Cis-cis and trans-perp HOONO: Action spectroscopy and isomerization kinetics

Abstract

The weakly bound HOONO product of the OHϩNO 2 ϩM reaction is studied using the vibrational predissociation that follows excitation of the first OH overtone (2 1 ). We observe formation of both cis-cis and trans-perp conformers of HOONO. The trans-perp HOONO 2 1 band is observed under thermal ͑223-238 K͒ conditions at 6971 cm Ϫ1 . We assign the previously published ͑warmer temperature͒ HOONO spectrum to the 2 1 band at 6365 cm Ϫ1 and 2 1 -containing combination bands of the cis-cis conformer of HOONO. The band shape of the trans-perp HOONO spectrum is in excellent agreement with the predicted rotational contour based on previous experimental and theoretical results, but the apparent origin of the cis-cis HOONO spectrum at 6365 cm Ϫ1 is featureless and significantly broader, suggesting more rapid intramolecular vibrational redistribution or predissociation in the latter isomer. The thermally less stable trans-perp HOONO isomerizes rapidly to cis-cis HOONO with an experimentally determined lifetime of 39 ms at 233 K at 13 hPa ͑in a buffer gas of predominantly Ar͒. The temperature dependence of the trans-perp HOONO lifetime in the range 223-238 K yields an isomerization barrier of 33Ϯ12 kJ/mol. New ab initio calculations of the structure and vibrational mode frequencies of the transition state perp-perp HOONO are performed using the coupled cluster singles and doubles with perturbative triples ͓CCSD͑T͔͒ model, using a correlation consistent polarized triple basis set ͑cc-pVTZ͒. The energetics of cis-cis, trans-perp, and perp-perp HOONO are also calculated at this level ͓CCSD͑T͒/ cc-pVTZ͔ and with a quadruple basis set using the structure determined at the triple basis set ͓CCSD͑T͒/cc-pVQZ//CCSD͑T͒/cc-pVTZ͔. These calculations predict that the anti form of perp-perp HOONO has an energy of ⌬E 0 ϭ42.4 kJ/mol above trans-perp HOONO, corresponding to an activation enthalpy of ⌬H 298 ‡0 ϭ41.1 kJ/mol. These results are in good agreement with statistical simulations based on a model developed by Golden, Barker, and Lohr. The simulated isomerization rates match the observed decay rates when modeled with a trans-perp to cis-cis HOONO isomerization barrier of 40.8 kJ/mol and a strong collision model. The quantum yield of cis-cis HOONO dissociation to OH and NO 2 is also calculated as a function of photon excitation energy in the range 3500-7500 cm Ϫ1 , assuming D 0 ϭ83 kJ/mol. The quantum yield is predicted to vary from 0.15 to 1 over the observed spectrum at 298 K, leading to band intensities in the action spectrum that are highly temperature dependent; however, the observed relative band strengths in the cis-cis HOONO spectrum do not change substantially with temperature over the range 193-273 K. Semiempirical calculations of the oscillator strengths for 2 1 (cis-cis HOONO) and 2 1 (trans-perp HOONO) are performed using ͑1͒ a one-dimensional anharmonic model and ͑2͒ a Morse oscillator model for the OH stretch, and ab initio dipole moment functions calculated using Becke, Lee, Yang, and Parr density functional theory ͑B3LYP͒,...

Role of OH-stretch/torsion coupling and quantum yield effects in the first OH overtone spectrum of cis-cis HOONO

“…1, 6,16 and isomerizes quickly to the more stable cis-cis conformer ͑millisecond time scale at room temperature and ϳ10 Torr͒. 6 Fry et al therefore attribute all of the bands reported by Nizkorodov and Wennberg to the lowest energy conformer, cis-cis HOONO.…”

“…1, 6,16 and isomerizes quickly to the more stable cis-cis conformer ͑millisecond time scale at room temperature and ϳ10 Torr͒. 6 Fry et al therefore attribute all of the bands reported by Nizkorodov and Wennberg to the lowest energy conformer, cis-cis HOONO. Matthews et al 17 conclude that the strong 6970 cm −1 band is due to trans-perp HOONO; however, this is at odds with the experimental observations of Fry et al…”

“…4,[8][9][10][11][12] The calculations of Golden et al at the QCISD͑T͒/cc-pVDZ level 5 ͑QCISD-quadratic configuration interaction with single and double excitation͒ and Bean et al at the CCSD͑T͒/cc-pVTZ level 2 demonstrate that higher levels of theory are required for a good description of HOONO. Attempts to locate a cis-perp HOONO minimum at the CCSD͑T͒ level have been unsuccessful, 5,6,13 calling into question earlier predictions of a cis-perp minimum. In the first spectroscopic detection of HOONO, Lee and co-workers reported matrix-isolation spectra of two of the HOONO isomers, cis-cis and trans-perp HOONO, but did not observe the cis-perp isomer.…”

“…The trans-perp conformer, with the heavy atoms in a plane in a trans configuration and the OH bond perpendicular to that plane, has been predicted to lie 15.3 kJ/mol ͑1280 cm −1 ͒ higher in energy at the CCSD͑T͒/cc-pVTZ level of theory. 5,6 A third postulated minimum corresponds to the cis-perp geometry in which the heavy atoms are in a plane in a cis configuration and the OH bond is perpendicular to that plane. This conformer was first investigated by McGrath and Rowland at the MP2/6-311+G͑3df ,2p͒ ͑MP2-Møller-Plesset perturbation theory truncated at the second order͒ level of theory, 7 and has since been studied by several groups at various levels of theory.…”

Inferring ozone production in an urban atmosphere using measurements of peroxynitric acid

“…The model does not include the HOONO branch of the OH+NO 2 +M reaction due to the short lifetime (seconds to minutes) of HOONO under MILAGRO conditions. The formation and breakdown of HOONO is considered to be a null cycle under these conditions (Fry et al, 2004). A reduction in the rate constant for Reaction (R8) leads to an increase in OH and NO 2 mixing ratios, leading to an increase in HO 2 NO 2 levels.…”