The electronic structure of NO2. II. The ? 2B2← ? 2A1 and ? 2B1←? 2A1 absorption systems

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Abstract
The results of an accurate ab initio study of the electronic structure of NO2 have been applied to an analysis of the two important visible and near infrared absorption systems of this molecule. The long wavelength absorption (λ≳6000 Å) arises from an ? 2B2←? 2A1 transition. A theoretical absorption spectrum that is generated from the C2V ab initio potential surfaces of these two states qualitatively reproduces most of the features of an experimental low resolution absorption spectrum between 9000 and 6000 Å. The (0–0) band of the transition is predicted to be several times less intense than nearby hot bands even at temperatures as low as 300 K. The computed ? 2B2 spectroscopic parameters are Te=1.18 eV, Re=1.26 Å, ϑe=102°, ω1=1461 cm−1, ω2=739 cm−1, and μ=0.46 D. There is a marked difference between experimentally determined ? 2B2 rotational constants and those deduced from the ab initio equilibrium geometry; this datum adds to the rapidly increasing evidence for strong vibronic coupling of the ? 2B2 state with high vibrational levels of the ground electronic state. The theoretical spectroscopic parameters of the experimentally better understood ? 2B1 excited state are Te=1.66 eV, Re=1.20 Å, ϑe=180°, ω1=1192 cm−1, 2 ω2=960 cm−1, ω3=2040 cm−1, and μ=0.0 D. The adiabatic excitation energy, bond angle, and bending frequency are in good agreement with experiment. However, the theoretical equilibrium bond length is significantly shorter than the value deduced from the experimental spectroscopic data and possible reasons for this discrepancy are discussed.