Hydration motifs of ammonium bisulfate clusters of relevance to atmospheric new particle formation

Abstract

Clusters of ammonia or amines and sulfuric acid are predicted to drive the formation of new particles from atmospheric trace gases, but the role of water in this process, or even the extent of hydration of the clusters, is not well known. We present vibrational spectra of exactly mass-selected clusters of ammonia or dimethylamine and sulfuric acid with zero to four water molecules bound. These hydrated clusters are synthesized by storing initially-dry clusters in an ion trap held at 180 K with a small partial pressure of water vapor. Analysis of the spectra shows that hydration occurs first at the ammonium NH groups, rather than the bisulfate OH or between the conjugate acids and bases, and that binding to the bisulfate OH only becomes favorable if an additional hydrogen bond acceptor is in the vicinity. These spectra are compared to quantum chemical predictions to evaluate the specific structural motifs present. Broadly speaking, they reveal classes of isomers with similar overall binding motifs but different specific arrangements of water within these motifs. The structures determined for each ensuing hydrate can be explained by addition of one water to the next lower hydrate, without the need for significant rearrangement of the cluster or hydrate structures. Taken together, these observations suggest that multiple hydration isomers may play a role in atmospheric conditions, but substantial barriers to water rearrangement may direct the mechanism of sequential hydration, and entropic contributions to the heats of formation may play an important role in models of new particle formation.