Abstract

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, D ₀ ≤ 5.3 kcal·mol⁻¹, which is in good accord with ab initio predictions.

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