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 (2nu(1)). We observe formation of both cis-cis and traps-perp conformers of HOONO. The traps-perp HOONO 2 nu(1) band is observed under thermal (223-238 K) conditions at 6971 cm(-1). We assign the previously published (warmer temperature) HOONO spectrum to the 2nu(1) band at 6365 cm(-1) and 2nu(1)-containing combination bands of the cis-cis conformer of HOONO. The band shape of the traps-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 traps-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 traps-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, traps-perp, and perp-perp HOONO are also calculated at this level [CCSD(T)/ cc-pVTZ] and with a quadruple C basis set using the structure determined at the triple C 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 DeltaE(o)=42.4 kJ/mol above traps-perp HOONO, corresponding to an activation enthalpy of DeltaH(298)(double dagger0)=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 traps-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 D-o= 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 2nu(1)(cis-cis HOONO) and 2nu(1)(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), Moller-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 2nu(1) oscillator strengths of traps-perp to cis-cis HOONO is 3.7:1. The observed intensities indicate that the concentration of traps-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 traps-perp HOONO from the OH+NO2+M reaction. In the atmosphere, traps-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. (C) 2004 American Institute of Physics.