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 (950-3900 cm-1) of D2-tagged [F, (HFIP)n]ˉ 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 OH 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]ˉ. OH 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. The pronounced dependence of HFIPs hydrogen bond donor ability on its direct solvation environment helps in understanding its unusual role in fluorination reactions.