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

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 a(2)'' vibrational levels is simple, with only the v(2) 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 v(2) mode) are calculated. The major features are: the v(4) fundamental has a large (positive) anharmonicity, several combination bands involving v(4) and its overtones are prominent in the spectrum, and the v(3) 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 v(3) ranks nineteenth. The results are broadly consistent with an earlier study that was based on a diabatic representation that also indicated that v(3) 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 v(3) + v(4) character, with some mixing of v(1) + v(4).