Proton mobility and stability of water clusters containing alkali metal ions

Abstract

The results of experiments with water clusters of the type M+(H2O)n (M=Li, Na, K, Rb and Cs, n=1–30) are presented. On the basis of analysis of two complementary data sets, namely the abundance spectra and the metastable ion dissociation spectra, both obtained using a quadrupole time-of-flight mass spectrometer, we report consistent rates for water evaporation. The results show enhanced kinetic stability for n=20 and 27 for all alkali metals except sodium, and for n=25 for Li, K and Rb. The structural relationships among alkali-metal ion centred water-clusters and protonated water cluster in terms of these so-called magic numbers are discussed.Reactions of size-selected M+(H2O)n (n=1–30) with D2O were also investigated. It was observed that the rates of protium/deuterium exchange for water clusters containing alkali metal ions, when properly corrected for contamination of the D2O, are consistently extremely low, even for the largest clusters. These results are valuable in providing solid support for the fact that alkali-metal–water clusters are essentially inert in H/D exchange reactions with water, making them ideal for benchmarking in such studies.Quantum chemical calculations performed for A(H2O)n (A=H2O, Li+, Na+, n=3–8) revealed significant energy barriers towards proton transfer, in good agreement with the experimental observations. Interestingly, it appears that proton mobility in a water cluster containing an alkali metal ion is lower than in pure water clusters of the same size. Furthermore, the lithium ion behaves like the other alkali metals, showing no similarity with hydrated H+ or D+.