On the basis of high-level molecular quantum mechanics, the zero-point-corrected energy for formation of the anti-anti conformer of carbonic acid from water and carbon dioxide is compensated by formation of the most stable dimer. Therefore, the zero-point-corrected energy for formation of the carbonic acid dimer from its constituents water and carbon dioxide is close to zero, and even slightly negative values seem to be possible. These results include new CCSD(T)/6-311++G(3df,3pd) calculations of the formation energy of carbonic acid from water and carbon dioxide, which gave an energy difference of between 8.2 and 9.2 kcal/mol which is lower than those reported before. Furthermore, the dimerization energy of carbonic acid and formic acid is reported up to the MP2/aug-cc-pVTZ level of theory, leading to a best estimate of −16.9 kcal/mol for the carbonic acid dimer relying on convergence considerations for the MP2/aug-cc-pVxZ level of theory that also clearly reveal the failure of counterpoise correction for basis set superposition error. We conclude that in our atmosphere formation of a dimer of carbonic acid is disfavored mainly by entropy, and not by enthalpy.

The reactions of hydroxyl radical with ethene, fluoroethene, and chloroethene have been studied by quantum chemical methods. Reactants, prereaction complexes, transition-state structures, and products were optimized and vibrational frequencies were calculated at the UMP2/6-311+G(2d,p) level. Transition-state structures are significantly different from the prereaction complexes formed on the reactant side of the MEP. The convergence of barrier heights and reaction enthalpies has been systematically investigated with respect to the size and quality of basis set and the treatment of correlation energy. The best agreement with experimental results is found at the MP2/aug-cc-pVTZ level of theory. Regioselectivity is discussed in terms of two properties of the radical and the investigated alkenes. The first factor is the relative spin density in the 3ππ* state of the alkene. The second factor is the relative strengths of the product C−O bond, i.e., relative stability of the corresponding radical product. In the case of fluoroethene these two effects oppose each other and regioselectivity is negligible. In the case of chloroethene spin density is the dominant factor and the addition of OH radicals to the unsubstituted carbon atom is preferred.

The concerted proton transfer hypersurface of cyclic water and hydrogen fluoride clusters has been described by high-level molecular quantum mechanical calculations. For the cyclic water clusters the concerted proton transfer transition states have been investigated for the first time with methods including treatment of dynamic electron correlation. The crucial importance of dynamic electron correlation for the barrier heights is demonstrated. A detailed analysis of the minimum energy path has been performed. The reaction swath of the concerted proton transfer was examined indicating a reasonable description by harmonic approximation of the energy hypersurface. Transfer rates have been calculated by means of variational transition state theory with interpolated corrections (VTST-IC) and dual-level direct dynamics (DLDD), both with semiclassical tunneling corrections. Tunneling is very efficient in the concerted proton exchange reaction of the cyclic hydrogen-bonded clusters under investigation. Rate constants for the concerted exchange of hydrogens in important hydrogen fluoride vapor phase species are reported for the first time. In the hydrogen fluoride tetramer and pentamer the concerted proton exchange of four and five protons, respectively, takes place with reaction rates that are comparable with the concerted exchange rates in carboxylic acid dimers and is not hindered by the large number of simultanously moving protons. The concerted proton exchange rates in the studied water clusters are comparably low because of higher exchange barriers. It is shown that hydrogen fluoride clusters can be used to a large extent as "simplified" experimental and theoretical models for water clusters.

The reaction C 2 H 5 Clϩ•OH→C 2 H 5 Cl•ϩH 2 O ͑␣ and ␤ abstraction͒ has been investigated by ab initio molecular orbital theory with several basis sets and levels of correlation. Optimized geometries and harmonic vibrational frequencies have been calculated for all reactants, transition states, and products at the ͑U͒HF/6-31G(d,p) and ͑U͒MP2/6-31G(d,p) levels of theory. The correlation energy is found to play an important role in determining the barrier heights and reaction enthalpies as well as the geometry and the vibrational frequencies of the transition states. A pseudocyclic transition state is found to be favorable to the ␤-abstraction reaction since the participation of the chlorine substituent reduces the barrier height by 0.95 kcal/mol, through a relatively large inductive through-space effect. The best results for the barrier heights and reaction enthalpies have been obtained using the second-order Mo "ller-Plesset perturbation theory with spin projection employing the 6-311ϩG(2d,p) basis set. A satisfactory agreement is found with available experimental values.