If a negative ion has vibrational energy in excess of the binding energy of its most weakly bound electron, the anion can undergo vibrational autodetachment, similar to thermionic emission. When this effect occurs after targeted infrared excitation of a specific vibrational mode in the anion, it encodes information on the intramolecular vibrational relaxation processes that take place between excitation and electron emission. We present examples on how vibrational autodetachment can be used to obtain infrared spectra of molecular anions, and we discuss how a vibrational autodetachment photoelectron spectrum can be modeled, using vibrational autodetachment after excitation of CH stretching modes of nitromethane anions as a case study.

The energetic and magnetic properties of the tubular cluster Au24, doped endohedrally by a 3d transition-metal atom M (M = V, Cr, Mn, Fe, Co, and Ni) have been investigated by the scalar relativistic density functional simulations. It is found that (1) these 3d transition-metal atoms can be encapsulated stably into the tubular Au24 and do not significantly perturb the atomic and electronic structures of the parent tubular Au24, (2) the infrared (IR) spectra of the tubular Au24 cluster are significantly changed by the dopant atoms, inducing a characteristic absorption peak in the IR spectra of all the M@Au24, and (3) protected by the tubular Au24, the 3d states of the dopant atoms are largely localized, and the atom-like magnetism is retained for all the doped gold clusters, exhibiting 3, 6, 5, 4, 3, and 2 μB for V−Ni, respectively.

We have assembled a global CASSCF potential energy surface for the excited 2A state of the cyclohexadiene -hexatriene system, in two degrees of freedom, with full relaxation in all other degrees of freedom. Quantum wavepacket dynamics on this surface yields simple interpretations of recent experimental data on the ultrafast photochemical ring-opening of cyclohexadiene as well as predictions on preferred product configurations. The feasibility of this system as a model for fulgide molecular switches is discussed.

Earlier steady-state fluorescence studies showed that 2-butylamino-6-methyl-4-nitropyridine N-oxide (2B6M) can undergo fast excited-state intramolecular proton transfer (ESIPT). In a nonpolar solvent such as n-octane, both normal and tautomeric fluorescence was observed. Strikingly, the relative ratio of those two emission bands and the fluorescence quantum yield of the normal emission were found to depend on the excitation wavelength in violation of the Kasha−Vavilov rule. In this work, the system was investigated further by means of transient absorption spectroscopy, followed by global and target analysis. Upon excitation at 420 nm, a normal excited singlet state S1(N) is reached, which decays in about 12 ps via fluorescence and ESIPT (minor pathways) and to a long-lived “dark” state (major pathway) that is most probably the triplet T1(N). Upon 330 nm excitation, however, a more complex pattern emerges and additional decay channels are opened. A set of four excited-state species is required to model the data, including a hot state S1(N)* that decays in about 3 ps to the tautomer, to the long-lived “dark” state and to the relaxed S1(N) state. A kinetic scheme is presented that can explain the observed transient absorption results as well as the earlier fluorescence data.

It is generally expected that ions in an aqueous ionic solution in contact with a hydrophobic phase enter the hydrophobic phase accompanied by a hydration shell. This expectation suggests that the ion mole fraction in the hydrophobic phase is less than, or at most, equal to that of water. Both gravimetric and spectroscopic evidence shows that for a model hydrophobic phase, carbon tetrachloride, this is not the case: In contact with a 1 M simple salt solution (sodium or potassium halide), the salt concentration in carbon tetrachloride ranges from 1.4 to nearly 3 times that of water. Infrared spectra of the OH stretch region support a model in which water associates with the cation, primarily as water monomers. Salts containing larger, more polarizable anions can form outer-sphere ion pairs that support water dimers, giving rise to a spectral signature at 3440 cm−1. In CCl4, the infrared spectral signature of the normally strongly ionized acid HCl clearly shows the presence of molecular HCl. Additionally, the presence of a Q branch for HCl indicates restricted rotational motion. The spectral and gravimetric data provide compelling evidence for ion clusters in the hydrophobic phase, which is a result that may have implications for hydrophobic matter in both biological and environmental systems.

Substituent effects on dynamics at conical intersections are investigated by means of femtosecond time-resolved photoelectron spectroscopy for cyclopentadiene and its substituted analogues 1,2,3,4-tetramethylcyclopentadiene, 1,2,3,4,5-pentamethylcyclopentadiene, and 1,2,3,4-tetramethyl-5-propylcyclopentadiene. By UV excitation to the S2 (11B2) state, the influence of these substitutions on dynamics for the initially excited S2 (11B2) surface and the spectroscopically dark S1 (21A1) surface were investigated. We observed that the dynamics depend only on a small number of specific vibrations. Whereas dynamics at the S2/S1-conical intersection are independent of substitution at the 5-position, internal conversion dynamics on the S1 (21A1) surface slow down as the inertia of the 5-substituent increases. Contrary to the expectations of simple models of radiationless transitions, an increasing density of states does not lead to faster processes, suggesting that a true dynamical picture of vibrational motions at conical intersections will be required.

In this study, we used TD-PBE0 calculations to investigate the first singlet excited state (S1) behavior of 2-(2′-hydroxyphenyl)benzimidazole (HBI) and its amino derivatives. We employed the potential energy surfaces (PESs) at the S1 state covering the normal syn, tautomeric (S1−Tsyn), and intramolecular charge-transfer (S1−TICT) states in ethanol and cyclohexane to investigate the reaction mechanisms, including excited-state intramolecular proton transfer (ESIPT) and intramolecular charge-transfer (ICT) processes. Two new S1−TICT states, stable in ethanol and cyclohexane, were found for HBI and its amino derivatives; they are twisted and pyramidalized. The flat PES of the ICT process makes the S1−TICT states accessible. The S1−TICT state is effective for radiationless relaxation, which is responsible for quenching the fluorescence of the S1−Tsyn state. In contrast to the situation encountered conventionally, the S1−TICT state does not possess a critically larger dipole moment than its precursor, S1−Tsyn state; hence, it is not particularly stable in polar solvents. On the basis of the detailed PESs, we rationalize various experimental observations complementing previous studies and provide insight to understand the excited-state reaction mechanisms of HBI and its amino derivatives.

We have used photoionization efficiency spectroscopy to determine ionization energies (IEs) of the gas-phase tantalum−carbide clusters Ta5Cy (y = 0−6). The structures of the clusters observed in the experiment are assigned by comparing the experimental IEs with those of candidate isomers, calculated by density functional theory. Two competing geometries of the underlying Ta5 cluster are found to be present in the assigned Ta5Cy structures; either a “prolate” or “distorted oblate” trigonal bipyramid geometry. The onset of carbon−carbon bonding in the Ta5Cy series is proposed to occur at y = 6, with the structure of Ta5C6 containing two molecular C2 units.

