On the vibronic level structure in the NO₃ radical: II. Adiabatic calculation of the infrared spectrum

Abstract

The infrared spectrum of the nitrate radical (NO3) is studied on the basis of ab initio calculations that use the equation-of-motion coupled-cluster method known as EOMIP-CCSD. While the spectrum associated with vibrational levels is simple, with only the ν2 fundamental having appreciable intensity, that for the e′ vibrational states is extremely rich. The first 20 levels of e′ symmetry (not including those that feature two quanta of excitation in the out-of-plane ν2 mode) are calculated. The major features are: the ν4 fundamental has a large (positive) anharmonicity, several combination bands involving ν4 and its overtones are prominent in the spectrum, and the ν3 fundamental is calculated to lie at 1067 cm−1 and have a small intensity. In fact, of the 20 levels studied, the predicted dipole strength associated with the transition to ν3 ranks nineteenth. The results are broadly consistent with an earlier study that was based on a diabatic representation that also indicated that ν3 is well below the 1492 cm−1 band to which it has been assigned. The present study suggests that the 1492 cm−1 experimental feature is due to a strongly absorbing level that has mostly ν3 + ν4 character, with some mixing of ν1 + ν4.