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 CHn F3-nSH (n = 0-3) were calculated at the G2 level of theory. The variation o f the geometry of the neutral species and products of heterolytic and homol ytic proton abstraction processes, as well as the energetics (gas-phase aci dities and homolytic bond dissociation energies of the title compounds), up on successive fluorine substitution has been studied. The results show that the progressive introduction of fluorine atoms into methanol and methaneth iol reduces significantly the acidity gap between the representatives of th ese two series, Therefore, the calculations indicate that the acidities of CF3OH and CF3SH are predicted to be rather close: the former compound is ex pected to be 0.1 kcal/mol more acidic than its SH counterpart. It was concl uded that negative (anionic) hyperconjugation and electrostatic effects are mainly responsible for such behavior.