H-atom abstraction from thiols by C-centered radicals. A theoretical and experimental study of reaction rates

Abstract

The Arrhenius parameters and rates of reaction of three hydroxyradicals, methyl radical, and the hindered primary C-centred radical from t-butyl alcohol with dithiothreitol were measured by pulse radiolysis in water. The bimolecular rate constants were found to be in the order: ˙C(CH₃)₂OH > ˙CH(CH₃)OH > ˙CH₂OH > ˙CH₃ > ˙CH₂C(CH₃)₂OH. The reaction of three of these, ˙C(CH₃)₂OH, ˙CH₂OH, and ˙CH₃, with methanethiol were examined at the ab initio B3LYP/6311+G(d,p) level, coupled with transition state theory, both in the gas phase and in solution. The solvent effects are evaluated by two different continuum models (SCIPCM, CPCM), coupled with a novel approach to the calculation of the solution phase entropy. The reaction is discussed in terms of the charge and spin polarization in the transition state, as determined by AIM analysis, and in terms of orbital interaction theory. Rate constants, calculated by transition state theory are in good agreement with the experimental data.