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

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 $(2ν1).$ We observe formation of both cis-cis and trans-perp conformers of HOONO. The trans-perp HOONO $2ν1$ band is observed under thermal (223–238 K) conditions at 6971 cm⁻¹. We assign the previously published (warmer temperature) HOONO spectrum to the $2ν1$ band at 6365 cm⁻¹ and $2ν1$-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 ζ 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 ζ basis set using the structure determined at the triple ζ 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 $ΔE0=42.4 kJ/mol$ above trans-perp HOONO, corresponding to an activation enthalpy of $ΔH298‡0=41.1 kJ/mol.$ 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 $2ν1(cis-cis HOONO)$ and $2ν1(trans-perp HOONO)$ 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 $2ν1$ 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.