Precipitation Reactions in Water Clusters

The reactions of hydrated metal cations Ag + (H 2 O) n, n = 11−26, and Na + (H 2 O) n, n = 12−28, with HCl were studied by FT-ICR mass spectrometry. One observes an efficient fragmentation due both to collisions and to blackbody radiation, as well as ligand exchange reactions. The number of HCl molecules which can be stabilized in a cluster depends on the number of H 2 O ligands. In the case of Na + (H 2 O) n clusters, 12 and 15 H 2 O ligands are needed to “dissolve” 1 and 2 HCl molecules, respectively, results very similar to H + (H 2 O) n . On the other hand, in the case of hydrated silver cations, only 4 and 7 H 2 O molecules are necessary to stabilize 1 and 2 HCl molecules. The difference is due to the fact that many water ligands are needed to stabilize the 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 cluster a covalently bound AgCl. The observed behavior reflects the large difference in the solubility products of NaCl and AgCl and shows that an analogue to bulk precipitation reaction occurs on a molecular level in clusters.