Spectroscopic Signature of Proton Location in Proton Bound HSO4-·H+·X- (X = F, Cl, Br, and I) Clusters.

Proton transfer plays a vital role in a variety of chemical and biological processes. The proton location in proton bound clusters, specifically, in the anions of HSO4-·H+·X- (X = F, Cl, Br, and I), has been studied by negative ion photoelectron spectroscopy and ab initio theoretical calculations. The measured photoelectron spectra of HSO4-·H+·X- (X = Cl, Br, and I) closely resemble those of X- by simply shifting to higher electron binding energies, suggesting that X- is the chromophore of the electron photodetachment, a fact clearly violating gas-phase acidity predictions. However, in the X = F case, the spectrum of HSO4-·H+·F- is more similar to that of HSO4-, indicating that H+ stays with F- and that the HSO4- moiety carries the extra electron. Accompanying theoretical analyses are in excellent agreement with the experimental measurements and observations. This work provides direct spectroscopic evidence of the proton locations, clearly showing cases in which proton affinities of the constituent bases cannot correctly predict the right chemistry involving proton transfer processes.