺ species in Table 8. A similar process has been proposed for the solvation of H⁺ ions in an aqueous medium, where an interconversion between H₅O₂⁺ and (H₃O⁺ + H₂O) is observed using Car–Parrinello MD. (35)

4 Conclusions

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We have used DFT with dispersion corrections to study the adsorption of TMPO at the internal and external surfaces of zeolite MFI, with the T2-site substituted by Al. We initiated relaxation of the structures using local geometry optimization, followed by 10 ps of MD centered at 300 K; we considered the MD results as the final criterion to characterize the acid site. The acid should perform as strong, according to the ³¹P chemical shift, when the proton is shared between the TMPO’s oxygen atom and one of the framework O atoms that bind the Al. This trend holds when the number of TMPO molecules around the BA site increases, being the only exception the adsorption of three molecules at the internal surface. In that case, one of the nonprotonated TMPO molecules directly interacts with the Al center through direct contact between O_(P) and Al. This interaction weakens the O_(Al)–Al bond, making the O_(Al)–H bond stronger and less prone to be broken; in consequence an O_(P)–H bond length of 1.2 Å is calculated, which translates into an acid strength classified as weak by ³¹P NMR. The TMPO–Al interaction is more likely at the internal surface, where confinement effects keep the TMPO within close proximity of the Al atom. At the external surface, the nonprotonated TMPO molecules tend to migrate from the acid site, and establish H-bonds with nearby silanol groups.

We only observed O_(P)–H bond lengths smaller than 1.01 Å (acid classified as very strong by ³¹P NMR) when a protonated TMPO was assumed to migrate from the acid site. In the absence of H-bonding between O_(P) and O_(Al), the O_(P)–H bond becomes shorter. Furthermore, if the acid proton is shared between two TMPO molecules, forming (TMPO)₂H⁺, the O_(P)–H bond length increases to an average value between 1.1 and 1.2 Å, producing ³¹P chemical shifts that classify the acid as weak. Therefore, the three main regions in the ³¹P spectrum that are used to classify the acid sites as very strong (90–80 ppm), strong (80–70 ppm), and weak (70–60 ppm) may be derived from the coexistence of the species TMPOH⁺···O_(Si) (very strong), TMPOH⁺···O_(Al) (strong), and (TMPO)₂H⁺ (weak).

Author Information

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Corresponding Author
- Nora H. de Leeuw - Department of Chemistry, University College London, 20 Gordon Street, London, United Kingdom WC1H 0AJ; School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff, United Kingdom CF10 3AT; Email: [email protected]
Authors
- Carlos E. Hernandez-Tamargo - Department of Chemistry, University College London, 20 Gordon Street, London, United Kingdom WC1H 0AJ; School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff, United Kingdom CF10 3AT
- Alberto Roldan - School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff, United Kingdom CF10 3AT
Notes
The authors declare no competing financial interest.

Acknowledgment

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The authors acknowledge the use of the UCL Legion High Performance Computing Facility (Legion@UCL), and associated support services, in the completion of this work. Via our membership of the UK’s HEC Materials Chemistry Consortium, which is funded by EPSRC (EP/L000202), this work used the ARCHER UK National Supercomputing Service. This work was also performed using the computational facilities of the Advanced Research Computing@Cardiff (ARCCA) Division, Cardiff University. We acknowledge UCL and the UK Engineering and Physical Sciences Research Council (Grant EP/K009567/2) for funding. Nora H. de Leeuw thanks the Royal Society for an Industry Fellowship. All data created during this research is openly available from the University of Cardiff Research Portal at http://dx.doi.org/10.17035/d.2016.0009738222.