Gas-phase deprotonation enthalpies were measured for chloric and perchloric acids and found to be 313.2 ± 3.3 and 299.9 ± 5.7 kcal mol−1, respectively. These values were combined with the previously reported electron affinities of ClO3 and ClO4 to obtain BDE(H−OClO2) = 97.6 ± 4.0 kcal mol−1 and BDE(H−OClO3) = 107.4 ± 6.1 kcal mol−1. These energetic determinations represent the first measurements of these quantities or extensive revisions of the currently available values. B3LYP, M06, M06−2X, G3, and G3B3 computations also were carried out to provide acidities, electron affinities, bond dissociation energies, and heats of formation via atomization energies for ClOx and HClOx, where x = 1−4. All of the methods do a reasonable job for the first three thermodynamic quantities but only M06 does a satisfactory job for the heats of formation, and its performance is similar to the highly accurate but extremely time intensive W4 method.

A peak appearing at the L2,3 X-ray absorption edge often provides the number of empty d states of the X-ray absorbing atoms. Ag+ compounds have a d10 state (no d empty states) but show a small peak at the edge. In this research, we systematically studied the edge peak of Ag+ compounds to understand its origin on the basis of the molecular orbital picture and to obtain a relation of the edge peak intensity to chemical and physical quantities. The edge peak can be formally assigned to the transition from 2p to 5s enhanced by the s−d hybridization. The peak intensity has a negative correlation with a coordination charge but has a positive correlation with the strength of the covalent bond, which is in the reverse order to the other dn (n < 10) elements.

Given the particular importance of dye photostability for single-molecule investigations, fluorescence fluctuation spectroscopy, and laser-scanning microscopy, refined strategies were explored for enhancing the fluorescence signal by selectively quenching the first excited triplet state of the laser dye Rhodamine 123 (Rh123). The strategy is to quench the T1 state by Dexter triplet energy transfer, while undesired quenching of the singlet state via Förster or Dexter singlet energy transfer and the generation of free radicals through electron transfer should be avoided. Diphenylhexatrienes (DPHs) were tested in ethanol for their beneficial effects as a novel class of photoprotective agents using fluorescence correlation spectroscopy. A library of DPHs with electron-donating (dimethlyamino) and withdrawing substituents (e.g., trifluormethyl) was synthesized to optimize the electronic properties. Quantum chemical calculations, optical spectroscopy, and cyclic voltammetry were used to determine the electronic properties. The computed T1 emission energy of Rh123 and the T1 excitation energies of all DPHs allow for exergonic triplet energy transfer to the quencher. The parent compound quenches the T1 state of Rh123 nearly diffusion controlled (4.9 × 109 M−1 s−1). All electron-deficient DPHs significantly increase (3×) the fluorescence rate of Rh123 by reducing the triplet state population and by avoiding the formation of other long-lived dark radical states. The quenching constants are reduced by more than a factor of 2, if substituents with increasing size or electronegativity are introduced. The beneficial effect of triplet quenching of substituted DPHs is governed by a delicate interplay of steric, electronic, and intermolecular Coulombic effects.

Although identical in formula, trans- and gauche-difluoroacetic acids behave as different molecules in rotational spectra due to their distinct sets of rotational constants. In this study, high-resolution microwave spectra of monohydrates, dihydrates, and trihydrates of both trans- and gauche-difluoroacetic acids were recorded using a Fourier transform microwave spectrometer. Rotational and centrifugal distortion constants of these hydrates were obtained with high accuracy via fitting their microwave spectra. It was found that the subtle structural difference between the trans and gauche forms of the acid gives different tunneling motions in their monohydrates. An unusual mixing of wave functions of energetically nondegenerate conformations was observed in the monohydrate of gauche-difluoroacetic acid. Ab initio calculations using the second-order Møller−Plesset perturbation method were also performed to optimize structures and to predict magnitudes of dipole moments of these hydrates. Close agreement between experimental and theoretical rotational constants confirms the feature of hydrogen-bonded ring structures in which all incoming H2O molecules bind to the carboxylic group for all hydrates.

The investigation of N-phenyl-4-amino- and N-phenyl-4-acetamido-1,8-naphthalimides containing N-benzo-15-crown-5 ether substituent showed that the presence of ionophoric fragment as N-substituent in naphthalimide molecule provides the design of compound possessing the properties of fluorescent receptor. The addition of metal cations does not change the position of absorption and emission bands but substantial increases the fluorescence intensity. The study of molecules included the theoretical and experimental (optical, NMR) methods, analysis of intramolecular charge (electron) transfer and fluorescence properties in the presence and absence of metal ions.

A second-order Møller−Plesset perturbation theory (MP2) and density functional theory (DFT) investigation of the dehalogenation reactions of thionyl chloride is reported, in which water molecules (up to seven) were explicitly involved in the reaction complex. The dehalogenation processes of thionyl chloride were found to be dramatically catalyzed by water molecules. The reaction rate became significantly faster as more water molecules became involved in the reaction complex. The dehalogenation processes can be reasonably simulated by the gas-phase water cluster models, which reveals that water molecules can help to solvate the thionyl chloride molecules and activate the release of the Cl− leaving group. The computed activation energies were used to compare the calculations to available experimental data.

The microwave spectra of seven isotopomers of fluoromethylsilane, CH2FSiH3, in the ground vibrational state were measured and analyzed in the frequency range 18−40 GHz. The rotational and centrifugal distortion constants were evaluated by the least-squares treatment of the observed frequencies of a- and b-type R- and b-type Q-transitions. The values for the components of the dipole moment were obtained from the measurements of Stark effects from both a- and b-type transitions and the determined values are: |μa| = 1.041(5), |μb| = 1.311(6), and |μt| = 1.674(4) D. Structural parameters have been determined and the heavy atom distances (r0) in Angstroms are: Si−C = 1.8942(57) and C−F = 1.4035(55) and the angle in degree, ∠SiCF = 109.58(14). A semi-experimental re structure was also determined from experimental ground state rotational constants and vibration−rotation constants derived from ab initio force fields. The internal torsional fundamental, SiH3, was observed at 149.2 cm−1 with two accompanying hot bands at 138.8 and 127.5 cm−1. The barrier to internal rotation was obtained as 717.3(16) cm−1 (2.051(46) kcal mol−1) by combining the analysis of the microwave A and E splittings and the torsional fundamental and hot band frequencies. Ab initio calculations have been carried out with full electron correlation by the second-order perturbation method with several different basis sets up to MP2/6-311+G(d,p) to obtain geometrical parameters, barriers to internal rotation, and centrifugal distortion constants. Adjusted r0 structural parameters have been obtained by combining the ab initio MP2/6-311+G(d,p) predicted values with the determined rotational constants for the fluoride as well as with the previously reported microwave data for the chloro- and bromo- compounds. These experimental results are compared to the corresponding parameters for the carbon analogues.

