Can O-H acid be more acidic than its S-H analog? A G2 study of fluoromethanols and fluoromethanethiols

Acidities and homolytic O-H bond dissociation energies of CHnF3-nOH and CHnF3-nSH (n = 0-3) were calculated at the G2 level of theory. The variation of the geometry of the neutral species and products of heterolytic and homolytic proton abstraction processes, as well as the energetics (gas-phase acidities and homolytic bond dissociation energies of the title compounds), upon successive fluorine substitution has been studied. The results show that the progressive introduction of fluorine atoms into methanol and methanethiol reduces significantly the acidity gap between the representatives of these two series, Therefore, the calculations indicate that the acidities of CF3OH and CF3SH are predicted to be rather close: the former compound is expected to be 0.1 kcal/mol more acidic than its SH counterpart. It was concluded that negative (anionic) hyperconjugation and electrostatic effects are mainly responsible for such behavior.