Enhanced Acidity of Zn2+ in the Presence of Small Numbers of Water Molecules

Experiments performed on gas phase [Zn(H2O)N](2+) complexes, for N in the range 4-7, show the ions readily undergo unimolecular (metastable) decay with respect to proton release via the reaction [Zn(H2O)N](2+) -> Zn2+OH-(H2O)(M) + H3O+(H2O)(N-M-2). To account for these products, it is proposed that the larger complexes have a stable [Zn(H2O)(4)](2+) core to which additional molecules are retained in an outer shell through hydrogen bonding. At N = 7, this arrangement would make it possible for proton release to be associated with a chain of up to four water molecules, which equates with ideas proposed for the activity of Zn2+ in metalloenzymes.