INFRARED ACTION SPECTROSCOPY AND STABILITY OF THE OH-HONO$_{2}$ REACTION INTERMEDIATE

Author Institution: Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323The reaction of nitric acid (HONO$_{2}$) with the hydroxyl radical (OH) is of significant atmospheric interest. Kinetic studies have revealed unusual behavior including a negative temperature dependence, pressure dependence, and an overall reaction rate strongly affected by isotopic substitution.} This suggests that the reaction occurs through an intermediate, theoretically predicted to be a hydrogen-bonded OH-HONO$_{2}$ complex in a six-membered ring-like configuration.} In this study, OH-HONO$_{2}$ is produced in the gas phase by the association of photolytically generated OH radicals with HONO$_{2}$ in the collisional region of a supersonic expansion. Infrared action spectroscopy is used to identify the fundamental OH radical stretch (\nub{1}) and OH nitric acid stretch (\nub{2}) of the complex. The rotationally structured \nub{1} band} is centered at 3216 \wn, while the extensively broadened \nub{2} band is centered at 3260 \wn, both shifted from their respective monomer. Following infrared excitation, the OH fragments resulting from vibrational predissociation are detected by laser-induced fluorescence on the OH {\it A\,}$^2\Sigma^+$-{\it X\,}$^2\Pi$ (1,0) transition. The highest observed OH (v=0) product channels, {\it J\,}=17/2, $\Omega$=1/2 for the OH radical stretch and {\it J\,}=15/2, $\Omega$=1/2 for the OH nitric acid stretch, lead to determination of an upper limit for the binding energy of OH-HONO$_{2}$, {\it D$_{0}$}$\le$6.2 kcal mol$^{-1}$, which is consistent with {\it ab initio} predictions.$^b