What influences barrier heights in hydrogen abstraction from thiols by carbon-centered radicals? A curve-crossing study

High-level ab initio molecular orbital calculations have been used to study the barriers and enthalpies for hydrogen atom abstraction reactions of the form RS-H + ^.R′ → RS^. + H-R′ for combinations of R, R′ = CH₃, CH₂Cl, CHCl ₂, CCl₃ CH₂F, CH₂OH, CH ₂SH, CH₂CN, CH₂CH₃) CH ₂CH₂CH₃, CH₂Ph, and CH ₂C(CH₃)₃. The results are analyzed with the aid of the curve-crossing model. Hydrogen abstraction by carbon-centered radicals from thiols is generally an exothermic process in which a strong C-H bond is formed at the expense of the weaker S-H bond of the thiol. However, the exothermicities are strongly influenced by substituents on the attacking radical (and, to a lesser extent, the thiol), and the reverse reaction could be thermodynamically preferred for appropriately substituted systems. The barrier heights are predominantly influenced by polar factors, with the reactions of nucleophilic radicals (such as ^.CH₂OH) being favored over reactions with electrophilic radicals (such as ^.CH₂CN). However, other factors, such as the reaction exothermicity, the strength of the forming and breaking bonds, and (in some cases) direct H-bonding interactions in the transition structures, also contribute to the trends in the barriers.