Infrared Photodissociation Spectroscopy of Fluoride-Anion Hexafluoroisopropanol Complexes: Solvation-Suppressed Proton Transfer.

We characterize the interaction in fluoride-anion complexes with up to three hexafluoroisopropanol (HFIP) molecules using gas-phase ion vibrational spectroscopy combined with electronic structure calculations and an energy decomposition analysis. Infrared photodissociation spectra (1000-3350 cm-1) of D2-tagged [F, (HFIP)n]- complexes with n = 1-3 allow for a detailed characterization of the extent of charge transfer and hence the extent of proton transfer in these prototypical systems. The OH stretching frequency for n = 1 is observed at 1436 cm-1. This dramatic red-shift ΔνOH by more than 2230 cm-1 relative to that of free HFIP is associated with the presence of a very strong, single-well ionic hydrogen bond and, consequently, a shared proton motif in [F, HFIP]-. ΔνOH is reduced to 1878 cm-1 (n = 2) and 1067 cm-1 (n = 3) in the larger complexes, indicating a substantial weakening of the ionic hydrogen bond concomitant with a suppression of proton transfer to the fluoride anion upon microsolvation with multiple HFIP molecules. This pronounced dependence of fluoride's hydrogen-bond acceptor ability on its direct solvation environment contributes to a better understanding of HFIP's role in promoting fluorination reactions.