Chemical Dynamics Special Feature: Spectroscopic identification and stability of the intermediate in the OH + HONO2 reaction

摘要

The reaction of nitric acid with the hydroxyl radical influences the residence time of HONO2 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–HONO2 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 HONO2 and stabilized in a pulsed supersonic expansion. Infrared action spectroscopy is used to identify the intermediate by the OH radical stretch (ν1) and OH stretch of nitric acid (ν2) in the OH–HONO2 complex. Two vibrational features are attributed to OH–HONO2: a rotationally structured ν1 band at 3516.8 cm⁻¹ and an extensively broadened ν2 feature at 3260 cm⁻¹, both shifted from their respective monomers. These same transitions are identified for OD–DONO2. 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 ν1 and ν2 excitation are used to determine the binding energy of OH–HONO2, D0 ≤ 5.3 kcal·mol⁻¹, which is in good accord with ab initio predictions.