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Characterization of the Surface Acidity of MFI Zeolite Nanosheets by 31P NMR of Adsorbed Phosphine Oxides and Catalytic Cracking of Decalin
Seo, Yongbeom; Cho, Kanghee; Jung, Younjae; Ryoo, Ryong
ACS Catalysis (2013), 3 (4), 713-720CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
MFI zeolite nanosheets tailored to 2.5-nm thickness were synthesized using a surfactant-type zeolite structure-directing agent, [C22H45-N+(CH3)2-C6H12-N+(CH3)2-C6H13](Br-)2. The zeolite nanosheets possessed Bronsted acid sites on their external surfaces as well as in the internal micropore walls. The acid strength and concn. was characterized by the 31P NMR signals of the adsorbed trimethylphosphine oxide and tributylphosphine oxide. The 31P NMR investigation identified three types of Bronsted acid sites with different strengths on external surfaces; there were four types inside the micropores. A linear correlation has been established between the no. of the external strongest acid sites and the catalytic activity in decalin cracking for the MFI zeolite catalysts investigated in this work.
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Ni, Y.; Sun, A.; Wu, X.; Hai, G.; Hu, J.; Li, T.; Li, G. The Preparation of Nano-Sized H[Zn,Al]ZSM-5 Zeolite and Its Application in the Aromatization of Methanol Microporous Mesoporous Mater. 2011, 143, 435– 442 DOI: 10.1016/j.micromeso.2011.03.029
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Costa, C.; Dzikh, I. P.; Lopes, J. M.; Lemos, F.; Ribeiro, F. R. Activity–acidity Relationship in Zeolite ZSM-5. Application of Brönsted-Type Equations J. Mol. Catal. A: Chem. 2000, 154, 193– 201 DOI: 10.1016/S1381-1169(99)00374-X
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Activity-acidity relationship in zeolite ZSM-5. Application of Bronsted-type equations
Costa, C.; Dzikh, I. P.; Lopes, J. M.; Lemos, F.; Ribeiro, F. R.
Journal of Molecular Catalysis A: Chemical (2000), 154 (1-2), 193-201CODEN: JMCCF2; ISSN:1381-1169. (Elsevier Science B.V.)
In this paper the relation between activity and acidity in a variety of ZSM-5 zeolite catalysts, with different Si/Al ratios and different protonic content, is analyzed and a quant. correlation is obtained. The acid site strength distribution was estd. using temp.-programmed desorption (TPD) of ammonia by applying a digital deconvolution method to the curves. These data were then correlated with exptl. catalytic activity data for the same catalysts towards n-heptane cracking reaction, by means of a Bronsted-type equation similar to the ones used for homogeneous acid catalysis and already used for other zeolites. It can be noticed that the same types of equation that are used for homogeneous acid catalysis also hold for heterogeneous acid catalysis and that the activation energy for ammonia desorption can be used as acid-strength scale for the purpose of correlation with catalytic activity.
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Kwok, T. J.; Jayasuriya, K.; Damavarapu, R.; Brodman, B. W. Application of H-ZSM-5 Zeolite for Regioselective Mononitration of Toluene J. Org. Chem. 1994, 59, 4939– 4942 DOI: 10.1021/jo00096a042
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Wang, Y.; Jian, G.; Peng, Z.; Hu, J.; Wang, X.; Duan, W.; Liu, B. Preparation and Application of Ba/ZSM-5 Zeolite for Reaction of Methyl Vinyl Ether and Methanol Catal. Commun. 2015, 66, 34– 37 DOI: 10.1016/j.catcom.2015.03.012
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Preparation and application of Ba/ZSM-5 zeolite for reaction of methyl vinyl ether and methanol
Wang, Yang; Jian, Guixing; Peng, Zhihong; Hu, Jiaji; Wang, Xin; Duan, Wubiao; Liu, Bo
Catalysis Communications (2015), 66 (), 34-37CODEN: CCAOAC; ISSN:1566-7367. (Elsevier B.V.)
The ZSM-5 zeolite is widely used to catalyze the reactions of methanol to olefins. Herein, we have prepd. the H-ZSM-5 doped with barium (Ba/ZSM-5) using incipient wetness impregnation method. The Ba modified catalysts were used to catalyze a new reaction of methanol with Me vinyl ether to improve the selectivity of ethylene and propylene (C=2 + C=3). The reaction catalyzed by Ba doped H-ZSM-5 shows higher propylene selectivity over H-ZSM-5. The reaction mechanism is discussed.
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Zheng, A.; Han, B.; Li, B.; Liu, S.-B.; Deng, F. Enhancement of Brønsted Acidity in Zeolitic Catalysts due to an Intermolecular Solvent Effect in Confined Micropores Chem. Commun. 2012, 48, 6936– 6938 DOI: 10.1039/c2cc32498a
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Enhancement of Bronsted acidity in zeolitic catalysts due to an intermolecular solvent effect in confined micropores
Zheng, Anmin; Han, Bing; Li, Bojie; Liu, Shang-Bin; Deng, Feng
Chemical Communications (Cambridge, United Kingdom) (2012), 48 (55), 6936-6938CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
Solid-state NMR and DFT calcn. studies certified the presence of an intermol. solvent effect for mols. confined in microporous zeolite, leading to a notable increase in Bronsted acidity of the solid acid catalyst.
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Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 558– 561 DOI: 10.1103/PhysRevB.47.558
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Ab initio molecular dynamics of liquid metals
Kresse, G.; Hafner, J.
Physical Review B: Condensed Matter and Materials Physics (1993), 47 (1), 558-61CODEN: PRBMDO; ISSN:0163-1829.
The authors present ab initio quantum-mech. mol.-dynamics calcns. based on the calcn. of the electronic ground state and of the Hellmann-Feynman forces in the local-d. approxn. at each mol.-dynamics step. This is possible using conjugate-gradient techniques for energy minimization, and predicting the wave functions for new ionic positions using sub-space alignment. This approach avoids the instabilities inherent in quantum-mech. mol.-dynamics calcns. for metals based on the use of a factitious Newtonian dynamics for the electronic degrees of freedom. This method gives perfect control of the adiabaticity and allows one to perform simulations over several picoseconds.
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Kresse, G.; Hafner, J. Ab Initio Molecular-Dynamics Simulation of the Liquid-Metal–amorphous-Semiconductor Transition in Germanium Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 49, 14251– 14269 DOI: 10.1103/PhysRevB.49.14251
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Ab initio molecular-dynamics simulation of the liquid-metal-amorphous-semiconductor transition in germanium
Kresse, G.; Hafner, J.
Physical Review B: Condensed Matter and Materials Physics (1994), 49 (20), 14251-69CODEN: PRBMDO; ISSN:0163-1829.
The authors present ab initio quantum-mech. mol.-dynamics simulations of the liq.-metal-amorphous-semiconductor transition in Ge. The simulations are based on (a) finite-temp. d.-functional theory of the 1-electron states, (b) exact energy minimization and hence calcn. of the exact Hellmann-Feynman forces after each mol.-dynamics step using preconditioned conjugate-gradient techniques, (c) accurate nonlocal pseudopotentials, and (d) Nose' dynamics for generating a canonical ensemble. This method gives perfect control of the adiabaticity of the electron-ion ensemble and allows the authors to perform simulations over >30 ps. The computer-generated ensemble describes the structural, dynamic, and electronic properties of liq. and amorphous Ge in very good agreement with expt.. The simulation allows the authors to study in detail the changes in the structure-property relation through the metal-semiconductor transition. The authors report a detailed anal. of the local structural properties and their changes induced by an annealing process. The geometrical, bounding, and spectral properties of defects in the disordered tetrahedral network are studied and compared with expt.
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Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set Comput. Mater. Sci. 1996, 6, 15– 50 DOI: 10.1016/0927-0256(96)00008-0
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Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set
Kresse, G.; Furthmuller, J.
Computational Materials Science (1996), 6 (1), 15-50CODEN: CMMSEM; ISSN:0927-0256. (Elsevier)
The authors present a detailed description and comparison of algorithms for performing ab-initio quantum-mech. calcns. using pseudopotentials and a plane-wave basis set. The authors will discuss: (a) partial occupancies within the framework of the linear tetrahedron method and the finite temp. d.-functional theory, (b) iterative methods for the diagonalization of the Kohn-Sham Hamiltonian and a discussion of an efficient iterative method based on the ideas of Pulay's residual minimization, which is close to an order N2atoms scaling even for relatively large systems, (c) efficient Broyden-like and Pulay-like mixing methods for the charge d. including a new special preconditioning optimized for a plane-wave basis set, (d) conjugate gradient methods for minimizing the electronic free energy with respect to all degrees of freedom simultaneously. The authors have implemented these algorithms within a powerful package called VAMP (Vienna ab-initio mol.-dynamics package). The program and the techniques have been used successfully for a large no. of different systems (liq. and amorphous semiconductors, liq. simple and transition metals, metallic and semi-conducting surfaces, phonons in simple metals, transition metals and semiconductors) and turned out to be very reliable.
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Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169– 11186 DOI: 10.1103/PhysRevB.54.11169
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Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set
Kresse, G.; Furthmueller, J.
Physical Review B: Condensed Matter (1996), 54 (16), 11169-11186CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
The authors present an efficient scheme for calcg. the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrixes will be discussed. This approach is stable, reliable, and minimizes the no. of order Natoms3 operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special "metric" and a special "preconditioning" optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent and self-consistent calcns. It will be shown that the no. of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order Natoms2 scaling is found for systems contg. up to 1000 electrons. If we take into account that the no. of k points can be decreased linearly with the system size, the overall scaling can approach Natoms. They have implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large no. of different systems (liq. and amorphous semiconductors, liq. simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable.
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Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple Phys. Rev. Lett. 1996, 77, 3865– 3868 DOI: 10.1103/PhysRevLett.77.3865
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Generalized gradient approximation made simple
Perdew, John P.; Burke, Kieron; Ernzerhof, Matthias
Physical Review Letters (1996), 77 (18), 3865-3868CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
Generalized gradient approxns. (GGA's) for the exchange-correlation energy improve upon the local spin d. (LSD) description of atoms, mols., and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental consts. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential.
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Grimme, S. Semiempirical GGA-Type Density Functional Constructed with a Long-Range Dispersion Correction J. Comput. Chem. 2006, 27, 1787– 1799 DOI: 10.1002/jcc.20495
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Semiempirical GGA-type density functional constructed with a long-range dispersion correction
