Spectroscopic identification and stability of the intermediate in the OH + HONO 2 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
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0800320105

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 (ν 1 ) and OH stretch of nitric acid (ν 2 ) in the OH–HONO 2 complex. Two vibrational features are attributed to OH–HONO 2 : a rotationally structured ν 1 band at 3516.8 cm −1 and an extensively broadened ν 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 ν 1 and ν 2 excitation are used to determine the binding energy of OH–HONO 2 , D 0 ≤ 5.3 kcal·mol −1 , which is in good accord with ab initio predictions.