LETTER pubs.acs.org/JPCL Infrared Spectroscopy of Hydrated Bisulfate Anion Clusters: HSO4h(H2O)116

Amolecular-level understanding of the solvation of acids insize-selected clusters can lend insight into their behavior in bulk solution. Experimental14 and theoretical5,6 studies have explored the extent of acid dissociation in clusters and the minimum number of water molecules needed for this process to occur. Analogous experiments on clusters of water with ionic acids (or bases) are also of considerable interest7,8 because the size and composition of a charged cluster is more readily determined than that for a neutral cluster. This paper focuses on hydrated clusters of the bisulfate anion, HSO4h, a weak acid in aqueous solution with a pKa of 2.0. Bisulfate is also among the most prevalent negative ions in the troposphere and the strato-sphere due to its high stability with respect to electron detach-ment.911 It may also play a role in seeding aerosols by homo-geneous nucleation.9,1214 These considerations motivate the work presented here, in which we report infrared multiple photo