Equilibrium constants for clustering of neutral molecules about gaseous ions

A study of the clustering of neutral molecules to gas-phase ions was made using mass spectrometry. Equilibrium constants and the associated standard Gibbs free energy changes were determined for the attachment of H2O, SO2, and CO2 to a number of positive and negative ions. Mixed clusters were studied, as well as clusters containing only one type of ligand. A total of 49 constants were measured for ligand association and switching reactions; a further 21 new constants could be derived from thermodynamic cycles. Clustering of N2 about hydrated positive and negative ions was observed, but no equilibrium constants were determined. The observed trends in the cluster stabilities are explained by consideration of the properties of the ions and molecules. The ions used were produced by a high pressure source using aqueous solutions. This technique extends considerably the range of ions that can be studied.