Spectroscopic identification and stability of the intermediate in the OH + HONO2 reaction.

The reaction of nitric acid with the hydroxyl radical influences the residence time of HONO(2) in the lower atmosphere. Prior studies [Brown SS, Burkholder JB, Talukdar RK, Ravishankara AR (2001) J Phys Chem A 105:1605-1614] have revealed unusual kinetic behavior for this reaction, including a negative temperature dependence, a complex pressure dependence, and an overall reaction rate strongly affected by isotopic substitution. This behavior suggested 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, the intermediate is generated directly by the association of photolytically generated OH radicals with HONO(2) and stabilized in a pulsed supersonic expansion. Infrared action spectroscopy is used to identify the intermediate by the OH radical stretch (nu(1)) and OH stretch of nitric acid (nu(2)) in the OH-HONO(2) complex. Two vibrational features are attributed to OH-HONO(2): a rotationally structured nu(1) band at 3516.8 cm(-1) and an extensively broadened nu(2) feature at 3260 cm(-1), both shifted from their respective monomers. These same transitions are identified for OD-DONO(2). Assignments of the features are based on their vibrational frequencies, analysis of rotational band structure, and comparison with complementary high level ab initio calculations. In addition, the OH (v = 0) product state distributions resulting from nu(1) and nu(2) excitation are used to determine the binding energy of OH-HONO(2), D(0) <or= 5.3 kcal x mol(-1), which is in good accord with ab initio predictions.