Infrared spectroscopic studies of molecular structures, intramolecular vibrational energy redistribution, and dissociation dynamics of peroxynitrous acid and nitric acid

The three-body association reaction, OH + NO₂ + M → HONO ₂ + M [M≡bath gas], is a primary termination step for atmospheric hydroxyl (OH) and nitrogen dioxide (NO₂) species. Peroxynitrous acid (HOONO), a less stable isomer of nitric acid (HONO₂), is a secondary product of this reaction that may have significant impact on HO _x and NO_x chemistry in the lower atmosphere. There are at least two stable conformers of HOONO, namely, cis-cis and trans-perp, where the labels refer to the OONO and HOON torsional angles, respectively. HOONO is generated via photolysis of HONO₂, followed by recombination of OH and NO₂ and subsequent cooling in a pulsed supersonic expansion. High resolution infrared action spectroscopy is utilized to identify the first (2ν_OH) and second (3ν_OH) OH overtone transitions of the trans-perp conformer. A binding energy of 16.2(1) kcal mol-1 is deduced from the statistical quantum state distributions of the OH products following unimolecular dissociation. The observed transitions are assigned based on vibrational frequency, rotational band structure, transition type and stability. The cis-cis conformer (2ν_OH) was not detected by action spectroscopy under jet-cooled conditions, presumably due to insufficient energy for dissociation. In addition, a feature observed in the vicinity of trans-perp HOONO (2ν_OH) is attributed to an OH overtone transition originating from a state with predominately cis-perp character, based on comparison with predicted spectral properties and stability. Homogenous linewidth broadening observed for the overtone transitions of trans-perp and cis-perp HOONO are attributed to intramolecular vibrational redistribution (IVR) and/or dissociation. For comparison, the IVR dynamics of HONO₂ (2ν_OH) is examined. Spectra are recorded using a sequential IR-UV excitation method that leads to emission from electronically excited NO₂*. High resolution infrared spectra of HONO₂ (2ν_OH) reveal three vibrational bands, which are attributed to strongly mixed states involving a Fermi resonance. A vibrational deperturbation analysis of the experimental data yields the energies and couplings between the zero-order states, with the identity of these states obtained from complementary theoretical calculations.