Knowledge Bank

Studies of solvated cluster ions can provide insights into the microscopic aspects of solvent-induced intracluster reactions. We have obtained infrared spectra of mass-selected $NO+(H2O)n$ cluster ions for $n=1-5$ in the 2700-3800 $cm-1$ region by vibrational predissociation spectroscopy. On the basis of the observed photodissociation behavior and infrared spectra, qualitative structural information about $NO+(H2O)n$ was derived. In the smaller clusters, the spectra indicate that $H2O$ ligands are bound to a nitrosonium ion core, but there is a dramatic change in the infrared spectrum at $n=5$, indicating that upon sufficient hydration an intracluster reaction occurs: $NO-(H2O)4+H2O\to H3O+(H2O)3(HONO)$. In the larger clusters, hydrogen-bonding stabilization strongly favors formation of the hydronium ion core, $H3O+$. We have also performed {ab initio} calculations to estimate vibrational frequencies and binding for $n=1$ and $n=2$ in order to aid in interpreting the observed spectra and photodissociation processes.