The recombination reactions of CH2Br and CH2Cl radicals have been used to generate vibrationally excited CH2BrCH2Br and CH2BrCH2Cl molecules with 91 kcal mol−1 of energy in a room-temperature bath gas. The experimental unimolecular rate constants for elimination of HBr and HCl were compared to calculated statistical rate constants to assign threshold energies of 58 kcal mol−1 for HBr elimination from C2H4Br2 and 58 and 60 kcal mol−1, respectively, for HBr and HCl elimination from C2H4BrCl. The Br−Cl interchange reaction was demonstrated and characterized by studying the CH2BrCD2Cl system generated by the recombination of CH2Br and CD2Cl radicals. The interchange reaction was identified from the elimination of HBr and DCl from CH2ClCD2Br. The interchange reaction rate is much faster than the rates of either DBr or HCl elimination from CH2BrCD2Cl, and a threshold energy of ≅43 kcal mol−1 was assigned to the interchange reaction. The statistical rate constants were calculated from models of the transition states that were obtained from density functional theory using the B3PW91 method with the 6-31G(d′,p′) basis set. The model for HBr elimination was tested versus published thermal and chemical activation data for C2H5Br. A comparison of Br−Cl interchange with the Cl−F and Br−F interchange reactions in 1,2-haloalkanes is presented.

We have studied Förster energy transfer between O−H vibrations in H2O/D2O ice Ih using femtosecond, two-color, mid-infrared pump−probe spectroscopy. We found that as a result of couplings to nearby O−H stretch modes, the vibrational relaxation time decreases from 480 fs for dilute HDO in D2O down to 300 fs for pure H2O ice. The anisotropy shows an initial 140 fs decay down to a concentration-dependent end level. This end level for low concentrations can be explained from the limited rotational freedom (∼20°) of a water molecule in the ice lattice over time scales > 15 ps. The decreasing end levels for higher concentrations of H2O result from Förster energy transfer to the next-nearest six O−H groups. No Förster transfer beyond these neighbors is observed. Variation of the ice temperature between 200 and 270 K was found to have negligible effect on the dynamics.

In the present study, photoinduced electron transfer (ET) processes of supramolecular donor−acceptor dyads of Zn 2,3,7,8,11,12,17,18-octaethylhemiporphycene (ZnHPc) and axial ligands were investigated by using various spectroscopic methods. The formation of 1:1 dyads was confirmed by absorption spectral change during titration of axial ligand. The association constants were determined from the spectral change. Quenching of the fluorescence intensity was observed when electron acceptor ability of the axial ligand increased. The driving forces for ET were estimated based on the estimated redox potentials and structural parameters. It became clear that various ZnHPc dyads showed ET because of slightly higher donor-ability and larger excitation energy of ZnHPc when compared to the corresponding dyads of Zn porphycene (ZnPcn). The transient absorption spectra during the sub-picosecond laser flash photolysis showed the formation of charge separated state, that is, radical cation of ZnHPc and radical anion of axial ligand, from the singlet excited ZnHPc. The observed ET rates were compared with the previously reported values for Zn porphyrins and ZnPcn. The ET rates of ZnHPc were located between those observed with porphyrins and ZnPcn supramolecular dyads, even when the −ΔG values were similar to each other. This observation was explained on the basis of the variation in reorganization energy and electronic coupling (V) values. Furthermore, distribution of HOMO electron density gave a plausible explanation for the variation in V values of these dyads.

We have investigated the photoluminescence properties of porphyrin-based erbium and gadolinium complexes at different levels of halogen substitution. Both the intensity and the decay time of the erbium near-infrared emission correlate with the degree of the halogenation. Conversely, no clear correlation is found with the triplet-state energy levels nor with the intensity of the residual visible emission. Such findings confirm that the key role in the low efficiency of the near-infrared emission is played by the nonradiative quenching of the erbium emitting level due to the vibrational modes of the surrounding C−H bonds.

In the 3D network [Rh(bpy)3][NaCr(ox)3]ClO4 (ox = oxalate, bpy = 2,2′-bipyridine) phonon-assisted as well as resonant energy migration within the R1 line of the 4A2 → 2E transition of Cr3+ has been identified. The latter is dominant below 4.2 K, and in a fluorescence line narrowing spectrum, it manifests itself in a multiline pattern across the inhomogeneous line width with spacings corresponding to the zero-field splitting of the 4A2 ground state (Milos, M.; Kairouani, S.; Rabaste, S.; Hauser, A. Coord. Chem. Rev. 2008, 252, 2540). H. Riesen demonstrated efficient spectral hole burning within the R1 line of Cr3+ doped at low concentrations into partially deuterated NaMg[Al(ox)3]·9H2O (Riesen, H. Coord. Chem. Rev. 2006 250, 1737). Here we show that at higher Cr3+ concentrations in the same host, both phenomena can be observed simultaneously, the resonant energy migration thus creating an additional series of persistent side holes.

The conversions of NiAs-type structures of transition metal chalcogenides (FeS and CoSe) to pyrite-type structures of dichalcogenides (FeS2 and CoSe2, respectively) under irradiation by HeNe laser (wavelength, 632.8 nm; intensity, 6 × 104 W/cm2) have been investigated using Raman spectroscopy. The laser-induced conversions give rise to Raman peaks corresponding to vibrations of S−S or Se−Se bonds of respective pyrite structures. The results are of interest for the characterization and fabrication of pyrite-like structures necessary for applications as oxygen reduction reaction catalysts. Material modifications at the micrometer and submicrometer levels are attainable. The structural conversions are accompanied by self-polymerization of excess chalcogen. Extended laser irradiation (>500 s) in air induces the substitution of chalcogen (S or Se) by oxygen in the chalcogenide materials and the subsequent formation of transition metal (Fe or Co) oxides. Excess chalcogen appears to prevent further oxidation. The article also presents conditions necessary to avoid laser-induced structural changes and oxidation of metal chalcogenide materials during Raman measurements.

We report three new noble-gas molecules prepared in low-temperature Kr and Xe matrices from the HCCF precursor by UV photolysis and thermal annealing. The identified molecules are two noble-gas hydrides HNgCCF (Ng = Kr and Xe) and a molecule of another type, HCCKrF. These molecules are assigned with the help of ab initio calculations. All strong absorptions predicted by theory are found in experiments with proper deuteration shifts. The experiments and theory suggest a higher stability against dissociation of HNgCCF molecules compared to HNgCCH reported previously. Surprisingly, only very tentative traces of HCCXeF, which is computationally very stable, are found in experiments. No strong evidence of similar argon compounds is found here.

Three water-insoluble, micelle-anchored flavylium salts, 7-hydroxy-3-octyl-flavylium chloride, 4′-hexyl-7-hydroxyflavylium chloride, and 6-hexyl-7-hydroxy-4-methyl-flavylium chloride, have been employed to probe excited-state prototropic reactions in micellar sodium dodecyl sulfate (SDS). In SDS micelles, the fluorescence decays of these three flavylium salts are tetraexponential functions in the pH range from 1.0 to 4.6 at temperatures from 293 to 318 K. The four components of the decays are assigned to four kinetically coupled excited species in the micelle: specifically, promptly deprotonable (AH+*) and nonpromptly deprotonable (AHh+*) orientations of the acid in the micelle, the base-proton geminate pair (A*···H+), and the free conjugate base (A*). The initial prompt deprotonation to form the geminate pair occurs at essentially the same rate (kd ∼ 6−7 × 1010 s−1) for all three photoacids. Recombination of the geminate pair is ∼3-fold faster than the rate of proton escape from the pair (krec ∼ 3 × 1010 s−1 and kdiss ∼ 1 × 1010 s−1), corresponding to an intrinsic recombination efficiency of the pair of ∼75%. Finally, the reprotonation of the short-lived free A* (200−350 ps, depending on the photoacid) has two components, only one of which depends on the proton concentration in the intermicellar aqueous phase. Ultrafast transfer of the proton to water and substantial compartmentalization of the photogenerated proton at the micelle surface on the picosecond time scale strongly suggest preferential transfer of the proton to preformed hydrogen-bonded water bridges between the photoacid and the anionic headgroups. This localizes the proton in the vicinity of the excited base much more efficiently than in bulk water, resulting in the predominance of geminate reprotonation at the micelle surface.

