Water clusters studied by electrospray mass spectrometry

Abstract

By a slight modification of the electrospray ionization method, positively charged water clusters H+(H2O)n can conveniently be studied by high-resolution mass spectrometry over a large range of masses (from 1 to well over 100 water molecules). Inclusion of an ammonium ion instead of a hydronium ion in the clusters does not lead to a change in the distribution, and this indicates similar structures for the two types of clusters. In small ammonia–water clusters the frequency of ammonia is higher than predicted by statistics, whereas the opposite is true for large ammonia–water clusters. Similarities in the distribution of clusters generated from pure water and from an ammonia–water mixture gives doubt about proton delocalization on the cage surface. Instead, structures which have a central hydronium or ammonium ion are more likely. A new suggestion is made about structures of the well-known clusters H3O+(H2O)20 and NH4+(H2O)20.