15min:

BRIDGET A. O'DONNELL, EUNICE X. J. LI AND MARSHA I. LESTER, Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323.

The reaction of nitric acid (HONO₂) 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-HONO₂ complex in a six-membered ring-like configuration. In this study, OH-HONO₂ is produced in the gas phase by the association of photolytically generated OH radicals with HONO₂ in the collisional region of a supersonic expansion. Infrared action spectroscopy is used to identify the fundamental OH radical stretch ( ₁) and OH nitric acid stretch ( ₂) of the complex. The rotationally structured ₁ band is centered at 3216 cm^-1, while the extensively broadened ₂ band is centered at 3260 cm^-1, 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 A ^{2 +}- X ² (1,0) transition. The highest observed OH (v=0) product channels, J =17/2, =1/2 for the OH radical stretch and J =15/2, =1/2 for the OH nitric acid stretch, lead to determination of an upper limit for the binding energy of OH-HONO₂, D₀ \le6.2 kcal mol^-1, which is consistent with ab initio predictions.^b