Density functional studies of hydrogen atom addition to the C=S bond

The results of density functional theory calculations on the structures, en ergetics, and hyperfine properties of radical H adducts to the C and S atom s of several thioketones and related species are presented. Despite the gre ater thermochemical stability of the C adduct in several cases, the isotrop ic proton couplings (and, where data are available, N-14 couplings) make it clear that the species observed in mu SR experiments are S adducts, counte r to the assignments made by the experimentalists. The preference for S add ition is probably a result of the barrier associated with the geometrical d istortion required for addition at C or may indicate a formation mechanism involving a charged intermediate. The distribution of the magnitudes of the couplings of both C and S adducts shows that previous intuitive arguments are inadequate in the assignment of experimental data. Calculation of the d ynamically averaged temperature-dependent coupling within a one-mode approx imation corrected for zero-point bond stretching yields almost-quantitative agreement with experiment for thioacetamide assuming addition is to S.