Theoretical study of the organosulfur systems CSHn (n=0–4) and CSHn+ (n=0–5): Dissociation energies, ionization energies, and enthalpies of formationa)

Abstract

The Gaussian-2 (G2) theoretical procedure, based on ab initio molecular orbital theory, is used to calculate the energies of the CSHn (n=0–4) hydrides and CSHn+ (n=0–5) cations. The G2 adiabatic ionization energies of CH3SH, CH2SH, CH3S, CH2S, HCS, and CS as well as the proton affinity of CH3SH are all in satisfactory agreement with experimental values. G2 enthalpies of formation of the neutrals and cations have also been calculated and are used to help assess the reliability of the experimental values, which in some cases cover a rather wide range. The calculated enthalpy of formation of CH3S at 0 K is 31.6 kcal/mol. This supports the value of 31.44 kcal/mol reported in a recent kinetics study over values of 34.2–35.5 kcal/mol obtained in recent photofragmentation studies. Theoretical S–H and C–H bond dissociation energies for CH3SH are 86.0 and 95.0 kcal/mol, respectively. G2 theory predicts the CH2SH+ cation to lie 33.1 kcal/mol lower in energy than the CH3S+ cation, in good agreement with the value of 33.5±2 kcal/mol obtained in a recent photoionization study, but differing significantly from results of previous experimental work that gave a much larger energy difference. The G2 enthalpies of formation at 0 K of CH2S, CH2S+, HCS, and HCS+ are calculated to be 28.7, 245.0, 70.8, and 241.7 kcal/mol, respectively.