Grimme, Stefan
Journal of Computational Chemistry (2006), 27 (15), 1787-1799CODEN: JCCHDD; ISSN:0192-8651. (John Wiley & Sons, Inc.)
A new d. functional (DF) of the generalized gradient approxn. (GGA) type for general chem. applications termed B97-D is proposed. It is based on Becke's power-series ansatz from 1997 and is explicitly parameterized by including damped atom-pairwise dispersion corrections of the form C6·R-6. A general computational scheme for the parameters used in this correction has been established and parameters for elements up to xenon and a scaling factor for the dispersion part for several common d. functionals (BLYP, PBE, TPSS, B3LYP) are reported. The new functional is tested in comparison with other GGAs and the B3LYP hybrid functional on std. thermochem. benchmark sets, for 40 noncovalently bound complexes, including large stacked arom. mols. and group II element clusters, and for the computation of mol. geometries. Further cross-validation tests were performed for organometallic reactions and other difficult problems for std. functionals. In summary, it is found that B97-D belongs to one of the most accurate general purpose GGAs, reaching, for example for the G97/2 set of heat of formations, a mean abs. deviation of only 3.8 kcal mol-1. The performance for noncovalently bound systems including many pure van der Waals complexes is exceptionally good, reaching on the av. CCSD(T) accuracy. The basic strategy in the development to restrict the d. functional description to shorter electron correlation lengths scales and to describe situations with medium to large interat. distances by damped C6·R-6 terms seems to be very successful, as demonstrated for some notoriously difficult reactions. As an example, for the isomerization of larger branched to linear alkanes, B97-D is the only DF available that yields the right sign for the energy difference. From a practical point of view, the new functional seems to be quite robust and it is thus suggested as an efficient and accurate quantum chem. method for large systems where dispersion forces are of general importance.
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Hernandez-Tamargo, C. E.; Roldan, A.; de Leeuw, N. H. A Density Functional Theory Study of the Structure of Pure-Silica and Aluminium-Substituted MFI Nanosheets J. Solid State Chem. 2016, 237, 192– 203 DOI: 10.1016/j.jssc.2016.02.006
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A density functional theory study of the structure of pure-silica and aluminium-substituted MFI nanosheets
Hernandez-Tamargo, Carlos E.; Roldan, Alberto; de Leeuw, Nora H.
Journal of Solid State Chemistry (2016), 237 (), 192-203CODEN: JSSCBI; ISSN:0022-4596. (Elsevier B.V.)
The authors present a theor. study of the structural features of silica and aluminum-substituted MFI nanosheets. The authors have analyzed the effects of aluminum substitution on the vibrational properties of silanols as well as the features of protons as counter-ions. The formation of the two-dimensional system did not lead to appreciable distortions within the framework. Moreover, the effects on the structure due to the aluminum dopants were the same in both the bulk and the slab. The principal differences were related to the silanol groups that form hydrogen-bonds with neighboring aluminum-substituted silanols, whereas intra-framework hydrogen-bonds increase the stability of aluminum-substituted silanols toward dehydration. Thus, the authors have complemented previous exptl. and theor. studies, showing the lamellar MFI zeolite to be a very stable material of high crystallinity regardless of its very thin structure.
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Blöchl, P. E. Projector Augmented-Wave Method Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953– 17979 DOI: 10.1103/PhysRevB.50.17953
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Projector augmented-wave method
Blochl
Physical review. B, Condensed matter (1994), 50 (24), 17953-17979 ISSN:0163-1829.
There is no expanded citation for this reference.
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Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1758– 1775 DOI: 10.1103/PhysRevB.59.1758
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From ultrasoft pseudopotentials to the projector augmented-wave method
Kresse, G.; Joubert, D.
Physical Review B: Condensed Matter and Materials Physics (1999), 59 (3), 1758-1775CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
The formal relationship between ultrasoft (US) Vanderbilt-type pseudopotentials and Blochl's projector augmented wave (PAW) method is derived. The total energy functional for US pseudopotentials can be obtained by linearization of two terms in a slightly modified PAW total energy functional. The Hamilton operator, the forces, and the stress tensor are derived for this modified PAW functional. A simple way to implement the PAW method in existing plane-wave codes supporting US pseudopotentials is pointed out. In addn., crit. tests are presented to compare the accuracy and efficiency of the PAW and the US pseudopotential method with relaxed-core all-electron methods. These tests include small mols. (H2, H2O, Li2, N2, F2, BF3, SiF4) and several bulk systems (diamond, Si, V, Li, Ca, CaF2, Fe, Co, Ni). Particular attention is paid to the bulk properties and magnetic energies of Fe, Co, and Ni.
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Ho, K.-M.; Fu, C. L.; Harmon, B. N.; Weber, W.; Hamann, D. R. Vibrational Frequencies and Structural Properties of Transition Metals via Total-Energy Calculations Phys. Rev. Lett. 1982, 49, 673– 676 DOI: 10.1103/PhysRevLett.49.673
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Vibrational frequencies and structural properties of transition metals via total-energy calculations
Ho, K. M.; Fu, C. L.; Harmon, B. N.; Weber, W.; Hamann, D. R.
Physical Review Letters (1982), 49 (9), 673-6CODEN: PRLTAO; ISSN:0031-9007.
The vibrational frequencies of selected normal modes can be obtained entirely from 1st principles with use of frozen phonon calcns. which involve the precise evaluation of crystal total energy as a function of lattice displacement. The calcns. allow a detailed anal. of the microscopic mechanism causing phonon anomalies and soft-mode phase transitions. Calcns. for Zr, Nb, and Mo were made using both tight-binding and pseudopotential methods.
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Fu, C.-L.; Ho, K.-M. First-Principles Calculation of the Equilibrium Ground-State Properties of Transition Metals: Applications to Nb and Mo Phys. Rev. B: Condens. Matter Mater. Phys. 1983, 28, 5480– 5486 DOI: 10.1103/PhysRevB.28.5480
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First-principles calculation of the equilibrium ground-state properties of transition metals: Application to niobium and molybdenum
Fu, C. L.; Ho, K. M.
Physical Review B: Condensed Matter and Materials Physics (1983), 28 (10), 5480-6CODEN: PRBMDO; ISSN:0163-1829.
A self-consistent pseudopotential method was used to calc. the equil. ground-state properties of Mo and Nb. The calcd. equil. lattice consts., cohesive energies, and bulk moduli agree with exptl. data.
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Quartieri, S.; Arletti, R.; Vezzalini, G.; Di Renzo, F.; Dmitriev, V. Elastic Behavior of MFI-Type Zeolites: 3 – Compressibility of Silicalite and Mutinaite J. Solid State Chem. 2012, 191, 201– 212 DOI: 10.1016/j.jssc.2012.03.039
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Elastic behavior of MFI-type zeolites: 3. Compressibility of silicalite and mutinaite
Quartieri, Simona; Arletti, Rossella; Vezzalini, Giovanna; Di Renzo, Francesco; Dmitriev, Vladimir
Journal of Solid State Chemistry (2012), 191 (), 201-212CODEN: JSSCBI; ISSN:0022-4596. (Elsevier B.V.)
We report the results of an in-situ synchrotron X-ray powder diffraction study, performed using silicone oil as "non-penetrating" pressure transmitting medium, of the elastic behavior of three zeolites with MFI-type framework: the natural zeolite mutinaite and two silicalites (labeled A and B) synthesized under different conditions. In mutinaite, no symmetry change is obsd. as a function of pressure; however, a phase transition from monoclinic (P21/n) to orthorhombic (Pnma) symmetry occurs at about 1.0 GPa in the silicalite samples. This phase transition is irreversible upon decompression. The second order bulk moduli of silicalite A and silicalite B, calcd. after the fulfillment of the phase transition, are: K 0=18.2(2) and K 0=14.3 (2) GPa, resp. These values makes silicalite the most compressible zeolite among those up to now studied in silicone oil. The structural deformations induced by HP in silicalite A were investigated by means of complete Rietveld structural refinements, before and after the phase transition, at P amb and 0.9 GPa, resp. The elastic behaviors of the three MFI-type zeolites here investigated were compared with those of Na-ZSM-5 and H-ZSM-5, studied in similar exptl. conditions: the two silicalites - which are the phases with the highest Si/Al ratios and hence the lowest extraframework contents - show the highest compressibility. On the contrary, the most rigid material is mutinaite, which has a very complex extraframework compn. characterized by a high no. of cations and water mols.
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Kokotailo, G. T.; Lawton, S. L.; Olson, D. H.; Meier, W. M. Structure of Synthetic Zeolite ZSM-5 Nature 1978, 272, 437– 438 DOI: 10.1038/272437a0
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Structure of synthetic zeolite ZSM-5
Kokotailo, G. T.; Lawton, S. L.; Olson, D. H.; Meier, W. M.
Nature (London, United Kingdom) (1978), 272 (5652), 437-8CODEN: NATUAS; ISSN:0028-0836.
The structure of synthetic zeolite ZSM-5 was detd. The crystals are orthorhombic, space group Pnma, with a 20.1, b 19.9, and c 13.4 Å. The unit contents of the Na form are NanAlnSi96-nO192.∼16H2O with n <27 and typically ∼3. The ZSM-5 framework contains 2 intersecting channel systems, 1 sinusoidal parallel to [001] and 1 straight parallel to [010]. The elliptical 10-membered ring openings controlling the channels have an effective diam. between those of zeolite Linde A and faujasite.
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Nosé, S. A Unified Formulation of the Constant Temperature Molecular Dynamics Methods J. Chem. Phys. 1984, 81, 511 DOI: 10.1063/1.447334
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A unified formulation of the constant-temperature molecular-dynamics methods
Nose, Shuichi
Journal of Chemical Physics (1984), 81 (1), 511-19CODEN: JCPSA6; ISSN:0021-9606.
Three recently proposed const. temp. mol. dynamics methods [N., (1984) (1); W. G. Hoover et al., (1982) (2); D. J. Evans and G. P. Morris, (1983) (2); and J. M. Haile and S. Gupta, 1983) (3)] are examd. anal. via calcg. the equil. distribution functions and comparing them with that of the canonical ensemble. Except for effects due to momentum and angular momentum conservation, method (1) yields the rigorous canonical distribution in both momentum and coordinate space. Method (2) can be made rigorous in coordinate space, and can be derived from method (1) by imposing a specific constraint. Method (3) is not rigorous and gives a deviation of order N-1/2 from the canonical distribution (N the no. of particles). The results for the const. temp.-const. pressure ensemble are similar to the canonical ensemble case.
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Nosé, S. Constant Temperature Molecular Dynamics Methods Prog. Theor. Phys. Suppl. 1991, 103, 1– 46 DOI: 10.1143/PTPS.103.1
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Constant-temperature molecular-dynamics methods
Nose, Shuichi
Progress of Theoretical Physics Supplement (1991), 103 (), 1-46CODEN: PTPSEP; ISSN:0375-9687.
