Theoretical studies of the kinetics, thermochemistry, and mechanism of H-abstraction from methanol and ethanol

The process of hydrogen abstraction from methanol and ethanol has been calculated with ab initio quantum chemical methods with extended basis sets (6-311G**) and with the inclusion of correlation up to MP4SDQ. These studies serve as a model of such processes in large molecules of biological importance including the sugar moiety of DNA. A comparison of geometries of ground and transition states optimized at UHF and MP2 levels with the 6-31G basis set shows that the UHF optimized geometries have lower energies at the highest level of theory used (MP4SDQ/6-311G**). The transition states occur at a somewhat later stage along the reaction coordinate at the UHF level than at the MP2 level. Energy barriers, along with zero-point energies, were used to calculate the rate constants for H-abstraction from Cα of methanol and ethanol. Tunneling corrections were applied according to an Eckart treatment of an unsymmetrical unidimensional barrier. The corrected rate constants are in very good agreement with experiment over a wide range of temperatures. The same approach was used to predict the rate constant for the abstraction of the hydrogen from Cβ, of ethanol, which is not known from experimental measurements. The calculated C-H bond strengths and heats of reactions are also in good agreement when the correlation energy is scaled according to the MPnSAC approach. The geometric and energetic parameters of the transition states behave according to Hammond's postulate, i.e., the more exothermic the H-abstraction, the closer is the transition state to the reactants. This relationship suggests that the C-H bond strength is one of the major factors that determine the barrier to H-abstraction. An analysis of the MCSCF wave function constructed from a CAS of three electrons distributed in three orbitals (σCH, σ*CH, and the orbital containing the unpaired electron) supports this conclusion.