VIBRATIONAL SPECTROSCOPY OF NO+(H2O)(N) - EVIDENCE FOR THE INTRACLUSTER REACTION NO+(H2O)(N)-]H3O+(H2O)(N-2) (HONO) AT N-GREATER-THAN-OR-EQUAL-TO-4

Infrared spectra of mass-selected clusters NO+(H2O)(n) for n = 1 to 5 were recorded from 2700 to 3800 cm(-1) by vibrational predissociation spectroscopy. Vibrational frequencies and intensities were also calculated for n = 1 and 2 at the second-order Moller-Plesset (MP2) level, to aid in the interpretation of the spectra, and at the singles and doubles coupled cluster (CCSD) level energies of n = 1 isomers were computed at the MP2 geometries. The smaller clusters (n = 1 to 3) were complexes of H2O ligands bound to a nitrosonium ion NO+ core. They possessed perturbed H2O stretch bands and dissociated by loss of H2O. The H2O antisymmetric stretch was absent in n = 1 and gradually increased in intensity with n. In the n = 4 clusters, we found evidence for the beginning of a second solvation shell as well as the: onset of an intracluster reaction that formed HONO. These clusters exhibited additional weak, broad bands between 3200 and 3400 cm(-1) and two new minor photodissociation channels, loss of HONO and loss of two H2O molecules. The reaction appeared to go to completion within the n = 5 clusters. The primary dissociation channel was loss of HONO, and seven vibrational bands were observed. From an analysis of the spectrum, we concluded that the n = 5 cluster rearranged to form H3O+(H2O)(3)(HONO), i.e., an adduct of the reaction products.