A review with 62 refs. on how the canonical distribution is realized in simulations based on deterministic dynamical equations. Basic formulations and extensions of two const.-temp. mol. dynamics methods, the constrained and the extended system methods, are discussed.
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Bylander, D. M.; Kleinman, L. Energy Fluctuations Induced by the Nosé Thermostat Phys. Rev. B: Condens. Matter Mater. Phys. 1992, 46, 13756– 13761 DOI: 10.1103/PhysRevB.46.13756
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Energy fluctuations induced by the Nose thermostat
Bylander; Kleinman
Physical review. B, Condensed matter (1992), 46 (21), 13756-13761 ISSN:0163-1829.
There is no expanded citation for this reference.
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Momma, K.; Izumi, F. VESTA 3 for Three-Dimensional Visualization of Crystal, Volumetric and Morphology Data J. Appl. Crystallogr. 2011, 44, 1272– 1276 DOI: 10.1107/S0021889811038970
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VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data
Momma, Koichi; Izumi, Fujio
Journal of Applied Crystallography (2011), 44 (6), 1272-1276CODEN: JACGAR; ISSN:0021-8898. (International Union of Crystallography)
VESTA is a 3D visualization system for crystallog. studies and electronic state calcns. It was upgraded to the latest version, VESTA 3, implementing new features including drawing the external morphpol. of crysals; superimposing multiple structural models, volumetric data and crystal faces; calcn. of electron and nuclear densities from structure parameters; calcn. of Patterson functions from the structure parameters or volumetric data; integration of electron and nuclear densities by Voronoi tessellation; visualization of isosurfaces with multiple levels, detn. of the best plane for selected atoms; an extended bond-search algorithm to enable more sophisticated searches in complex mols. and cage-like structures; undo and redo is graphical user interface operations; and significant performance improvements in rendering isosurfaces and calcg. slices.
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Tuckerman, M.; Laasonen, K.; Sprik, M.; Parrinello, M. Ab Initio Molecular Dynamics Simulation of the Solvation and Transport of Hydronium and Hydroxyl Ions in Water J. Chem. Phys. 1995, 103, 150 DOI: 10.1063/1.469654
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Ab initio molecular dynamics simulation of the solvation and transport of hydronium and hydroxyl ions in water
Tuckerman, M.; Laasonen, K.; Sprik, M.; Parrinello, M.
Journal of Chemical Physics (1995), 103 (1), 150-61CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
Charge defects in water created by excess or missing protons appear in the form of solvated hydronium H3O+ and hydroxyl OH- ions. Using the method of ab initio mol. dynamics, we have investigated the structure and proton transfer dynamics of the solvation complexes, which embed the ions in the network of hydrogen bonds in the liq. In our ab initio mol. dynamics approach, the interat. forces are calcd. each time step from the instantaneous electronic structure using d. functional methods. All hydrogen atoms, including the excess proton, are treated as classical particles with the mass of a deuterium atom. For the H3O+ ion we find a dynamic solvation complex, which continuously fluctuates between a (H5O2)+ and a (H9O4)+ structure as a result of proton transfer. The OH- has a predominantly planar fourfold coordination forming a (H9O5)- complex. Occasionally this complex is transformed in a more open tetrahedral (H7H4)- structure. Proton transfer is obsd. only for the more waterlike (H7O4)- complex. Transport of the charge defects is a concerted dynamical process coupling proton transfer along hydrogen bonds and reorganization of the local environment. The simulation results strongly support the structural diffusion mechanism for charge transport. In this model, the entire structure-and not the constituent particles-of the charged complex migrates through the hydrogen bond network. For H3O+, we propose that transport of the excess proton is driven by coordination fluctuations in the first solvation shell (i.e., second solvation shell dynamics). The rate-limiting step for OH- diffusion is the formation of the (H7O4)- structure, which is the solvation state showing proton transfer activity.
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Henkelman, G.; Arnaldsson, A.; Jónsson, H. A Fast and Robust Algorithm for Bader Decomposition of Charge Density Comput. Mater. Sci. 2006, 36, 354– 360 DOI: 10.1016/j.commatsci.2005.04.010
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Sanville, E.; Kenny, S. D.; Smith, R.; Henkelman, G. Improved Grid-Based Algorithm for Bader Charge Allocation J. Comput. Chem. 2007, 28, 899– 908 DOI: 10.1002/jcc.20575
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Improved grid-based algorithm for Bader charge allocation
Sanville, Edward; Kenny, Steven D.; Smith, Roger; Henkelman, Graeme
Journal of Computational Chemistry (2007), 28 (5), 899-908CODEN: JCCHDD; ISSN:0192-8651. (John Wiley & Sons, Inc.)
An improvement to the grid-based algorithm of Henkelman et al. for the calcn. of Bader vols. is suggested, which more accurately calcs. at. properties as predicted by the theory of Atoms in Mols. The CPU time required by the improved algorithm to perform the Bader anal. scales linearly with the no. of interat. surfaces in the system. The new algorithm corrects systematic deviations from the true Bader surface, calcd. by the original method and also does not require explicit representation of the interat. surfaces, resulting in a more robust method of partitioning charge d. among atoms in the system. Applications of the method to some small systems are given and it is further demonstrated how the method can be used to define an energy per atom in ab initio calcns.
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Tang, W.; Sanville, E.; Henkelman, G. A Grid-Based Bader Analysis Algorithm without Lattice Bias J. Phys.: Condens. Matter 2009, 21, 084204 DOI: 10.1088/0953-8984/21/8/084204
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A grid-based Bader analysis algorithm without lattice bias
Tang, W.; Sanville, E.; Henkelman, G.
Journal of Physics: Condensed Matter (2009), 21 (8), 084204/1-084204/7CODEN: JCOMEL; ISSN:0953-8984. (Institute of Physics Publishing)
A computational method for partitioning a charge d. grid into Bader vols. is presented which is efficient, robust, and scales linearly with the no. of grid points. The partitioning algorithm follows the steepest ascent paths along the charge d. gradient from grid point to grid point until a charge d. max. is reached. In this paper, we describe how accurate off-lattice ascent paths can be represented with respect to the grid points. This improvement maintains the efficient linear scaling of an earlier version of the algorithm, and eliminates a tendency for the Bader surfaces to be aligned along the grid directions. As the algorithm assigns grid points to charge d. maxima, subsequent paths are terminated when they reach previously assigned grid points. It is this grid-based approach which gives the algorithm its efficiency, and allows for the anal. of the large grids generated from plane-wave-based d. functional theory calcns.
39
Wernet, P.; Nordlund, D.; Bergmann, U.; Cavalleri, M.; Odelius, M.; Ogasawara, H.; Näslund, L. A.; Hirsch, T. K.; Ojamäe, L.; Glatzel, P. The Structure of the First Coordination Shell in Liquid Water Science 2004, 304, 995– 999 DOI: 10.1126/science.1096205
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The Structure of the First Coordination Shell in Liquid Water
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Science (Washington, DC, United States) (2004), 304 (5673), 995-999CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
X-ray absorption spectroscopy and x-ray Raman scattering were used to probe the mol. arrangement in the 1st coordination shell of liq. H2O. The local structure is characterized by comparison with bulk and surface of ordinary hexagonal ice Ih and with calcd. spectra. Most mols. in liq. H2O are in 2 H-bonded configurations with 1 strong donor and 1 strong acceptor H bond in contrast to the 4 H-bonded tetrahedral structure in ice. Upon heating from 25° to 90°, 5 to 10% of the mols. change from tetrahedral environments to 2 H-bonded configurations. Findings are consistent with neutron and x-ray diffraction data, and combining the results sets a strong limit for possible local structure distributions in liq. H2O. Serious discrepancies with structures based on current mol. dynamics simulations are obsd.
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Noncovalent interactions are ubiquitous in mol. and condensed-phase environments, and hence a reliable theor. description of these fundamental interactions could pave the way toward a more complete understanding of the microscopic underpinnings for a diverse set of systems in chem. and biol. In this work, we demonstrate that recent algorithmic advances coupled to the availability of large-scale computational resources make the stochastic quantum Monte Carlo approach to solving the Schrodinger equation an optimal contender for attaining "chem. accuracy" (1 kcal/mol) in the binding energies of supramol. complexes of chem. relevance. To illustrate this point, we considered a select set of seven host-guest complexes, representing the spectrum of noncovalent interactions, including dispersion or van der Waals forces, π-π stacking, hydrogen bonding, hydrophobic interactions, and electrostatic (ion-dipole) attraction. A detailed anal. of the interaction energies reveals that a complete theor. description necessitates treatment of terms well beyond the std. London and Axilrod-Teller contributions to the van der Waals dispersion energy.
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Huang, S.-J.; Yang, C.-Y.; Zheng, A.; Feng, N.; Yu, N.; Wu, P.-H.; Chang, Y.-C.; Lin, Y.-C.; Deng, F.; Liu, S.-B. New Insights into Keggin-Type 12-Tungstophosphoric Acid from 31P MAS NMR Analysis of Absorbed Trimethylphosphine Oxide and DFT Calculations Chem. - Asian J. 2011, 6, 137– 148 DOI: 10.1002/asia.201000572
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New Insights into Keggin-Type 12-Tungstophosphoric Acid from 31P MAS NMR Analysis of Absorbed Trimethylphosphine Oxide and DFT Calculations
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The acid and transport properties of the anhyd. Keggin-type 12-tungstophosphoric acid (H3PW12O40; HPW) have been studied by solid-state 31P magic-angle spinning NMR of absorbed trimethylphosphine oxide (TMPO) in conjunction with DFT calcns. Accordingly, 31P NMR resonances arising from various protonated complexes, such as TMPOH+ and (TMPO)2H+ adducts, could be unambiguously identified. It was found that thermal pretreatment of the sample at elevated temps. (≥423 K) is a prerequisite for ensuring complete penetration of the TMPO guest probe mol. into HPW particles. Transport of the TMPO absorbate into the matrix of the HPW adsorbent was found to invoke a desorption/absorption process assocd. with the (TMPO)2H+ adducts. Consequently, three types of protonic acid sites with distinct superacid strengths, which correspond to 31P chem. shifts of 92.1, 89.4, and 87.7 ppm, were obsd. for HPW samples loaded with less than three mols. of TMPO per Keggin unit. Together with detailed DFT calcns., these results support the scenario that the TMPOH+ complexes are assocd. with protons located at three different terminal oxygen (Od) sites of the PW12O403- polyanions. Upon increasing the TMPO loading to >3.0 mols. per Keggin unit, abrupt decreases in acid strength and the corresponding structural variations were attributed to the change in secondary structure of the pseudoliquid phase of HPW in the presence of excessive guest absorbate.

