Spectroscopic Identification and Stability of the Intermediate in the OH + HONO₂ Reaction

• Published in: Proceedings of the National Academy of Sciences - PNAS 2008-09, Vol.105 (35), p.12678-12683
• Main Authors: O'Donnell, Bridget A., Li, Eunice X. J., Lester, Marsha I., Francisco, Joseph S.
• Format: Article
• Language: English
• Subjects: Binding energy; Chemical Dynamics Special Feature; Hydrogen; Hydrogen bonds; Hydroxyl radicals; Infrared radiation; Infrared spectroscopy; Infrared spectrum; Kinetics; Lasers; Monomers; Physical Sciences
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0800320105

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.