Proton mobility and stability of waterclusters containing the bisulfate anion, HSO4−(H2O)n

Abstract

Bisulfate water clusters, HSO₄⁻(H₂O)_n, have been studied both experimentally by a quadrupole time-of-flight mass spectrometer and by quantum chemical calculations. For the cluster distributions studied, there are some possible “magic number” peaks, although the increase in abundance compared to their neighbours is small. Experiments with size-selected clusters with n = 0–25, reacting with D₂O at a center-of-mass energy of 0.1 eV, were performed, and it was observed that the rate of hydrogen/deuterium exchange is lower for the smallest clusters (n < 8) than for the larger (n > 11), with a transition taking place in the range n = 8–11. We propose that the protonic defect of the bisulfate ion remains rather stationary unless the degree of hydration reaches a given level. In addition, it was observed that H/D scrambling becomes close to statistically randomized for the larger clusters. Insight into this size dependency was obtained by B3LYP/6-311++G(2d,2p) calculations for HSO₄⁻(H₂O)_n with n = 0–10. In agreement with experimental observations, these calculations suggest pronounced effectiveness of a ‘‘see-saw mechanism’’ for pendular proton transfer with increasing HSO₄⁻(H₂O)_n cluster size.