Kinetics of Homolytic Substitutions by Hydrogen Atoms at Thiols and Sulfides



Thermodynamic and kinetic data in the temperature range 300–1500 K are calculated for 94 homolytic substitution reactions by a hydrogen atom at thiols and sulfides with the CBS-QB3//BMK/6-311G(2d,d,p) method. The studied reactions were found to proceed according to a one-step mechanism. A group additivity (GA) method is presented to model the Arrhenius parameters of this reaction family. The required GA values were derived from data obtained for a set containing 58 reactions. By using the developed GA scheme, rate coefficients at 300 K for 26 substitution reactions by the hydrogen atom are reproduced within a factor of 2.2. Mean absolute deviations on the activation energy and pre-exponential factor are limited to 1.1 kJ mol−1 and 0.19, respectively. Rate coefficients for the reverse reactions, that is, substitution reactions by C- and S-centered radicals with expulsion of a hydrogen atom, are reproduced within a factor of 6 by using thermodynamic consistency. At 1000 K, group additive and calculated rate coefficients for the forward and reverse reactions agree within a factor of 1.8 and 4, respectively. Experimental rate coefficients in the temperature range 300–400 K are reproduced within a factor of 5. Discrepancies between calculated and experimental data are discussed.