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Published August 8, 2016

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Abstract
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Figure 1
Figure 1. Representation of the BA sites at the internal (bottom, left panel) and at the external (bottom, right panel) surfaces. One of the two pentasil layers that form the slab is identified by a black-line rectangle (top, right panel). The Al-substituted T2-site (light blue ball) with the proton (white ball) at the O1 position (red ball) are shown. The rest of the O atoms and silanol OH groups were deleted for a better view; Si atoms are represented by orange sticks. A molecule of trimethylphosphine oxide is shown in the bottom-right corner; H is represented in white, C in gray, P in brown, and O in red.
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Figure 2
Figure 2. Correlation of the O_(P)–H distances and the ³¹P chemical shifts using Zheng et al. data (black squares linked with black lines). (11) The experimental classification of the acid sites of the zeolite MFI according to the TMPO ³¹P chemical shift is indicated by vertical dashed blue lines. (10, 12) The experimental classification is used to extrapolate the expected O_(P)–H distances from its interception with the theoretical curve (indicated by red circles); the corresponding distance values are written above the horizontal dashed black lines. The spectrum of O_(P)–H distances is divided into three zones taking the middle points between the extrapolated O_(P)–H distances. These zones are shaded alternately in light gray and white, corresponding to (top) weak acids, (center) strong acids, and (bottom) very strong acids; the limits of each range are indicated by the red numbers at the left-hand side of the graph.
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Figure 3
Figure 3. Representation of the interaction of a single TMPO molecule with (a, b) the internal and (c, d) the external BA sites after local optimization; H in white, C in gray, P in brown, O in red, Al in light blue, and Si represented by orange sticks. All the framework O atoms (except the protonated one) and silanol OH groups were deleted for an enhanced view. Related structural values are presented in Table 1.
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Figure 4
Figure 4. Configuration after 10 ps of MD simulation of the locally optimized structure in Figure 3b.
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Figure 5
Figure 5. Representation of the adsorption of two TMPO molecules on (a) the internal and (b) the external BA sites after local optimization. H in white, C in gray, P in brown, O in red, Al in light blue, and Si represented by orange sticks. All the framework O atoms (except the protonated one) and silanol OH groups were deleted for an enhanced view. Related structural values are presented in Table 3.
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Figure 6
Figure 6. Representation of two TMPO molecules on (a) the internal BA site and (b) the external BA site after 10 ps of MD simulation of the locally optimized structure in Figure 5.
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Figure 7
Figure 7. Representation of the adsorption of three TMPO molecules on (a) the internal and (b) the external BA sites after local optimization. H in white, C in gray, P in brown, O in red, Al in light blue, and Si represented by orange sticks. All the framework O atoms (except the protonated one) and silanol OH groups were deleted for an enhanced view. The interaction between one of the nonprotonated TMPO and the Al atom is represented by a stick connecting O_(P2) to the Al atom. Related structural values are presented in Table 5.
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Figure 8
Figure 8. Charge difference isosurfaces with values of 0.005 bohr^–3 calculated from eq 1. (a) Internal BA site; (b) external BA site. The structures correspond to those shown in Figure 7. H in white, C in gray, P in brown, O in red, Al in light blue, and Si represented by orange sticks. All the framework O atoms (except the protonated one) and silanol OH groups were deleted for an enhanced view.
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Figure 9
Figure 9. Projected density of states (PDOS) of (a) O_(P2)(2p), (b) Al(3s), and (c) Al(3p) for the 3TMPO/1BA configuration observed in the cavity according to Figure 7a. PDOS of (d) Al(3s) and (e) Al(3p) for the structure without the nonprotonated TMPO that interacts with the Al center.
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Figure 10
Figure 10. Representation of (a, c) TMPOH⁺ and (b, d) (TMPO)₂H⁺ at more than 5 Å away from the BA site after local optimization. (a, b) Internal surface; (c, d) external surface. H in white, C in gray, P in brown, O in red, and Si represented by orange sticks. All the framework O atoms (except the protonated one) and silanol OH groups were deleted for an enhanced view. Related structural values are presented in Table 8.
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  Zheng, Anmin; Liu, Shang-Bin; Deng, Feng
  Solid State Nuclear Magnetic Resonance (2013), 55-56 (), 12-27CODEN: SSNRE4; ISSN:0926-2040. (Elsevier)
  A review; characterization of the surface acidic properties of solid acid catalysts is a key issue in heterogeneous catalysis. Important acid features of solid acids, such as their type (Bronsted vs. Lewis acid), distribution and accessibility (internal vs. external sites), concn. (amt.), and strength of acid sites are crucial factors dictating their reactivity and selectivity. This short review provides information on different solid-state NMR techniques used for acidity characterization of solid acid catalysts. In particular, different approaches using probe mols. contg. a specific nucleus of interest, such as pyridine-d5, 2-13C-acetone, trimethylphosphine, and trimethylphosphine oxide, are compared. Incorporation of valuable information (such as the adsorption structure, deprotonation energy, and NMR parameters) from d. functional theory (DFT) calcns. can yield explicit correlations between the chem. shift of adsorbed probe mols. and the intrinsic acid strength of solid acids. Methods that combine exptl. NMR data with DFT calcns. can therefore provide both qual. and quant. information on acid sites.
8. 8
  Xu, J.; Zheng, A.; Yang, J.; Su, Y.; Wang, J.; Zeng, D.; Zhang, M.; Ye, C.; Deng, F. Acidity of Mesoporous MoOx/ZrO2 and WOx/ZrO2Materials: A Combined Solid-State NMR and Theoretical Calculation Study J. Phys. Chem. B 2006, 110, 10662– 10671 DOI: 10.1021/jp0614087
  8
  Acidity of Mesoporous MoOx/ZrO2 and WOx/ZrO2 Materials: A Combined Solid-State NMR and Theoretical Calculation Study
  Xu, Jun; Zheng, Anmin; Yang, Jun; Su, Yongchao; Wang, Jiqing; Zeng, Danlin; Zhang, Mingjin; Ye, Chaohui; Deng, Feng
  Journal of Physical Chemistry B (2006), 110 (22), 10662-10671CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
  The acidity of mesoporous MoOx/ZrO2 and WOx/ZrO2 materials was studied in detail by multinuclear solid-state NMR techniques as well as DFT quantum chem. calcns. The 1H MAS NMR expts. clearly revealed two different types of strong Bronsted acid sites on both MoOx/ZrO2 and WOx/ZrO2 mesoporous materials, which were able to prontonate adsorbed pyrine-d5 (resulting in 1H NMR signals at chem. shifts in the range 16-19 ppm) as well as adsorbed trimethylphosphine (giving rise to 31P NMR signal at ∼0 ppm). The 13C NMR of adsorbed 2-13C-acetone indicated that the av. Bronsted acid strength of the two mesoporous materials was stronger than that of zeolite HZSM-5 but still weaker than that of 100% H2SO4, which was in good agreement with theor. predictions. The quantum chem. calcns. revealed the detailed structures of the two distinct types of Bronsted acid sites formed on the mesoporous MoOx/ZrO2 and WOx/ZrO2. The existence of both monomer and oligomer Mo (or W) species contg. a Mo-OH-Zr (or W-OH-Zr) bridging OH group was confirmed with the former having an acid strength close to zeolite HZSM-5, with the latter having an acid strength similar to sulfated zirconia. From the authors' NMR exptl. and theor. calcn. results, a possible mechanism is proposed for the formation of acid sites on these mesoporous materials.
9. 9
  Li, S.; Huang, S.-J.; Shen, W.; Zhang, H.; Fang, H.; Zheng, A.; Liu, S.-B.; Deng, F. Probing the Spatial Proximities among Acid Sites in Dealuminated H-Y Zeolite by Solid-State NMR Spectroscopy J. Phys. Chem. C 2008, 112, 14486– 14494 DOI: 10.1021/jp803494n
  9
  Probing the Spatial Proximities among Acid Sites in Dealuminated H-Y Zeolite by Solid-State NMR Spectroscopy
  Li, Shenhui; Huang, Shing-Jong; Shen, Wanling; Zhang, Hailu; Fang, Hanjun; Zheng, Anmin; Liu, Shang-Bin; Deng, Feng
  Journal of Physical Chemistry C (2008), 112 (37), 14486-14494CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
  A comprehensive study has been made to probe the spatial proximities among different acid sites in dealuminated H-Y zeolites modified with various degrees of calcination, steam, and acid treatments by using a variety of different solid-state NMR techniques, including multinuclear MAS NMR and two-dimensional 1H double-quantum (DQ) MAS NMR spectroscopy. The effects of dealumination treatments on the nature, concn., and location of extraframework Al species in H-Y zeolites were followed by 1H DQ MAS NMR of hydroxyl protons in conjunction with 1H, 27Al, and 29Si MAS NMR results. It was found that the extraframework AlOH species (Lewis acid sites) are always in close proximity to the bridging AlOHSi hydroxyls (Bronsted acid sites) on the framework of dealuminated H-Y zeolites prepd. by thermal and hydrothermal treatments, indicating the presence of a Bronsted/Lewis acid synergy effect. However, such an effect is absent in acid-treated H-Y zeolites, as also confirmed by 13C CP/MAS NMR of adsorbed 2-13C-acetone.
10. 10
  Zhao, Q.; Chen, W.-H.; Huang, S.-J.; Wu, Y.-C.; Lee, H.-K.; Liu, S.-B. Discernment and Quantification of Internal and External Acid Sites on Zeolites J. Phys. Chem. B 2002, 106, 4462– 4469 DOI: 10.1021/jp015574k
  10
  Discernment and Quantification of Internal and External Acid Sites on Zeolites
  Zhao, Qi; Chen, Wen-Hua; Huang, Shing-Jong; Wu, Yu-Chih; Lee, Huang-Kuei; Liu, Shang-Bin
  Journal of Physical Chemistry B (2002), 106 (17), 4462-4469CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
  A new methodol. is reported for concurrent qual. and quant. characterization of internal and external acid sites in zeolitic catalysts. H-ZSM-5 zeolites with varied Si/Al ratios have been examd. by solid-state 31P MAS NMR using different adsorbed probe mols., namely trimethylphosphine oxides (TMPO) and tributylphosphine oxide (TBPO), in conjunction with elemental anal. Up to seven distinct 31P resonance peaks at 86, 75, 67, 63, 53, 43, and 30 ppm were identified from the 31P NMR spectra of adsorbed TMPO. The resonance peak at 30 ppm has never been obsd. previously and may be ascribed to mobile TMPO. The peak at 43 ppm is assigned to physisorbed TMPO. The rest of the peaks result from TMPOH+ complexes at Bronsted sites with peaks at higher chem. shifts reflecting acid sites of higher strengths. 31P NMR expts., performed with TMPO and TBPO adsorbed on a mesoporous MCM-41 sample, resp., provide further correlation of the internal and external acid sites. It is concluded that the peaks at 75 and 53 ppm arise exclusively from the internal Bronsted sites, whereas the peaks at 86, 67, and 63 ppm are assocd. with both internal and external acid sites. While the concn. and distribution of internal sites were found to increase with acid strengths, changing the Si/Al ratio of HZSM-5 has nearly no effect on the strength of the external acid sites.
11. 11
  Zheng, A.; Zhang, H.; Lu, X.; Liu, S.-B.; Deng, F. Theoretical Predictions of 31P NMR Chemical Shift Threshold of Trimethylphosphine Oxide Absorbed on Solid Acid Catalysts J. Phys. Chem. B 2008, 112, 4496– 4505 DOI: 10.1021/jp709739v
  11
  Theoretical Predictions of 31P NMR Chemical Shift Threshold of Trimethylphosphine Oxide Absorbed on Solid Acid Catalysts
  Zheng, Anmin; Zhang, Hailu; Lu, Xin; Liu, Shang-Bin; Deng, Feng
  Journal of Physical Chemistry B (2008), 112 (15), 4496-4505CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