In our previous work [Bren, U., et al. J. Phys. Chem. A 2008, 112, 166] we proposed a novel physical mechanism for microwave catalysis based on rotationally hot reactive species and verified its validity through a Monte Carlo simulation of a realistic chemical reaction: neutral ester hydrolysis. This article represents a continuation of our ongoing effort toward quantitative understanding of the microwave catalytic effect. It provides a derivation of an analytical solution for the microwave catalysis. The obtained expression is compared with the results of the Monte Carlo simulation and is applied to reproduce the microwave catalytic effect experimentally observed in the polyethylene terephthalate solvolysis. Implications for the interactions of microwaves with living organisms in the context of widespread mobile telephony are also discussed.

The rates of gas-phase elimination of trimethyl orthovalerate and trimethyl orthochloroacetate have been determined in a static system, and the reaction Pyrex vessels have been deactivated with the product of decomposition of allyl bromide. The reactions are unimolecular and follow a first-order rate law. The working temperature and pressure ranges were 313−410 °C and 40−140 Torr, respectively. The rate coefficients for the homogeneous reaction are given by the following Arrhenius expressions: for trimethyl orthovalerate: log k (s−1) = [(14.00 ± 0.28) − (196.3 ± 1.7) (kJ/mol)] (2.303RT)−1, (r = 0.9999); and for trimethyl orthochloroacetate: log k (s−1) = [(13.54 ± 0.21) − (209.3 ± 1.9)(kJ/mol)](2.303RT)−1, (r = 0.9998). The theoretical calculations of the kinetic and thermodynamic parameters were carried out by using B3LYP, B3PW91, MPW1PW91, and PBEPBE methods. The theoretical results show reasonably good agreement with the experimental energy and enthalpy of activation values when using the B3PW91/6-31++G** method for trimethyl orthovalerate and PBEPBE /6-31++G** for trimethyl orthochloroacetate. These calculations suggest a molecular concerted nonsynchronous mechanism where C−OCH3 bond polarization, in the sense Cδ+···δ−OCH3, is the rate-determining step. The increase in electron density of the oxygen atom at OCH3 eases the abstraction of the hydrogen of the adjacent C−H bond in a four-membered cyclic structure to give methanol and the corresponding unsaturated ketal. The electron-donor substituent enhances decomposition rates by stabilizing the positive charge developing in the transition state at the carbon bearing the three methoxy groups, whereas the electron-withdrawing substituent destabilizes this charge, thus retarding the reaction.

A systematic theoretical investigation was carried out to study the reactions of various germylenes with germane. Molecular structures of the reactants (GeX2 and GeHX, where X = H, F, Cl and Br) plus GeH4, transition states, and products have been optimized to understand the effects of halo-substituted germylenes. The basis set used is of double-ζ plus polarization quality with additional s- and p-type diffuse functions. Consistent with experiment, the theoretical gas-phase reaction GeH2 + GeH4 → Ge2H6 possesses a negative activation energy. The predicted activation energies reveal interesting trends for both mono- and di- halo-substituted germylenes, −1.5 [GeH2], +20.5 [GeHF], +59.9 [GeF2], +18.0 [GeHCl], +46.8 [GeCl2], +17.3 [GeHBr], and +42.9 kcal mol−1 [GeBr2]. There is a noteworthy relationship between the activation energies and the singlet−triplet splittings of the divalent germylenes. We report for the first time rate constants for the transfer of hydrogen, evaluated using standard transition-state theory with tunneling corrections. These results are analyzed and compared to the available experimental and previous theoretical findings for the gas-phase reactions involving germylene derivatives and germanium analogues.

The gas-phase reactions of Cl atoms with allyl alcohol (k1), 3-buten-2-ol (k2), and 2-methyl-3-buten-2-ol (k3) at 296 ± 2 K have been investigated using absolute and relative rate methods in 1−700 Torr of N2 diluent. Absolute rate studies were performed using pulsed laser photolysis/vacuum ultraviolet laser-induced fluorescence spectroscopy techniques. Relative rate studies were performed using smog chamber/Fourier transform infrared spectroscopy techniques. The absolute and relative rate studies gave consistent results. The kinetics of the reactions are dependent on pressure over the range studied. Molar yields for HCl production in 700 Torr of N2 for reactions of chlorine atoms with allyl alcohol, 3-buten-2-ol, and 2-methyl-3-buten-2-ol were measured to be 0.26 ± 0.03, 0.23 ± 0.03, and 0.12 ± 0.02, respectively. The chlorine-atom-initiated oxidation of 2-methyl-3-buten-2-ol in 700 Torr of air gave the following products (molar yields): acetone (47 ± 4%), chloroacetaldehyde (47 ± 5%), and HCHO (7.2 ± 0.6%). The observation of substantial and indistinguishable yields of acetone and chloroacetaldehyde products indicates that a major fraction of the reaction proceeds via addition of chlorine atoms to the terminal carbon atom. The results are discussed with respect to the literature data.

The hygroscopic behavior of atmospheric aerosols has a significant effect on the global climate change. In this study, a physisorption analyzer was used to measure the water adsorption capacity of Al2O3, NaCl, NH4NO3, and (NH4)2SO4 particles at 273.6 K. Qualitative and quantitative information about water adsorption on these particles was obtained with changing the temperature and/or relative humidity (RH). Uptake of water on Al2O3 showed a type-II BET adsorption isotherm with the monolayer formed at ∼18% relative humidity (RH). The hygroscopic properties of NaCl, (NH4)2SO4, and NH4NO3, including the deliquescence relative humidities (DRH), the temperature dependence of the DRH for NH4NO3, and the growth factors of NaCl and (NH4)2SO4 were determined. All these results were in good agreement with the results obtained by other methods and/or theoretical prediction with a deviation less than 2%. For NaCl, the water adsorption amount increase rate exhibits three stages (<30% RH, ∼30%−65% RH, and >65% RH) in the predeliquescence process and monolayer thin film water was formed at about 30% RH. It demonstrated that this instrument was practicable for studying the hygroscopic behavior of both soluble and insoluble but wettable atmospheric nonviolate aerosol particles.

