Rate Constants and Transition State Geometry of Reactions of Alkyl, Alkoxyl and Peroxyl Radicals with Thiols

Enthalpy, activation energy, and rate constant of 9 alkyl, 3 acyl, 3 alkoxyl, and 9 peroxyl radicals with alkanethiols, benzenethiol, and L-cysteine are calculated. The intersection parabolas model is used for activation energy calculations. Depending on the structure of attacking radical, the activation energy of reactions with alkylthiols varies from 3 to 43 kJ mol(-1) for alkyl radicals, from 7 to 9 kJ mol(-1) for alkoxyl, and from 18 to 35 kJ mol(-1) for peroxyl radicals. The influence of adjacent pi-bonds on activation energy is estimated. The polar effect is found in reactions of hydroxyalkyl and acyl radicals with alkylthiols. The steric effect is observed in reactions of alkyl radicals with tert-alkylthiols. All these factors are characterized via increments of activation energy. Quantum chemical calculations of activation energy and geometry of transition state were performed for model reactions: C(center dot)H(3) + CH(3)SH, CH(3)O(center dot) + CH(3)SH, and HO(2)(center dot) + CH(3)SH with using density functional theory and Gaussian-98.