  The 31P NMR chem. shifts of adsorbed trimethylphosphine oxide (TMPO) and the configurations of the corresponding TMPOH+ complexes on Bronsted acid sites with varying acid strengths in modeled zeolites have been predicted theor. by means of d. functional theory (DFT) quantum chem. calcns. The configuration of each TMPOH+ complex was optimized at the PW91/DNP level based on an 8T cluster model, whereas the 31P chem. shifts were calcd. with the gauge including AO (GIAO) approach at both the HF/TZVP and MP2/TZVP levels. A linear correlation between the 31P chem. shift of adsorbed TMPO and the proton affinity of the solid acids was obsd., and a threshold for superacidity (86 ppm) was detd. This threshold for superacidity was also confirmed by comparative investigations on other superacid systems, such as carborane acid and heteropolyoxometalate H3PW12O40. In conjunction with the strong correlation between the MP2 and the HF 31P isotropic shifts, the 8T cluster model was extended to more sophisticated models (up to 72T) that are not readily tractable at the GIAO-MP2 level, and a 31P chem. shift of 86 ppm was detd. for TMPO adsorbed on zeolite H-ZSM-5, which is in good agreement with the NMR exptl. data.
12. 12
  Seo, Y.; Cho, K.; Jung, Y.; Ryoo, R. Characterization of the Surface Acidity of MFI Zeolite Nanosheets by 31 P NMR of Adsorbed Phosphine Oxides and Catalytic Cracking of Decalin ACS Catal. 2013, 3, 713– 720 DOI: 10.1021/cs300824e
  12
  Characterization of the Surface Acidity of MFI Zeolite Nanosheets by 31P NMR of Adsorbed Phosphine Oxides and Catalytic Cracking of Decalin
  Seo, Yongbeom; Cho, Kanghee; Jung, Younjae; Ryoo, Ryong
  ACS Catalysis (2013), 3 (4), 713-720CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
  MFI zeolite nanosheets tailored to 2.5-nm thickness were synthesized using a surfactant-type zeolite structure-directing agent, [C22H45-N+(CH3)2-C6H12-N+(CH3)2-C6H13](Br-)2. The zeolite nanosheets possessed Bronsted acid sites on their external surfaces as well as in the internal micropore walls. The acid strength and concn. was characterized by the 31P NMR signals of the adsorbed trimethylphosphine oxide and tributylphosphine oxide. The 31P NMR investigation identified three types of Bronsted acid sites with different strengths on external surfaces; there were four types inside the micropores. A linear correlation has been established between the no. of the external strongest acid sites and the catalytic activity in decalin cracking for the MFI zeolite catalysts investigated in this work.
13. 13
  Ni, Y.; Sun, A.; Wu, X.; Hai, G.; Hu, J.; Li, T.; Li, G. The Preparation of Nano-Sized H[Zn,Al]ZSM-5 Zeolite and Its Application in the Aromatization of Methanol Microporous Mesoporous Mater. 2011, 143, 435– 442 DOI: 10.1016/j.micromeso.2011.03.029
  There is no corresponding record for this reference.
14. 14
  Costa, C.; Dzikh, I. P.; Lopes, J. M.; Lemos, F.; Ribeiro, F. R. Activity–acidity Relationship in Zeolite ZSM-5. Application of Brönsted-Type Equations J. Mol. Catal. A: Chem. 2000, 154, 193– 201 DOI: 10.1016/S1381-1169(99)00374-X
  14
  Activity-acidity relationship in zeolite ZSM-5. Application of Bronsted-type equations
  Costa, C.; Dzikh, I. P.; Lopes, J. M.; Lemos, F.; Ribeiro, F. R.
  Journal of Molecular Catalysis A: Chemical (2000), 154 (1-2), 193-201CODEN: JMCCF2; ISSN:1381-1169. (Elsevier Science B.V.)
  In this paper the relation between activity and acidity in a variety of ZSM-5 zeolite catalysts, with different Si/Al ratios and different protonic content, is analyzed and a quant. correlation is obtained. The acid site strength distribution was estd. using temp.-programmed desorption (TPD) of ammonia by applying a digital deconvolution method to the curves. These data were then correlated with exptl. catalytic activity data for the same catalysts towards n-heptane cracking reaction, by means of a Bronsted-type equation similar to the ones used for homogeneous acid catalysis and already used for other zeolites. It can be noticed that the same types of equation that are used for homogeneous acid catalysis also hold for heterogeneous acid catalysis and that the activation energy for ammonia desorption can be used as acid-strength scale for the purpose of correlation with catalytic activity.
15. 15
  Kwok, T. J.; Jayasuriya, K.; Damavarapu, R.; Brodman, B. W. Application of H-ZSM-5 Zeolite for Regioselective Mononitration of Toluene J. Org. Chem. 1994, 59, 4939– 4942 DOI: 10.1021/jo00096a042
  There is no corresponding record for this reference.
16. 16
  Wang, Y.; Jian, G.; Peng, Z.; Hu, J.; Wang, X.; Duan, W.; Liu, B. Preparation and Application of Ba/ZSM-5 Zeolite for Reaction of Methyl Vinyl Ether and Methanol Catal. Commun. 2015, 66, 34– 37 DOI: 10.1016/j.catcom.2015.03.012
  16
  Preparation and application of Ba/ZSM-5 zeolite for reaction of methyl vinyl ether and methanol
  Wang, Yang; Jian, Guixing; Peng, Zhihong; Hu, Jiaji; Wang, Xin; Duan, Wubiao; Liu, Bo
  Catalysis Communications (2015), 66 (), 34-37CODEN: CCAOAC; ISSN:1566-7367. (Elsevier B.V.)
  The ZSM-5 zeolite is widely used to catalyze the reactions of methanol to olefins. Herein, we have prepd. the H-ZSM-5 doped with barium (Ba/ZSM-5) using incipient wetness impregnation method. The Ba modified catalysts were used to catalyze a new reaction of methanol with Me vinyl ether to improve the selectivity of ethylene and propylene (C=2 + C=3). The reaction catalyzed by Ba doped H-ZSM-5 shows higher propylene selectivity over H-ZSM-5. The reaction mechanism is discussed.
17. 17
  Zheng, A.; Han, B.; Li, B.; Liu, S.-B.; Deng, F. Enhancement of Brønsted Acidity in Zeolitic Catalysts due to an Intermolecular Solvent Effect in Confined Micropores Chem. Commun. 2012, 48, 6936– 6938 DOI: 10.1039/c2cc32498a
  17
  Enhancement of Bronsted acidity in zeolitic catalysts due to an intermolecular solvent effect in confined micropores
  Zheng, Anmin; Han, Bing; Li, Bojie; Liu, Shang-Bin; Deng, Feng
  Chemical Communications (Cambridge, United Kingdom) (2012), 48 (55), 6936-6938CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
  Solid-state NMR and DFT calcn. studies certified the presence of an intermol. solvent effect for mols. confined in microporous zeolite, leading to a notable increase in Bronsted acidity of the solid acid catalyst.
18. 18
  Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 558– 561 DOI: 10.1103/PhysRevB.47.558
  18
  Ab initio molecular dynamics of liquid metals
  Kresse, G.; Hafner, J.
  Physical Review B: Condensed Matter and Materials Physics (1993), 47 (1), 558-61CODEN: PRBMDO; ISSN:0163-1829.
  The authors present ab initio quantum-mech. mol.-dynamics calcns. based on the calcn. of the electronic ground state and of the Hellmann-Feynman forces in the local-d. approxn. at each mol.-dynamics step. This is possible using conjugate-gradient techniques for energy minimization, and predicting the wave functions for new ionic positions using sub-space alignment. This approach avoids the instabilities inherent in quantum-mech. mol.-dynamics calcns. for metals based on the use of a factitious Newtonian dynamics for the electronic degrees of freedom. This method gives perfect control of the adiabaticity and allows one to perform simulations over several picoseconds.
19. 19
  Kresse, G.; Hafner, J. Ab Initio Molecular-Dynamics Simulation of the Liquid-Metal–amorphous-Semiconductor Transition in Germanium Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 49, 14251– 14269 DOI: 10.1103/PhysRevB.49.14251
  19
  Ab initio molecular-dynamics simulation of the liquid-metal-amorphous-semiconductor transition in germanium
  Kresse, G.; Hafner, J.
  Physical Review B: Condensed Matter and Materials Physics (1994), 49 (20), 14251-69CODEN: PRBMDO; ISSN:0163-1829.
  The authors present ab initio quantum-mech. mol.-dynamics simulations of the liq.-metal-amorphous-semiconductor transition in Ge. The simulations are based on (a) finite-temp. d.-functional theory of the 1-electron states, (b) exact energy minimization and hence calcn. of the exact Hellmann-Feynman forces after each mol.-dynamics step using preconditioned conjugate-gradient techniques, (c) accurate nonlocal pseudopotentials, and (d) Nose' dynamics for generating a canonical ensemble. This method gives perfect control of the adiabaticity of the electron-ion ensemble and allows the authors to perform simulations over >30 ps. The computer-generated ensemble describes the structural, dynamic, and electronic properties of liq. and amorphous Ge in very good agreement with expt.. The simulation allows the authors to study in detail the changes in the structure-property relation through the metal-semiconductor transition. The authors report a detailed anal. of the local structural properties and their changes induced by an annealing process. The geometrical, bounding, and spectral properties of defects in the disordered tetrahedral network are studied and compared with expt.
20. 20
  Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set Comput. Mater. Sci. 1996, 6, 15– 50 DOI: 10.1016/0927-0256(96)00008-0
  20
  Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set
  Kresse, G.; Furthmuller, J.
  Computational Materials Science (1996), 6 (1), 15-50CODEN: CMMSEM; ISSN:0927-0256. (Elsevier)
  The authors present a detailed description and comparison of algorithms for performing ab-initio quantum-mech. calcns. using pseudopotentials and a plane-wave basis set. The authors will discuss: (a) partial occupancies within the framework of the linear tetrahedron method and the finite temp. d.-functional theory, (b) iterative methods for the diagonalization of the Kohn-Sham Hamiltonian and a discussion of an efficient iterative method based on the ideas of Pulay's residual minimization, which is close to an order N2atoms scaling even for relatively large systems, (c) efficient Broyden-like and Pulay-like mixing methods for the charge d. including a new special preconditioning optimized for a plane-wave basis set, (d) conjugate gradient methods for minimizing the electronic free energy with respect to all degrees of freedom simultaneously. The authors have implemented these algorithms within a powerful package called VAMP (Vienna ab-initio mol.-dynamics package). The program and the techniques have been used successfully for a large no. of different systems (liq. and amorphous semiconductors, liq. simple and transition metals, metallic and semi-conducting surfaces, phonons in simple metals, transition metals and semiconductors) and turned out to be very reliable.
21. 21
  Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169– 11186 DOI: 10.1103/PhysRevB.54.11169
  21
  Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set
  Kresse, G.; Furthmueller, J.
  Physical Review B: Condensed Matter (1996), 54 (16), 11169-11186CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
  The authors present an efficient scheme for calcg. the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrixes will be discussed. This approach is stable, reliable, and minimizes the no. of order Natoms3 operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special "metric" and a special "preconditioning" optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent and self-consistent calcns. It will be shown that the no. of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order Natoms2 scaling is found for systems contg. up to 1000 electrons. If we take into account that the no. of k points can be decreased linearly with the system size, the overall scaling can approach Natoms. They have implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large no. of different systems (liq. and amorphous semiconductors, liq. simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable.
22. 22
  Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple Phys. Rev. Lett. 1996, 77, 3865– 3868 DOI: 10.1103/PhysRevLett.77.3865
  22
  Generalized gradient approximation made simple
  Perdew, John P.; Burke, Kieron; Ernzerhof, Matthias
  Physical Review Letters (1996), 77 (18), 3865-3868CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
  Generalized gradient approxns. (GGA's) for the exchange-correlation energy improve upon the local spin d. (LSD) description of atoms, mols., and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental consts. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential.