We investigated the abiotic reduction of inorganic Hg(II) by dissolved organic matter (DOM) and stannous(II) chloride (SnCl2) in the absence of light and quantified fractionation of Hg isotopes during these processes. The kinetics of reduction by DOM was characterized using multiple parallel pseudo-first-order reactions, implying different reactive Hg(II) species resulting from Hg−DOM complexation. Significant mass independent isotopic anomalies were observed in reduction by both reducing reagents. Isotopes with odd atomic masses (199Hg and 201Hg) showed less enrichment in reactants Hg(II) than expected for a mass dependent fractionation process. The fractionation factors (α) showed an odd−even staggering pattern that resembles the variation of nuclear charge radii. We demonstrated that these isotopic anomalies originated from nuclear field shift effect (NFS). The contribution of NFS to the measured fractionation factors was estimated and found to be as significant as the mass dependent effect. The observed Δ199Hg/Δ201Hg slope was explained by NFS and determined to be between 1.5 and 1.6 in abiotic nonphotochemical reduction, which is distinguishable from slopes determined for photochemical reduction. Therefore, we first demonstrated experimentally the significance of the nuclear field shift effect during reduction of Hg(II) and showed the application of isotope fractionation to distinguish between different reduction pathways.

Photochemical reduction of Hg(II) by various low-molecular-weight organic compounds (LMWOC) was investigated to evaluate the effect of specific functional groups that are typically encountered in natural dissolved organic matters (DOM) on the photoreactivity and isotope fractionation of Hg. LMWOC with reduced sulfur functional groups (e.g., cysteine, glutathione) resulted in slower photochemical reduction of Hg(II) than those without reduced sulfur groups (e.g., serine, oxalic acid). Reduction rate constants were specifically determined for two contrasting LMWOC: dl-serine (0.640 h−1) and l-cysteine (0.047 h−1). Different mass independent isotope effects of Hg were induced by the two types of LMWOC. S-containing ligands specifically enriched magnetic isotopes (199Hg and 201Hg) in the product (Hg(0)) while sulfurless ligands enriched 199Hg and 201Hg in the reactant (Hg(II)), suggesting that opposite magnetic isotope effects were produced by different types of ligands. The nuclear field shift effect was also observed in the photochemical reduction by serine. These isotope effects are related to specific functional groups and reduction mechanisms, and may be used to distinguish between primary and secondary photochemical reduction mechanisms of Hg(II) and to explain isotope fractionation during the photochemical reduction of Hg(II) by natural DOM, which provides mixed bonding conditions.

High level ab initio electronic structure calculations at the coupled cluster level with a correction for triples extrapolated to the complete basis set limit have been made for the thermodynamics of the BrBrO2, IIO2, ClBrO2, ClIO2, and BrIO2 isomers, as well as various molecules involved in the bond dissociation processes. Of the BrBrO2 isomers, BrOOBr is predicted to be the most stable by 8.5 and 9.3 kcal/mol compared to BrBrO2 and BrOBrO at 298 K, respectively. The weakest bond in BrOOBr is the O−Br bond with a bond dissociation energy (BDE) of 15.9 kcal/mol, and in BrBrO2, it is the Br−Br bond of 19.1 kcal/mol. The smallest BDE in BrOBrO is for the central O−Br bond with a BDE of 12.6 kcal/mol. Of the IIO2 isomers, IIO2 is predicted to be the most stable by 3.3, 9.4, and 28.9 kcal/mol compared to IOIO, IOOI, and OIIO at 298 K, respectively. The weakest bond in IIO2 is the I−I bond with a BDE of 22.2 kcal/mol. The smallest BDEs in IOIO and IOOI are the terminal O−I bonds with values of 19.0 and 5.2 kcal/mol, respectively.

A mixed molecular dynamics/quantum mechanics model has been applied to the ammonium/water clustering system. The use of the high level MP2 calculation method and correlated basis sets, such as aug-cc-pVDZ and aug-cc-pVTZ, lends confidence in the accuracy of the extrapolated energies. These calculations provide electronic and free energies for the formation of clusters of ammonium and 1−10 water molecules at two different temperatures. Structures and thermodynamic values are in good agreement with previous experimental and theoretical results. The estimated concentration of these clusters in the troposphere was calculated using atmospheric amounts of ammonium and water. Results show the favorability of forming these clusters and implications for ion-induced nucleation in the atmosphere.

Geometry parameters, frequencies, heats of formation, and bond dissociation energies are predicted for simple alkali metal compounds (hydrides, chlorides, fluorides, hydroxides and oxides) of Li, Na, and K from coupled cluster theory [CCSD(T)] calculations including core−valence correlation with the aug-cc-pwCVnZ basis set (n = D, T, Q, and 5). To accurately calculate the heats of formation, the following additional correction were included: scalar relativistic effects, atomic spin−orbit effects, and vibrational zero-point energies. For calibration purposes, the properties of some of the lithium compounds were predicted with iterative triple and quadruple excitations via CCSDT and CCSDTQ. The calculated geometry parameters, frequencies, heats of formation, and bond dissociation energies were compared with all available experimental measurements and are in excellent agreement with high-quality experimental data. High-level calculations are required to correctly predict that K2O is linear and that the ground state of KO is 2Σ+, not 2Π, as in LiO and NaO. This reliable and consistent set of calculated thermodynamic data is appropriate for use in combustion and atmospheric simulations.

We have studied the thermodynamic properties of the ammonium sulfate/malonic acid/water system using differential scanning calorimetry and infrared spectroscopy of thin films. Using the results from our experiments and literature data, we have created a solid/liquid phase diagram of the ammonium sulfate/malonic acid/water system for temperatures below 300 K. We also compare our results to the predictions of the aerosol inorganics model (AIM).

Recent experiments have revealed the existence of an excited state dissociative mechanism for certain peroxycarbonates, with the demonstration that the lifetime of the excited state matches the picosecond time scale for appearance of nascent carbon dioxide product. The data infer that the photoreaction proceeds via an effectively concerted three-body dissociation within the lifetime of the singlet excited state. Many other arylperoxides decay sequentially via [(aryloxy)carbonyl]oxy radical intermediates on nanosecond−microsecond time scales. Uncertainty as to the lifetime of the excited state relates to the character and the relative energetic ordering of states of the parent molecule, since the spectra and photochemistry imply that low-lying states may exist on each of the aryl, carbonate, and peroxide chemical functionalities. We employ many-body electronic structure calculations to determine the energies and characters of the low-lying valence states of a minimal aryl peroxycarbonate model germane to the above-mentioned experiments, methyl phenyl peroxycarbonate (MPC). Our results indicate that the lowest-lying state is an intrinsically nondissociative aryl ππ* excited state. We identify additional low-lying states that are expected to be dissociative in nature and propose that the time scales observed for the dissociation reaction may correspond to the time scale for transfer of excited state population to these states.

