Peroxynitrous acid (HOONO) is generated in a pulsed supersonic expansion through recombination of photolytically generated OH and NO(2) radicals. A rotationally resolved infrared action spectrum of HOONO is obtained in the OH overtone region at 6971.351(4) cm(-1) (origin), providing definitive spectroscopic identification of the trans-perp (tp) conformer of HOONO. Analysis of the rotational band structure yields rotational constants for the near prolate asymmetric top, the ratio of the a-type to c-type components of the transition dipole moment for the hybrid band, and a homogeneous linewidth arising from intramolecular vibrational energy redistribution and/or dissociation. The quantum state distribution of the OH (nu=0,J(OH)) products from dissociation is well characterized by a microcanonical statistical distribution constrained only by the energy available to products, 1304+/-38 cm(-1). This yields a 5667+/-38 cm(-1) [16.2(1) kcal mol(-1)] binding energy for tp-HOONO. An equivalent available energy and corresponding binding energy are obtained from the highest observed OH product state. Complementary high level ab initio calculations are carried out in conjunction with second-order vibrational perturbation theory to predict the spectroscopic observables associated with the OH overtone transition of tp-HOONO including its vibrational frequency, rotational constants, and transition dipole moment. The same approach is used to compute frequencies and intensities of multiple quantum transitions that aid in the assignment of weaker features observed in the OH overtone region, in particular, a combination band of tp-HOONO involving the HOON torsional mode.

The trans-perp (tp) conformer of peroxynitrous acid (HOONO), a secondary product of the OH + NO 2 reaction and a less stable isomer of nitric acid, has been identified by infrared action spectroscopy in the OH overtone region after being generated in a pulsed supersonic expansion. Rotational structure associated with the OH overtone band of tp-HOONO has been observed at 6971.4(1) cm-1 (origin) and assigned by simulating its band structure using a transition dipole moment and rotational constants derived from ab initio theory. The highest observed OH product state, OH 2 Π 1/2 (v=0, J=13/2), sets an upper limit for the tp-HOONO binding energy of 16.8 kcal/mol, confirming previous estimates for the O-O bond energy of HOONO.

The second OH overtone transition of the trans-perp conformer of peroxynitrous acid (tp-HOONO) is identified using infrared action spectroscopy. HOONO is produced by the recombination of photolytically generated OH and NO(2) radicals, and then cooled in a pulsed supersonic expansion. The second overtone transition is assigned to tp-HOONO based on its vibrational frequency (10 195.3 cm(-1)) and rotational band contour, which are in accord with theoretical predictions and previous observations of the first overtone transition. The transition dipole moment associated with the overtone transition is rotated considerably from the OH bond axis, as evident from its hybrid band composition, indicating substantial charge redistribution upon OH stretch excitation. The overtone band exhibits homogeneous line broadening that is attributed to intramolecular vibrational redistribution, arising from the coupling of the initially excited OH stretch to other modes that ultimately lead to dissociation. The quantum state distributions of the OH X (2)Pi (nu=0) products following first and second OH overtone excitation of tp-HOONO are found to be statistical by comparison with three commonly used statistical models. The product state distributions are principally determined by the tp-HOONO binding energy of 16.2(1) kcal mol(-1). Only a small fraction of the OH products are produced in nu=1 following the second overtone excitation, consistent with statistical predictions.

This paper presents experimental evidence, supported by two-dimensional theoretical calculations, that HOONO can be observed in cis-perp (cp) configurations in a pulsed supersonic expansion. The spectral properties (transition frequency, rotational constants, and transition type) of OH overtone transitions originating from a state with predominately cp character are predicted theoretically and compared with those associated with a weak feature at 6996.2 cm(-1) observed experimentally using infrared action spectroscopy. This spectral feature is attributed to HOONO in cp configurations based on its vibrational frequency, rotational band contour, and resultant OH product state distribution.

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 ( 1) and OH stretch of nitric acid ( 2) in the OH-HONO2 complex. Two vibrational features are attributed to OH-HONO2: a rotationally structured 1 band at 3516.8 cm ؊1 and an extensively broadened 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 1 and 2 excitation are used to determine the binding energy of OH-HONO2, D0 < 5.3 kcal⅐mol ؊1 , which is in good accord with ab initio predictions.atmospheric chemistry ͉ hydroxyl radical ͉ nitric acid ͉ reaction intermediate N itric acid is a chemically inactive and photochemically stable reservoir for reactive HO x and NO x species in the atmosphere. In the troposphere, HONO 2 is usually removed by dry deposition or rainout faster than it is photochemically converted back to NO x , making it a permanent sink for NO x (1, 2). In the upper troposphere and stratosphere, where there is no rain, HONO 2 can be removed by solar photolysis and reaction with OH (3, 4),This reaction results in the conversion of HONO 2 into reactive NO x species, because the unstable nitrate radical rapidly decomposes into NO or NO 2 .Kinetic studies have shown that the OH ϩ HONO 2 reaction is unusual in several respects (3, 5-11), particularly under the temperature and pressure conditions found in the lower atmosphere. Under these conditions, this reaction exhibits a negative temperature dependence, a pressure dependence, and a strong kinetic isotope effect. The indirect mechanism inferred from these studies consists of initial OH radical addition forming an energized OH-HONO 2 * intermediate, followed by redissociation or reaction to form products. The energized intermediate can also be temporarily stabilized by collisions with bath gas M, and then redissociate to reactants or decompose to products as shown in the following scheme:The unusual kinetic behavior arises because of enhanced formation and collisional st...