23. 23
  Grimme, S. Semiempirical GGA-Type Density Functional Constructed with a Long-Range Dispersion Correction J. Comput. Chem. 2006, 27, 1787– 1799 DOI: 10.1002/jcc.20495
  23
  Semiempirical GGA-type density functional constructed with a long-range dispersion correction
  Grimme, Stefan
  Journal of Computational Chemistry (2006), 27 (15), 1787-1799CODEN: JCCHDD; ISSN:0192-8651. (John Wiley & Sons, Inc.)
  A new d. functional (DF) of the generalized gradient approxn. (GGA) type for general chem. applications termed B97-D is proposed. It is based on Becke's power-series ansatz from 1997 and is explicitly parameterized by including damped atom-pairwise dispersion corrections of the form C6·R-6. A general computational scheme for the parameters used in this correction has been established and parameters for elements up to xenon and a scaling factor for the dispersion part for several common d. functionals (BLYP, PBE, TPSS, B3LYP) are reported. The new functional is tested in comparison with other GGAs and the B3LYP hybrid functional on std. thermochem. benchmark sets, for 40 noncovalently bound complexes, including large stacked arom. mols. and group II element clusters, and for the computation of mol. geometries. Further cross-validation tests were performed for organometallic reactions and other difficult problems for std. functionals. In summary, it is found that B97-D belongs to one of the most accurate general purpose GGAs, reaching, for example for the G97/2 set of heat of formations, a mean abs. deviation of only 3.8 kcal mol-1. The performance for noncovalently bound systems including many pure van der Waals complexes is exceptionally good, reaching on the av. CCSD(T) accuracy. The basic strategy in the development to restrict the d. functional description to shorter electron correlation lengths scales and to describe situations with medium to large interat. distances by damped C6·R-6 terms seems to be very successful, as demonstrated for some notoriously difficult reactions. As an example, for the isomerization of larger branched to linear alkanes, B97-D is the only DF available that yields the right sign for the energy difference. From a practical point of view, the new functional seems to be quite robust and it is thus suggested as an efficient and accurate quantum chem. method for large systems where dispersion forces are of general importance.
24. 24
  Hernandez-Tamargo, C. E.; Roldan, A.; de Leeuw, N. H. A Density Functional Theory Study of the Structure of Pure-Silica and Aluminium-Substituted MFI Nanosheets J. Solid State Chem. 2016, 237, 192– 203 DOI: 10.1016/j.jssc.2016.02.006
  24
  A density functional theory study of the structure of pure-silica and aluminium-substituted MFI nanosheets
  Hernandez-Tamargo, Carlos E.; Roldan, Alberto; de Leeuw, Nora H.
  Journal of Solid State Chemistry (2016), 237 (), 192-203CODEN: JSSCBI; ISSN:0022-4596. (Elsevier B.V.)
  The authors present a theor. study of the structural features of silica and aluminum-substituted MFI nanosheets. The authors have analyzed the effects of aluminum substitution on the vibrational properties of silanols as well as the features of protons as counter-ions. The formation of the two-dimensional system did not lead to appreciable distortions within the framework. Moreover, the effects on the structure due to the aluminum dopants were the same in both the bulk and the slab. The principal differences were related to the silanol groups that form hydrogen-bonds with neighboring aluminum-substituted silanols, whereas intra-framework hydrogen-bonds increase the stability of aluminum-substituted silanols toward dehydration. Thus, the authors have complemented previous exptl. and theor. studies, showing the lamellar MFI zeolite to be a very stable material of high crystallinity regardless of its very thin structure.
25. 25
  Blöchl, P. E. Projector Augmented-Wave Method Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953– 17979 DOI: 10.1103/PhysRevB.50.17953
  25
  Projector augmented-wave method
  Blochl
  Physical review. B, Condensed matter (1994), 50 (24), 17953-17979 ISSN:0163-1829.
  There is no expanded citation for this reference.
26. 26
  Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1758– 1775 DOI: 10.1103/PhysRevB.59.1758
  26
  From ultrasoft pseudopotentials to the projector augmented-wave method
  Kresse, G.; Joubert, D.
  Physical Review B: Condensed Matter and Materials Physics (1999), 59 (3), 1758-1775CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
  The formal relationship between ultrasoft (US) Vanderbilt-type pseudopotentials and Blochl's projector augmented wave (PAW) method is derived. The total energy functional for US pseudopotentials can be obtained by linearization of two terms in a slightly modified PAW total energy functional. The Hamilton operator, the forces, and the stress tensor are derived for this modified PAW functional. A simple way to implement the PAW method in existing plane-wave codes supporting US pseudopotentials is pointed out. In addn., crit. tests are presented to compare the accuracy and efficiency of the PAW and the US pseudopotential method with relaxed-core all-electron methods. These tests include small mols. (H2, H2O, Li2, N2, F2, BF3, SiF4) and several bulk systems (diamond, Si, V, Li, Ca, CaF2, Fe, Co, Ni). Particular attention is paid to the bulk properties and magnetic energies of Fe, Co, and Ni.
27. 27
  Ho, K.-M.; Fu, C. L.; Harmon, B. N.; Weber, W.; Hamann, D. R. Vibrational Frequencies and Structural Properties of Transition Metals via Total-Energy Calculations Phys. Rev. Lett. 1982, 49, 673– 676 DOI: 10.1103/PhysRevLett.49.673
  27
  Vibrational frequencies and structural properties of transition metals via total-energy calculations
  Ho, K. M.; Fu, C. L.; Harmon, B. N.; Weber, W.; Hamann, D. R.
  Physical Review Letters (1982), 49 (9), 673-6CODEN: PRLTAO; ISSN:0031-9007.
  The vibrational frequencies of selected normal modes can be obtained entirely from 1st principles with use of frozen phonon calcns. which involve the precise evaluation of crystal total energy as a function of lattice displacement. The calcns. allow a detailed anal. of the microscopic mechanism causing phonon anomalies and soft-mode phase transitions. Calcns. for Zr, Nb, and Mo were made using both tight-binding and pseudopotential methods.
28. 28
  Fu, C.-L.; Ho, K.-M. First-Principles Calculation of the Equilibrium Ground-State Properties of Transition Metals: Applications to Nb and Mo Phys. Rev. B: Condens. Matter Mater. Phys. 1983, 28, 5480– 5486 DOI: 10.1103/PhysRevB.28.5480
  28
  First-principles calculation of the equilibrium ground-state properties of transition metals: Application to niobium and molybdenum
  Fu, C. L.; Ho, K. M.
  Physical Review B: Condensed Matter and Materials Physics (1983), 28 (10), 5480-6CODEN: PRBMDO; ISSN:0163-1829.
  A self-consistent pseudopotential method was used to calc. the equil. ground-state properties of Mo and Nb. The calcd. equil. lattice consts., cohesive energies, and bulk moduli agree with exptl. data.
29. 29
  Quartieri, S.; Arletti, R.; Vezzalini, G.; Di Renzo, F.; Dmitriev, V. Elastic Behavior of MFI-Type Zeolites: 3 – Compressibility of Silicalite and Mutinaite J. Solid State Chem. 2012, 191, 201– 212 DOI: 10.1016/j.jssc.2012.03.039
  29
  Elastic behavior of MFI-type zeolites: 3. Compressibility of silicalite and mutinaite
  Quartieri, Simona; Arletti, Rossella; Vezzalini, Giovanna; Di Renzo, Francesco; Dmitriev, Vladimir
  Journal of Solid State Chemistry (2012), 191 (), 201-212CODEN: JSSCBI; ISSN:0022-4596. (Elsevier B.V.)
  We report the results of an in-situ synchrotron X-ray powder diffraction study, performed using silicone oil as "non-penetrating" pressure transmitting medium, of the elastic behavior of three zeolites with MFI-type framework: the natural zeolite mutinaite and two silicalites (labeled A and B) synthesized under different conditions. In mutinaite, no symmetry change is obsd. as a function of pressure; however, a phase transition from monoclinic (P21/n) to orthorhombic (Pnma) symmetry occurs at about 1.0 GPa in the silicalite samples. This phase transition is irreversible upon decompression. The second order bulk moduli of silicalite A and silicalite B, calcd. after the fulfillment of the phase transition, are: K 0=18.2(2) and K 0=14.3 (2) GPa, resp. These values makes silicalite the most compressible zeolite among those up to now studied in silicone oil. The structural deformations induced by HP in silicalite A were investigated by means of complete Rietveld structural refinements, before and after the phase transition, at P amb and 0.9 GPa, resp. The elastic behaviors of the three MFI-type zeolites here investigated were compared with those of Na-ZSM-5 and H-ZSM-5, studied in similar exptl. conditions: the two silicalites - which are the phases with the highest Si/Al ratios and hence the lowest extraframework contents - show the highest compressibility. On the contrary, the most rigid material is mutinaite, which has a very complex extraframework compn. characterized by a high no. of cations and water mols.
30. 30
  Kokotailo, G. T.; Lawton, S. L.; Olson, D. H.; Meier, W. M. Structure of Synthetic Zeolite ZSM-5 Nature 1978, 272, 437– 438 DOI: 10.1038/272437a0
  30
  Structure of synthetic zeolite ZSM-5
  Kokotailo, G. T.; Lawton, S. L.; Olson, D. H.; Meier, W. M.
  Nature (London, United Kingdom) (1978), 272 (5652), 437-8CODEN: NATUAS; ISSN:0028-0836.
  The structure of synthetic zeolite ZSM-5 was detd. The crystals are orthorhombic, space group Pnma, with a 20.1, b 19.9, and c 13.4 Å. The unit contents of the Na form are NanAlnSi96-nO192.∼16H2O with n <27 and typically ∼3. The ZSM-5 framework contains 2 intersecting channel systems, 1 sinusoidal parallel to [001] and 1 straight parallel to [010]. The elliptical 10-membered ring openings controlling the channels have an effective diam. between those of zeolite Linde A and faujasite.
31. 31
  Nosé, S. A Unified Formulation of the Constant Temperature Molecular Dynamics Methods J. Chem. Phys. 1984, 81, 511 DOI: 10.1063/1.447334
  31
  A unified formulation of the constant-temperature molecular-dynamics methods
  Nose, Shuichi
  Journal of Chemical Physics (1984), 81 (1), 511-19CODEN: JCPSA6; ISSN:0021-9606.
  Three recently proposed const. temp. mol. dynamics methods [N., (1984) (1); W. G. Hoover et al., (1982) (2); D. J. Evans and G. P. Morris, (1983) (2); and J. M. Haile and S. Gupta, 1983) (3)] are examd. anal. via calcg. the equil. distribution functions and comparing them with that of the canonical ensemble. Except for effects due to momentum and angular momentum conservation, method (1) yields the rigorous canonical distribution in both momentum and coordinate space. Method (2) can be made rigorous in coordinate space, and can be derived from method (1) by imposing a specific constraint. Method (3) is not rigorous and gives a deviation of order N-1/2 from the canonical distribution (N the no. of particles). The results for the const. temp.-const. pressure ensemble are similar to the canonical ensemble case.
32. 32
  Nosé, S. Constant Temperature Molecular Dynamics Methods Prog. Theor. Phys. Suppl. 1991, 103, 1– 46 DOI: 10.1143/PTPS.103.1
  32
  Constant-temperature molecular-dynamics methods
  Nose, Shuichi
  Progress of Theoretical Physics Supplement (1991), 103 (), 1-46CODEN: PTPSEP; ISSN:0375-9687.
  A review with 62 refs. on how the canonical distribution is realized in simulations based on deterministic dynamical equations. Basic formulations and extensions of two const.-temp. mol. dynamics methods, the constrained and the extended system methods, are discussed.
33. 33
  Bylander, D. M.; Kleinman, L. Energy Fluctuations Induced by the Nosé Thermostat Phys. Rev. B: Condens. Matter Mater. Phys. 1992, 46, 13756– 13761 DOI: 10.1103/PhysRevB.46.13756
  33
  Energy fluctuations induced by the Nose thermostat
  Bylander; Kleinman
  Physical review. B, Condensed matter (1992), 46 (21), 13756-13761 ISSN:0163-1829.
  There is no expanded citation for this reference.
34. 34
  Momma, K.; Izumi, F. VESTA 3 for Three-Dimensional Visualization of Crystal, Volumetric and Morphology Data J. Appl. Crystallogr. 2011, 44, 1272– 1276 DOI: 10.1107/S0021889811038970