The mechanism of polymerization of p-xylylene and its derivatives is analyzed at the theoretical level. The polymerization reaction takes place in vacuo without any catalyst. The first step is a pyrolytic decomposition of starting material for polymerization, p-cyclophane, a cyclic dimer of p-xylylene, into biradical linear dimer and finally into two quinonoid monomeric molecules of p-xylylene. The quinonoid monomer is diamagnetic; i.e., it has a singlet ground state. The monomers after pyrolysis, when the temperature is lowered, do not re-form cyclic dimers but instead polymerize into long chain molecules. The initiation of polymerization requires dimerization of two monomers leading to formation of a genuine noncoupled biradical dimer. The chain molecules grow through the propagation reaction only one unit at a time, by the attachment of a monomer to a radical chain end. In this work the pyrolysis reaction, the initiation reaction and the first propagation steps of parylene polymerization (up to pentamer) are studied in details using different quantum chemical methods: AM1 and PM6 semiempirical methods and density functional theory (DFT) approach using B3LYP functional with two basis sets of different size (SVP and TZVP).

The dynamics of four-centered HCl elimination from chloroethane are studied using a mixed quantum-classical method based on a reaction path Hamiltonian. Both the structural details of the reaction and the partitioning of the exit-channel potential energy to the products are analyzed. The minimum energy path was calculated at the B3LYP/6-311++G(2d,2p) level of theory, which was followed by energy-partitioning dynamics computations. Selective vibrational excitation of the HCl product was observed, leading to a vibrational state distribution in good agreement with experiment. Differences between HCl elimination from C2H5Cl and HF elimination from C2H5F, particularly in the ethylene fragment, were observed and are discussed.

The interaction between the fragment of carbon nanotube (CNT) and carbohydrates has been investigated using MP2 and M05−2X methods using various basis sets in gas phase. Three carbohydrates, viz., β-d-glucose, β-d-galactose, and β-d-xylose with different degree of hydrophobic nature have been selected for this investigation. With a view to assess the effect of curvature on the interaction between the carbohydrates and CNT, calculations on intermolecular complexes comprising of coronene (COR) and carbohydrates have also been carried out in gas phase. Results obtained from electronic structure calculations combined with the Bader’s electron density analysis reveal that CH···π interaction is the predominant one in the stabilization of the carbohydrate-CNT and carbohydrate-COR complexes. Furthermore, the importance of OH···π and lone pair···π (lp···π) interactions are also evident from the results. The calculated BEs for the various carbohydrate-CNT and carbohydrate-COR complexes at M05−2X with dual basis set [aug-cc-pVTZ for carbohydrate + cc-pVTZ for both CNT and COR] vary from −2.52 to −5.14 and from −4.14 to −8.04 kcal/mol, respectively. The corresponding BEs obtained from MP2/6-311++G(d,p)//M05−2X/6-31+G(d,p) level of calculation range from −4.92 to −9.93 and from −6.75 to −12.53 kcal/mol. Close scrutiny of the energetics of all the complexes elucidate that the electron correlation energy (dispersion energy) significantly contribute to the stability of these complexes. It is found from the analysis of geometrical parameters and BEs that the interplay of orientation of the X-H (X = C and O) bond to the π-surface is crucial for the recognition and further stabilization. Molecular electrostatic potential (MESP) isosurfaces of curved and planar surfaces have clearly provided the difference between the π-electron distributions. Evidences form the energy decomposition analysis elicit that the dispersive interaction plays a significant role in the overall stabilization of the complexes. And, it is possible to observe the delicate balance between the electrostatic interaction and the exchange-repulsion energy.

The mechanisms of the aromatic Claisen rearrangement of 1-(4-chloronaphthyl) 1,1-dimethylallyl ether (Re) under neat conditions and “on water” were investigated. The aromatic Claisen rearrangement usually involves the [3,3]-intramolecular shift followed by a proton transfer. The intermolecular proton transfer is the rate-limiting step under neat conditions with ΔΔEb and ΔΔG‡ values of 25.7 and 29.8 kcal/mol at the B3LYP/6-311++G(d,p) level, respectively. The on water condition was simply modeled by a combination of the “oil” droplet/water interface and neat condition inside the oil droplet. The MD simulation was used to obtain the most reliable interaction position between Re and solvent water, which was further used as a starting material for the water-catalyst mechanism to model the surface reaction. We found that the chairlike [3,3]-intramolecular shift became the rate-limiting step for the water-catalyst mechanism, with lower ΔΔEb (16.3 kcal/mol) and ΔΔG‡ (25.2 kcal/mol) values compared with those under neat condition. Their ΔΔEb and ΔΔG‡ values changed to be 22.0 and 24.9 kcal/mol, respectively, after considering the bulk water effect by QM/MM calculation. Hence, these calculated energy results strongly suggested that the on water reaction should be faster than the one under neat conditions. This can be explained by the following three key factors: (1) the interaction between the species and water clusters in the transition states, especially for the proton transfer process, is stronger than in other states, which was revealed by the binding energy calculation; (2) the two-water cluster enhanced the charge separation in the reaction center of the [3,3]-intramolecular shift, increasing the stability of the corresponding transition state; and (3) the donor−acceptor NBO results suggested that the hydrogen-bonded two-water cluster accelerated the proton transfer process by serving as a proton bridge.

The characteristics of an iodide ion (I−) in aqueous solution were investigated by means of HF/MM and B3LYP/MM molecular dynamics simulations, in which the ion and its surrounding water molecules were treated at HF and B3LYP levels using the LANL2DZdp and D95 V+ basis sets for I− and water, respectively. According to both the HF/MM and B3LYP/MM results, the ion−water interactions are relatively weak, compared to the water−water hydrogen bonds, thus causing an unstructured nature of the hydration shell. Comparing the HF and B3LYP treatments for the description of this hydrated ion, the overestimation of the ion−water hydrogen-bond strength by the B3LYP method is recognizable, yielding a remarkably more compact and too rigid ion−water complex.

A detailed theoretical investigation has been carried out at the density functional level of theories to investigate the nature of Raman intensities of the -P═O stretching mode of a model nerve agent DFP (diisopropylfluorophosphate) when bound to different gold (Au8, Au20) and oxide-supported gold (MgO···Au4, CaO···Au4, TiO2···Au4, Al2O3···Au4, M16O16···Au8, and [M16O15···Au8]2+, M = Ca, Mg) clusters. All of these clusters and the DFP-bound clusters are fully optimized, and the computed energetics shows that DFP attaches itself weakly to these clusters. The normal Raman spectra calculations on these clusters show that there is substantial enhancement of the -P═O stretching mode of DFP compared to the isolated species. This enhancement has been found to be due to the polarization of the -P═O bond of DFP when bound to the clusters. Significant enhancement in intensity has been observed in the case of Aun···DFP (n = 8, 20), M16O16···Au8···DFP, and [M16O15···Au8]2+···DFP (M = Ca, Mg) clusters. The resonance Raman calculations on the Aun···DFP (n = 8, 20) reveals that this enhancement could be made quite large and selective, which is a feature that is unique to the nerve agents and could be used as a property for detecting them.

