3nuOH Paper_rev.pdfAbstract 3nuOH figures.pdfSecond OH Overtone Excitation and Statistical Dissociation Dynamics of Peroxynitrous Acid Ian M. Konen, Eunice X. J. Li, Thomas A. Stephenson,* and Marsha I. Lester†Department of Chemistry, University of Pennsylvania Philadelphia, PA 19104-6323 Abstract 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 recombination of photolytically generated OH and NO2 radicals, and then cooled in a pulsed supersonic expansion. The second overtone transition is assigned to tp-HOONO based on its vibrational frequency (10195.3 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 2Π (v=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 * On sabbatical leave (2004-05) from the Department of Chemistry and Biochemistry, Swarthmore College, Swarthmore. PA 19081 † Author to whom all correspondence should be addressed; phone (215) 898-4640, fax (215) 573-2112, email [email protected] 1energy of 16.2(1) kcal mol-1. Only a small fraction of the OH products are produced in v=1 following second overtone excitation, consistent with statistical predictions. 2I. INTRODUCTION The three-body association reaction of OH with NO2 to produce stable nitric acid (HONO2) OH + NO2 + M → HONO2 + M is a key termination step of the radical chemistry that occurs the lower atmosphere.1 This process terminates many reactions involving HOx (HOx ≡ OH + HO2) and NOx (NOx ≡ NO + NO2) species that would otherwise lead to photochemical smog. Until recently, this termolecular reaction was believed to produce HONO2 exclusively, but kinetic studies over a wide range of pressure (for the bath gas M) and temperature conditions have shown that a secondary reaction product is also formed.2-5 The secondary product was proposed to be peroxynitrous acid (HOONO),2,6,7 a weakly bound isomer of HONO2. Under atmospheric conditions, HOONO is formed reversibly OH + NO2 + M HOONO + M Rand is estimated to account for up to 20% of the yield from the OH + NO2 reaction.5 Ab initio studies have predicted that there are at least two stable conformations of HOONO,5,8-10 namely, the trans-perp (tp) conformer with an extended open structure and the cis-cis (cc) conformer with a five member ring-like structure. The latter has an intramolecular hydrogen bond between the terminal hydrogen and oxygen atoms, which increases the rigidity and stability of the cc-conformer, and also has the effect of significantly shifting its OH stretching frequency to lower energy. As illustrated in Fig. 1, experiments have shown that the tp-conformer has a stability of 16.2(1) kcal mol-1 relative to the OH + NO2 asymptote,11 the cc-conformer has a stability of ∼19.8 kcal mol-1,4,12 and a substantial barrier (tp-cc) separates the two conformers.13 There has been considerable debate in the literature (as yet unresolved) as to 3whether or not there is a third stable conformer of HOONO in the cis-perp (cp) configuration.8,9,13-15 Nevertheless, there is ample evidence that torsional excitation of cc-HOONO can break the intramolecular hydrogen bond and thereby access cp-like configurations.12,16 Although HOONO was detected under matrix-isolation conditions some time ago,17,18 it eluded gas-phase spectroscopic detection until very recently. The cc-HOONO conformer has subsequently been observed under thermal flow cell conditions using infrared action spectroscopy in the first and second OH overtone regions,13,19,20 cavity ring down spectroscopy in the fundamental and first OH overtone regions,10,16 and pure rotational spectroscopy.21 The tp-HOONO conformer has been characterized by this laboratory using infrared action spectroscopy in the first OH overtone region under jet-cooled conditions,11,22 and has also been detected using this same method under thermal conditions.13 The cc-HOONO conformer was not detected in the first overtone region under analogous jet-cooled conditions,11 because a combination of its lower OH overtone transition frequency, 6365 cm-1 (cc) vs. 6971 cm-1 (tp), and greater binding energy prevents dissociation, which is required for action spectroscopy measurements. The cc-HOONO overtone spectrum obtained under thermal conditions also exhibits a reduced quantum yield for features below ∼6940 cm-1 (19.8 kcal mol-1).13 This skews the appearance of the action spectrum, suppressing lower energy features relative to hot bands originating from excited torsional levels that appear at higher frequency (∼6935 cm-1).12,16 This paper extends our previous work11,22 by examining HOONO in the second OH overtone region under jet-cooled conditions. The overtone transition associated with the tp-conformer is identified by its vibrational frequency and rotational band contour. Other aspects of the overtone spectrum provide detailed information on the transition dipole moment and the 4initial step of the intramolecular vibrational redistribution process that ultimately leads to dissociation. Finally, the quantum state distributions of the OH products resulting from first and second OH overtone excitation of tp-HOONO are thoroughly characterized, and compared with the predictions of several commonly used statistical models. II. EXPERIMENTAL METHOD The second OH overtone transition of tp-HOONO is detected in a pulsed supersonic expansion using an IR pump–UV probe technique similar to that used previously for the first OH overtone transition.11,22 For the second overtone, IR excitation occurs at 0.98 μm and leads to dissociation of tp-HOONO. The OH fragments are then detected by UV