  34
  VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data
  Momma, Koichi; Izumi, Fujio
  Journal of Applied Crystallography (2011), 44 (6), 1272-1276CODEN: JACGAR; ISSN:0021-8898. (International Union of Crystallography)
  VESTA is a 3D visualization system for crystallog. studies and electronic state calcns. It was upgraded to the latest version, VESTA 3, implementing new features including drawing the external morphpol. of crysals; superimposing multiple structural models, volumetric data and crystal faces; calcn. of electron and nuclear densities from structure parameters; calcn. of Patterson functions from the structure parameters or volumetric data; integration of electron and nuclear densities by Voronoi tessellation; visualization of isosurfaces with multiple levels, detn. of the best plane for selected atoms; an extended bond-search algorithm to enable more sophisticated searches in complex mols. and cage-like structures; undo and redo is graphical user interface operations; and significant performance improvements in rendering isosurfaces and calcg. slices.
35. 35
  Tuckerman, M.; Laasonen, K.; Sprik, M.; Parrinello, M. Ab Initio Molecular Dynamics Simulation of the Solvation and Transport of Hydronium and Hydroxyl Ions in Water J. Chem. Phys. 1995, 103, 150 DOI: 10.1063/1.469654
  35
  Ab initio molecular dynamics simulation of the solvation and transport of hydronium and hydroxyl ions in water
  Tuckerman, M.; Laasonen, K.; Sprik, M.; Parrinello, M.
  Journal of Chemical Physics (1995), 103 (1), 150-61CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
  Charge defects in water created by excess or missing protons appear in the form of solvated hydronium H3O+ and hydroxyl OH- ions. Using the method of ab initio mol. dynamics, we have investigated the structure and proton transfer dynamics of the solvation complexes, which embed the ions in the network of hydrogen bonds in the liq. In our ab initio mol. dynamics approach, the interat. forces are calcd. each time step from the instantaneous electronic structure using d. functional methods. All hydrogen atoms, including the excess proton, are treated as classical particles with the mass of a deuterium atom. For the H3O+ ion we find a dynamic solvation complex, which continuously fluctuates between a (H5O2)+ and a (H9O4)+ structure as a result of proton transfer. The OH- has a predominantly planar fourfold coordination forming a (H9O5)- complex. Occasionally this complex is transformed in a more open tetrahedral (H7H4)- structure. Proton transfer is obsd. only for the more waterlike (H7O4)- complex. Transport of the charge defects is a concerted dynamical process coupling proton transfer along hydrogen bonds and reorganization of the local environment. The simulation results strongly support the structural diffusion mechanism for charge transport. In this model, the entire structure-and not the constituent particles-of the charged complex migrates through the hydrogen bond network. For H3O+, we propose that transport of the excess proton is driven by coordination fluctuations in the first solvation shell (i.e., second solvation shell dynamics). The rate-limiting step for OH- diffusion is the formation of the (H7O4)- structure, which is the solvation state showing proton transfer activity.
36. 36
  Henkelman, G.; Arnaldsson, A.; Jónsson, H. A Fast and Robust Algorithm for Bader Decomposition of Charge Density Comput. Mater. Sci. 2006, 36, 354– 360 DOI: 10.1016/j.commatsci.2005.04.010
  There is no corresponding record for this reference.
37. 37
  Sanville, E.; Kenny, S. D.; Smith, R.; Henkelman, G. Improved Grid-Based Algorithm for Bader Charge Allocation J. Comput. Chem. 2007, 28, 899– 908 DOI: 10.1002/jcc.20575
  37
  Improved grid-based algorithm for Bader charge allocation
  Sanville, Edward; Kenny, Steven D.; Smith, Roger; Henkelman, Graeme
  Journal of Computational Chemistry (2007), 28 (5), 899-908CODEN: JCCHDD; ISSN:0192-8651. (John Wiley & Sons, Inc.)
  An improvement to the grid-based algorithm of Henkelman et al. for the calcn. of Bader vols. is suggested, which more accurately calcs. at. properties as predicted by the theory of Atoms in Mols. The CPU time required by the improved algorithm to perform the Bader anal. scales linearly with the no. of interat. surfaces in the system. The new algorithm corrects systematic deviations from the true Bader surface, calcd. by the original method and also does not require explicit representation of the interat. surfaces, resulting in a more robust method of partitioning charge d. among atoms in the system. Applications of the method to some small systems are given and it is further demonstrated how the method can be used to define an energy per atom in ab initio calcns.
38. 38
  Tang, W.; Sanville, E.; Henkelman, G. A Grid-Based Bader Analysis Algorithm without Lattice Bias J. Phys.: Condens. Matter 2009, 21, 084204 DOI: 10.1088/0953-8984/21/8/084204
  38
  A grid-based Bader analysis algorithm without lattice bias
  Tang, W.; Sanville, E.; Henkelman, G.
  Journal of Physics: Condensed Matter (2009), 21 (8), 084204/1-084204/7CODEN: JCOMEL; ISSN:0953-8984. (Institute of Physics Publishing)
  A computational method for partitioning a charge d. grid into Bader vols. is presented which is efficient, robust, and scales linearly with the no. of grid points. The partitioning algorithm follows the steepest ascent paths along the charge d. gradient from grid point to grid point until a charge d. max. is reached. In this paper, we describe how accurate off-lattice ascent paths can be represented with respect to the grid points. This improvement maintains the efficient linear scaling of an earlier version of the algorithm, and eliminates a tendency for the Bader surfaces to be aligned along the grid directions. As the algorithm assigns grid points to charge d. maxima, subsequent paths are terminated when they reach previously assigned grid points. It is this grid-based approach which gives the algorithm its efficiency, and allows for the anal. of the large grids generated from plane-wave-based d. functional theory calcns.
39. 39
  Wernet, P.; Nordlund, D.; Bergmann, U.; Cavalleri, M.; Odelius, M.; Ogasawara, H.; Näslund, L. A.; Hirsch, T. K.; Ojamäe, L.; Glatzel, P. The Structure of the First Coordination Shell in Liquid Water Science 2004, 304, 995– 999 DOI: 10.1126/science.1096205
  39
  The Structure of the First Coordination Shell in Liquid Water
  Wernet, Ph.; Nordlund, D.; Bergmann, U.; Cavalleri, M.; Odelius, M.; Ogasawara, H.; Naeslund, L. A.; Hirsch, T. K.; Ojamaee, L.; Glatzel, P.; Pettersson, L. G. M.; Nilsson, A.
  Science (Washington, DC, United States) (2004), 304 (5673), 995-999CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
  X-ray absorption spectroscopy and x-ray Raman scattering were used to probe the mol. arrangement in the 1st coordination shell of liq. H2O. The local structure is characterized by comparison with bulk and surface of ordinary hexagonal ice Ih and with calcd. spectra. Most mols. in liq. H2O are in 2 H-bonded configurations with 1 strong donor and 1 strong acceptor H bond in contrast to the 4 H-bonded tetrahedral structure in ice. Upon heating from 25° to 90°, 5 to 10% of the mols. change from tetrahedral environments to 2 H-bonded configurations. Findings are consistent with neutron and x-ray diffraction data, and combining the results sets a strong limit for possible local structure distributions in liq. H2O. Serious discrepancies with structures based on current mol. dynamics simulations are obsd.
40. 40
  Ambrosetti, A.; Alfè, D.; Robert, A.; DiStasio, J.; Tkatchenko, A. Hard Numbers for Large Molecules: Toward Exact Energetics for Supramolecular Systems J. Phys. Chem. Lett. 2014, 5, 849– 855 DOI: 10.1021/jz402663k
  40
  Hard Numbers for Large Molecules: Toward Exact Energetics for Supramolecular Systems
  Ambrosetti, Alberto; Alfe, Dario; DiStasio, Robert A.; Tkatchenko, Alexandre
  Journal of Physical Chemistry Letters (2014), 5 (5), 849-855CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
  Noncovalent interactions are ubiquitous in mol. and condensed-phase environments, and hence a reliable theor. description of these fundamental interactions could pave the way toward a more complete understanding of the microscopic underpinnings for a diverse set of systems in chem. and biol. In this work, we demonstrate that recent algorithmic advances coupled to the availability of large-scale computational resources make the stochastic quantum Monte Carlo approach to solving the Schrodinger equation an optimal contender for attaining "chem. accuracy" (1 kcal/mol) in the binding energies of supramol. complexes of chem. relevance. To illustrate this point, we considered a select set of seven host-guest complexes, representing the spectrum of noncovalent interactions, including dispersion or van der Waals forces, π-π stacking, hydrogen bonding, hydrophobic interactions, and electrostatic (ion-dipole) attraction. A detailed anal. of the interaction energies reveals that a complete theor. description necessitates treatment of terms well beyond the std. London and Axilrod-Teller contributions to the van der Waals dispersion energy.
41. 41
  Huang, S.-J.; Yang, C.-Y.; Zheng, A.; Feng, N.; Yu, N.; Wu, P.-H.; Chang, Y.-C.; Lin, Y.-C.; Deng, F.; Liu, S.-B. New Insights into Keggin-Type 12-Tungstophosphoric Acid from 31P MAS NMR Analysis of Absorbed Trimethylphosphine Oxide and DFT Calculations Chem. - Asian J. 2011, 6, 137– 148 DOI: 10.1002/asia.201000572
  41
  New Insights into Keggin-Type 12-Tungstophosphoric Acid from 31P MAS NMR Analysis of Absorbed Trimethylphosphine Oxide and DFT Calculations
  Huang, Shing-Jong; Yang, Chih-Yi; Zheng, Anmin; Feng, Ningdong; Yu, Ningya; Wu, Pei-Hao; Chang, Yu-Chi; Lin, Ying-Chih; Deng, Feng; Liu, Shang-Bin
  Chemistry - An Asian Journal (2011), 6 (1), 137-148CODEN: CAAJBI; ISSN:1861-4728. (Wiley-VCH Verlag GmbH & Co. KGaA)
  The acid and transport properties of the anhyd. Keggin-type 12-tungstophosphoric acid (H3PW12O40; HPW) have been studied by solid-state 31P magic-angle spinning NMR of absorbed trimethylphosphine oxide (TMPO) in conjunction with DFT calcns. Accordingly, 31P NMR resonances arising from various protonated complexes, such as TMPOH+ and (TMPO)2H+ adducts, could be unambiguously identified. It was found that thermal pretreatment of the sample at elevated temps. (≥423 K) is a prerequisite for ensuring complete penetration of the TMPO guest probe mol. into HPW particles. Transport of the TMPO absorbate into the matrix of the HPW adsorbent was found to invoke a desorption/absorption process assocd. with the (TMPO)2H+ adducts. Consequently, three types of protonic acid sites with distinct superacid strengths, which correspond to 31P chem. shifts of 92.1, 89.4, and 87.7 ppm, were obsd. for HPW samples loaded with less than three mols. of TMPO per Keggin unit. Together with detailed DFT calcns., these results support the scenario that the TMPOH+ complexes are assocd. with protons located at three different terminal oxygen (Od) sites of the PW12O403- polyanions. Upon increasing the TMPO loading to >3.0 mols. per Keggin unit, abrupt decreases in acid strength and the corresponding structural variations were attributed to the change in secondary structure of the pseudoliquid phase of HPW in the presence of excessive guest absorbate.

DFT Modeling of the Adsorption of Trimethylphosphine Oxide at the Internal and External Surfaces of Zeolite MFIClick to copy article linkArticle link copied!

The Journal of Physical Chemistry C

Abstract

1 Introduction

Figure 1

2 Computational Methods

2.1 Classification of the Acid Strength

Figure 2

3 Results and Discussion

3.1 Adsorption of One TMPO Molecule

Figure 3

Figure 4

3.2 Adsorption of Two TMPO Molecules

Figure 5

Figure 6

3.3 Adsorption of Three TMPO Molecules

Figure 7

Figure 8

Figure 9

3.4 Full Deprotonation of the Brønsted Acid Site

Figure 10

4 Conclusions

Author Information

Acknowledgment

References

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The Journal of Physical Chemistry C

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DFT Modeling of the Adsorption of Trimethylphosphine Oxide at the Internal and External Surfaces of Zeolite MFI
Click to copy article linkArticle link copied!