The performance of some recently proposed DFT functionals by Truhlar’s group (mPW1B95, mPWLYP1W, PBELYP1W, and PBE1W [Dahlke, E. E.; Truhlar, D. G. J. Phys. Chem. B 2005, 109, 317. Zhao, Y.; Truhlar, D. G. J. Phys. Chem. A 2004, 108, 6908.]) was tested primarily with respect to computation of anharmonic vibrational frequency shifts upon hydrogen bond formation in small molecular/ionic dimers. Five hydrogen-bonded systems with varying hydrogen bond strengths were considered: methanol−fluorobenzene, phenol−carbon monoxide in ground neutral (S0) and cationic (D0) electronic states, phenol−acetylene, and phenol−benzene(+). Anharmonic OH stretching frequency shifts were calculated from the computed vibrational potentials for free and hydrogen-bonded proton-donor molecules. To test the basis set convergence properties, all calculations were performed with 6-31++G(d,p) and 6-311++G(2df,2pd) basis sets. The mPW1B95 functional was found to perform remarkably better in comparison to more standard functionals (such as B3LYP, mPW1PW91, PBE1PBE) in the case of neutral dimers. In the case of cationic dimers, however, this is not always the case. With respect to prediction of anharmonic OH stretching frequency shifts upon ionization of free phenol, all DFT levels of theory outperform MP2. Some other aspects of the functional performances with respect to computation of interaction and dissociation energies were considered as well.

The terahertz (THz) spectrum of (S)-(+)-ketamine hydrochloride has been investigated from 10 to 100 cm−1 (0.3−3.0 THz) at both liquid-nitrogen (78 K) and room (294 K) temperatures. Complete solid-state density functional theory structural analyses and normal-mode analyses are performed using a single hybrid density functional (B3LYP) and three generalized gradient approximation density functionals (BLYP, PBE, PW91). An assignment of the eight features present in the well-resolved cryogenic spectrum is provided based upon solid-state predictions at a PW91/6-31G(d,p) level of theory. The simulations predict that a total of 13 infrared-active vibrational modes contribute to the THz spectrum with 26.4% of the spectral intensity originating from external lattice vibrations.

Cardiolipin is a key lipid component in many biological membranes. Proton conduction and proton−lipid interactions on the membrane surface are thought to be central to mitochondrial energy production. However, details on the cardiolipin headgroup structure are lacking and the protonation state of this lipid at physiological pH is not fully established. Here we present ab initio DFT calculations of the cardiolipin (CL) headgroup and its 2′-deoxy derivative (dCL), with the aim of establishing a connection between structure and acid−base equilibrium in CL. Furthermore, we investigate the effects of solvation on the molecular conformations. In our model, both CL and dCL showed a significant gap between the two pKa values, with pKa2 above the physiological range, and intramolecular hydrogen bonds were found to play a central role in the conformations of both molecules. This behavior was also observed experimentally in CL. Structures derived from the DFT calculations were compared with those obtained experimentally, collected for CL in the Protein Data Bank, and conformations from previous as well as new molecular dynamics simulations of cardiolipin bilayers. Transition states for proton transfer in CL were investigated, and we estimate that protons can exchange between phosphate groups with an approximate 4−5 kcal/mol barrier. Computed NMR and IR spectral properties were found to be in reasonable agreement with experimental results available in the literature.

We present here a theoretical methodology that exploits quantum mechanical calculations, molecular mechanics calculations, and Monte Carlo simulations to predict the time-of-flight measurement mobilities in films of phenyl-cored conjugated thiophene dendrimers. Our aim is to reveal structure−property relationships in amorphous films of organic π-conjugated materials. The simulations show that both hole and electron mobilities increase with the size of dendrimer, and that the former is larger than latter in all dendrimers. Internal reorganization energies are inversely correlated with the mobilities. Our simulations also indicate that dendrimers have small density of states for energetic disorder (<60 meV), and both hole and electron mobilities possess weak electric field dependence. We examine the influence of external reorganization energy as well as the possible trap sites on charge transport in these materials.

The C3h conformation of the trimethylsilicenium ion 1 was established to be the preferred global energy minimum structure based on energy calculations. Because C−H hyperconjugation occurs least favorably in this conformation of the analogous tert-butyl cation, it may not contribute in large part to the stabilization of this cation, especially given the ineffectiveness of the 3p−2sp3 overlap that would need to be involved. This is in contrast with the preferred Cs global energy conformers of the tert-butyl cation. The C2v structure 4 and C2 enantiomers 6 and 7 are the preferred conformations of the dimethylsilicenium ion based on energy comparison. None of these structures have C−H bonds ideally oriented for hyperconjugation with the empty p orbital of the cationic silicon, indicating that it does not likely stabilize the ion to any significant extent. The computed IR spectra and 29Si, 13C, and 1H NMR chemical shifts of the isomers were also discussed. Whereas the studied alkylsilicenium ions are thermodynamically stable, their observation as persistent ions in solution is much more difficult because of their kinetic instability toward varied electron-donating solvents.

The main purpose of the present work is to simulate from many-body quantum mechanical calculations the results of experimental studies of the valence electronic structure of n-hexane employing photoelectron spectroscopy (PES) and electron momentum spectroscopy (EMS). This study is based on calculations of the valence ionization spectra and spherically averaged (e, 2e) electron momentum distributions for each known conformer by means of one-particle Green’s function [1p-GF] theory along with the third-order algebraic diagrammatic construction [ADC(3)] scheme and using Kohn−Sham orbitals derived from DFT calculations employing the Becke 3-parameters Lee−Yang−Parr (B3LYP) functional as approximations to Dyson orbitals. A first thermostatistical analysis of these spectra and momentum distributions employs recent estimations at the W1h level of conformational energy differences, by Gruzman et al. [J. Phys. Chem. A 2009, 113, 11974], and of correspondingly obtained conformer weights using MP2 geometrical, vibrational, and rotational data in thermostatistical calculations of partition functions beyond the level of the rigid rotor−harmonic oscillator approximation. Comparison is made with the results of a focal point analysis of these energy differences using this time B3LYP geometries and the corresponding vibrational and rotational partition functions in the thermostatistical analysis. Large differences are observed between these two thermochemical models, especially because of strong variations in the contributions of hindered rotations to relative entropies. In contrast, the individual ionization spectra or momentum profiles are almost insensitive to the employed geometry. This study confirms the great sensitivity of valence ionization bands and (e, 2e) momentum distributions on the molecular conformation and sheds further light on spectral fingerprints of through-space methylenic hyperconjugation, in both PES and EMS experiments.

Using ab initio information at the CCSD(T)/cc-pVTZ level, the reaction path for the Cl + NH3 hydrogen abstraction reaction was traced, and the coupling terms between the reaction coordinate and the normal modes were analyzed along it. Two intermediate complexes were located in the entry channel and characterized close to the reactants. One of them presents a typical Cl···H−N bond, while the second presents a two-center/three-electron Cl∴N bond. Both complexes are on the reaction path and contribute to the final rate constants. With this information, the rate constants were calculated over the temperature range 200−2000 K, using variational transition state theory with multidimensional tunneling contributions, and were found to reproduce the experimental evidence in the common temperature range. Finally, analysis of the coupling terms showed qualitatively that vibrational excitation of the N−H stretch and the bending and umbrella modes in the reactant NH3 enhances the forward thermal rate constants, and that, in the products, the H−Cl stretch mode and the bending mode in NH2 could appear vibrationally excited, although the randomization of the energy in the well in the exit channel might diminish this excitation.

