Substituent effects on the S-H bond dissociation energies of thiophenols

Density function UB3LYP/6-311++g(d,p) and perturbation theory ROMP2/6-311++g(d,p) calculations were performed on 4-substituted thiophenols and their corresponding radicals. It was found that although UB3LYP and ROMP2 methods underestimated the absolute S-H bond dissociation energies, they could predict almost as good relative S-H bond dissociation energies as a method of a considerably higher level, UCCSD(T)/6-311++(d,p). From the calculation results it was determined that the S-H bond dissociation energies of thiophenols should have a positive correlation with the substituent sigma(p)(+) constants whose slope was ca. 2.5 kcal mol(-1). Such a slope indicated that the experimental S-H bond dissociation energies obtained from a previous solution phase measurement were reasonably accurate for para H, CH3, OCH3, Cl, and NO2 substituted thiophenols. However, the solution phase bond dissociation energy for 4-aminothiophenol was too low, which was found by the calculation in this study to be caused by the hydrogen bonding between the amino group and the solvent molecules. Finally, through the studies on the isodesmic reactions it was found that the substituent effects on the stability of neutral thiophenols had a fair and positive correlation with the substituent sigma(p)(+) constants; the slope was 0.5 kcal mol(-1). On the other hand, the substituent effects on the stability of thiophenol radicals had an excellent and negative correlation with the substituent sigma(p)(+) constants and gave a slope of -1.8 kcal mol(-1). Therefore, the major source of the substituent effects on S-H bond dissociation energies of thiophenols was the stability of the homolysis products, namely, thiophenol radicals.