Vibrational spectroscopy of NO⁺(H₂O)_n: Evidence for the intracluster reaction NO+(H₂O)_n→ H₃O⁺(H₂O)_n-2 (HONO) at n≥4

Infrared spectra of mass-selected clusters NO⁺(H ₂O)_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 Møller-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 H ₃O⁺(H₂O)₃(HONO), i.e., an adduct of the reaction products.