Precipitation reactions in water clusters

The reactions of hydrated metal cations Ag+(H2O)(n), n = 11-26, and Na+(H2O )(n), n = 12-28, with HCl were studied by FT-ICR mass spectrometry. One obs erves an efficient fragmentation due both to collisions and to blackbody ra diation, as well as ligand exchange reactions. The number of HCl molecules which can be stabilized in a cluster depends on the number of H2O ligands. In the case of Na+(H2O), clusters, 12 and 15 H2O ligands are needed to "dis solve" 1 and 2 HCl molecules, respectively, results very similar to H+(H2O) (n). On the other hand, in the case of hydrated silver cations, only 4 and 7 H2O molecules are necessary to stabilize 1 and 2 HCl molecules. The diffe rence is due to the fact that many water ligands are needed to stabilize th e HCl and NaCl species which are ionically dissolved, while much less water is needed in the case of hydrated Ag+ cations, which form within the clust er a covalently bound AgCl. The observed behavior reflects the large differ ence in the solubility products of NaCl and AgCl and shows that an analogue to bulk precipitation reaction occurs on a molecular level in clusters.