Optimal structures, interaction energies, harmonic and anharmonic vibrational frequencies, and NMR chemical shifts of the dimers LA···H2O and trimers LA···(H2O)2 (where LA is lactic acid) have been determined from the second-order Møller-Plesset perturbation theory and B3LYP with the aug-cc-pVDZ calculations. The nature of the pairwise and nonadditive three-body interactions was investigated by the SAPT method. As revealed by SAPT analysis, the main two-body binding contributions in the LA···H2O dimers and LA···(H2O)2 trimers result from a delicate balance of the attractive and repulsive terms. The three-body nonadditivity for LA···(H2O)2 is stabilizing and dominated by the exchange and induction effects but small.

The radical HSO is an oxidation product of pollutants such as H2S and CH3SH in Earth’s atmosphere. For the first time, the interaction of HSO and its tautomer HOS with single water molecules to yield the hydrates HSO·nH2O and HOS·nH2O was studied for n = 1−3, applying the high-level G3X(MP2) theory. A large number of structures corresponding to local minima on the potential energy surfaces has been identified. While gaseous HSO is more stable than HOS, the enthalpy diffference between HSO·nH2O and HOS·nH2O decreases with increasing degree of hydration and becomes practically zero for n = 3. Thus, in aqueous solution as well as in fog and rain droplets, HOS is expected to compete with HSO. The barrier for the tautomerization of HSO to HOS is dramatically lowered by the presence of water molecules since a cyclic transition state allows a concerted proton shift within the system of neighboring hydrogen bonds. The corresponding activation enthalpy of only 73.5 kJ mol−1 predicted for the transformation of HSO·2H2O into HOS·2H2O may be compared to the 202 kJ mol−1 reported for the tautomerization of the unhydrated gaseous HSO/HOS molecules. The impact of water of hydration on the fundamental vibrational modes of HSO and HOS has also been studied. Furthermore, HOS is predicted to dimerize at low temperatures to give two van der Waals molecules with singlet (symmetry C2) or triplet configuration (symmetry C2h), the latter being more stable than the singlet isomer. The disproportionation of 2HSO to H2S and SO2 is predicted to be exothermic by −263.5 kJ mol−1. The reaction of HSO with ozone to HSO2 and O2 is also strongly exothermic by −274.0 kJ mol−1 and seems to proceed without any barrier. HOS forms a 1:1 van der Waals complex with O3; the redox reaction of its two components is calculated as exothermic by −410.9 kJ mol−1 and results in a rather stable adduct between HOSO and O2 with the structure of a peroxo isomer of HOSO3. This unprecedented hydrogen peroxosulfite radical might open a novel route to atmospheric sulfate without the intermediate formation of SO2 and SO3.

Calculations on the He···MX, Ne···MX, and Ar···MX (M = Cu, Ag, Au; X = F, Cl) complexes at the CCSD and CCSD(T) levels of theory have been carried out. The geometries of the Ar−MF complexes are in good agreement to those determined via microwave spectroscopy. The RG···MX (RG = He, Ne, and Ar) dissociation energies for these complexes have been evaluated by extrapolation to the complete basis set limit. The importance of the inclusion of diffuse functions to the determined dissociation energies of these complexes are discussed with comparison to recent work. For the He···CuF and He···AuF complexes, the dissociation energies have been found to be significant, at ≈26 kJ mol−1, while the bonding in the chlorine analogues is only ≈15 kJ mol−1. The bonding between the helium and the metal atoms in the He···CuF and He···AuF complexes has been investigated by using an atoms-in-molecules (AIM) analysis together with an evaluation of the dipole/induced-dipole and ion/induced-dipole interactions. This analysis has shown that the bonding in these complexes is slightly covalent in nature. For the He···AgF and Ne···MF (M = Cu, Ag, Au) complexes the dissociation energy is much smaller and the AIM analysis shows the bonding is more electrostatic in nature. Calculations have also been carried out on He···AgCl and Ne···MCl (M = Cu, Ag, Au) complexes for the first time in addition to the Ar···MX (M = Cu, Ag, Au; X = F, Cl) complexes. The RG···MCl complexes are found to be more weakly bound than the corresponding RG···MF complexes as a result of the difference in electronegativity of the halogen. For each complex, bond lengths, rotational constants, and harmonic vibrational frequencies have also been evaluated.

Based on accurate electronic structure calculations, a new analytical potential energy surface (PES) was fitted to simultaneously describe the hydrogen abstraction reaction from ammonia by a hydrogen atom, and the ammonia inversion. Using a wide spectrum of properties of the reactive system (equilibrium geometries, vibrational frequencies, and relative energies of the stationary points, topology of the reaction paths, and points on the reaction swaths) as reference, the resulting analytical PES reproduces reasonably well the input ab initio information obtained at the CCSD(T)/cc-pVTZ level, which represents a severe test for the new surface. As a first application, on this analytical PES we perform an extensive kinetics study using variational transition-state theory with semiclassical transmission coefficients over a wide temperature range, 200−2000 K. For the hydrogen abstraction reaction, the forward rate constants reproduce the experimental measurements, while the reverse ones are slightly underestimated. Another severe test of the new surface is the analysis of the kinetic isotope effects (KIEs). The KIEs between unsubstituted and all deuterated reactions agree with experiment in the common temperature range. For the ammonia inversion reaction, the splitting of the degenerate vibrational levels of the double well due to the tunneling contribution, which is very important in this reaction representing 93% of the reactivity at 200 K, was calculated for the NH3 and ND3 species. The values found were 3.6 and 0.37 cm−1, respectively, which although higher than experimental values, reproduce the experimental behavior on isotopic substitution.

Electronic structure, charge distribution, and vibrational frequencies of cucurbit[n]uril, CB[n] (n = 5−12), hosts have been derived using the density functional methods. CB[n] conformers with different orientations of methylene group connecting glycouril units have been investigated. The conformers that possess uniform CB[n] cavity turn out to be of lowest energy, and molecular electrostatic potential (MESP) herein engender shallow minima near ureido oxygens along the series. MESP topography has been utilized to estimate the cavity height and diameter; the ratio of which governs the shape (circular or elliptical) of the cavity. When this ratio is larger than unity (for CB[n] with n ≥ 8), an elliptical host cavity is noticed. Calculated vibrational spectra reveal that carbonyl stretching frequency shift in successive CB[n] homologue decreases steadily from 1760 cm−1 in CB[5] to 1742 cm−1 in CB[12]. An increase in glycouril units along the CB[n] series influences significantly the intensity profile of C═O and C−N stretching vibrations in the calculated infrared spectra. Furthermore, calculated 1H chemical shifts predict that one of methylene protons directing outside the host cavity are deshielded, whereas the remaining proton near the carbonyl group exhibits downshifted signal in the NMR spectra.