Fry, Juliane L. and Nizkorodov, Sergey A. and Okumura, Mitchio and Roehl, Coleen M. and Francisco, Joseph S. and Wennberg, Paul O. (2004) Cis-cis and trans-perp HOONO: Action spectroscopy and isomerization kinetics. Journal of Chemical Physics, 121 (3). pp. 1432-1448. ISSN 0021-9606 http://resolver.caltech.edu/CaltechAUTHORS:FRYjcp04
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The weakly bound HOONO product of the OH + NO2 + M reaction is studied using the vibrational predissociation that follows excitation of the first OH overtone (2nu1). We observe formation of both cis-cis and trans-perp conformers of HOONO. The trans-perp HOONO 2nu1 band is observed under thermal (223–238 K) conditions at 6971 cm–1. We assign the previously published (warmer temperature) HOONO spectrum to the 2nu1 band at 6365 cm–1 and 2nu1-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–1 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 Ar). The temperature dependence of the trans-perp HOONO lifetime in the range 223–238 K yields an isomerization barrier of 33±12 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 [CCSD(T)] model, using a correlation consistent polarized triple zeta basis set (cc-pVTZ). The energetics of cis-cis, trans-perp, and perp-perp HOONO are also calculated at this level [CCSD(T)/cc-pVTZ] and with a quadruple zeta basis set using the structure determined at the triple zeta basis set [CCSD(T)/cc-pVQZ//CCSD(T)/cc-pVTZ]. These calculations predict that the anti form of perp-perp HOONO has an energy of DeltaE0 = 42.4 kJ/mol above trans-perp HOONO, corresponding to an activation enthalpy of DeltaH<sub>298</sub><sup>[double-dagger]0</sup> = 41.1 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 NO2 is also calculated as a function of photon excitation energy in the range 3500–7500 cm–1, assuming D0 = 83 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 2nu1(cis-cis HOONO) and 2nu1(trans-perp HOONO) are performed using (1) a one-dimensional anharmonic model and (2) a Morse oscillator model for the OH stretch, and ab initio dipole moment functions calculated using Becke, Lee, Yang, and Parr density functional theory (B3LYP), Møller-Plesset pertubation theory truncated at the second and third order (MP2 and MP3), and quadratic configuration interaction theory using single and double excitations (QCISD). The QCISD level calculated ratio of 2nu1 oscillator strengths of trans-perp to cis-cis HOONO is 3.7:1. The observed intensities indicate that the concentration of trans-perp HOONO early in the OH + NO2 reaction is significantly greater than predicted by a Boltzmann distribution, consistent with statistical predictions of high initial yields of trans-perp HOONO from the OH + NO2 + M reaction. In the atmosphere, trans-perp HOONO will isomerize nearly instantaneously to cis-cis HOONO. Loss of HOONO via photodissociation in the near-IR limits the lifetime of cis-cis HOONO during daylight to less than 45 h, other loss mechanisms will reduce the lifetime further.
|Additional Information:||©2004 American Institute of Physics. (Received 26 January 2004; accepted 21 April 2004) This material is based upon work supported under a National Science Foundation Graduate Research Fellowship (J.L.F.) and supported by NSF's Atmospheric Chemistry program (Contract No. ATM-0094670) and the NASA Upper Atmospheric Research Program (Contract No. NAG5-11657). S.A.N. thanks the Camille and Henry Dreyfus Postdoctoral Program in Environmental Chemistry for support. The authors thank J. R. Barker for providing MULTIWELL input files and H. G. Kjaergaard, I. M. Konen, I. B. Pollack, E. X. J. Li, and M. I. Lester for insightful discussions.|
|Subject Keywords:||isomerisation; ab initio calculations; vibrational states; molecular configurations; density functional theory; photodissociation; configuration interactions; oscillator strengths; spectral line intensity; molecular moments; coupled cluster calculations; predissociation; molecule-molecule reactions; photoexcitation; spectral line breadth; atmospheric chemistry; oxygen compounds; reaction kinetics|
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|Deposited On:||07 Apr 2006|
|Last Modified:||26 Dec 2012 08:49|
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