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

Abstract

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 C{sub 1} 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 C{sub {beta}} of ethanol, which is not known from experimental measurements. The calculated C-H bond strengths and heatsmore » 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 ({sigma}{sub CH}, {sigma}{sub CH}, and the orbital containing the unpaired electron) supports this conclusion. 38 refs., 4 figs., 6 tabs.« less