Infrared Spectroscopy of Hydrated Bisulfate Anion Clusters: HSO4¯(H2O)1–16

Abstract

Gas-phase infrared photodissociation spectra of the microhydrated bisulfate anions HSO4¯(H2O)n, with n = 1–16, are reported in the spectral range of 550–1800 cm–1. The spectra show extensive vibrational structure assigned to stretching and bending modes of the bisulfate core, as well as to water bending and librational modes. Comparison with electronic structure calculations suggests that the acidic proton of HSO4– is involved in the formation of a hydrogen bond from n ≥ 1 and that water–water hydrogen bonds form for n ≥ 2. The water network for the larger clusters forms hydrogen-bonded “bands” about the bisulfate core. The blue shifting of the SOH bending mode from 1193 (n = 1) to 1381 cm–1 (n = 12) accompanied by a dramatic decrease in its IR intensity suggests increased incorporation of the bisulfate hydrogen atom into the hydrogen-bonding network, the first step toward acid dissociation.