Ab initio study of NO2. V. Nonadiabatic vibronic states and levels of the X 2A1/Ã 2B2 conical intersection

We have computed 1500 nonadiabatic levels of the X̃ 2A1/Ã 2B2 conical intersection of NO2, up to 18 700 cm−1. By using a bond lengths–bond angle Hamiltonian, the molecular states have been expanded in a diabatic electronic basis and in primitive, optimized, and Born–Oppenheimer vibrational basis functions. We have optimized the diabatic potentials with respect to 191 observed bands up to 10 000 cm−1, with a root mean square deviation (RMSD) of 17.8 cm−1, and 691 nonadiabatic bands up to 15 000 cm−1 and 1060 up to 17 000 cm−1 have been converged within 1.9 and 4.4 cm−1, respectively, by using 6117 basis functions per symmetry, and several states have been assigned. Up to 9500 cm−1 we have essentially found 2A1 vibrational states, some of them mixed by the Delon–Jost resonances. The nonadiabatic coupling then begins near the 2B2 (0,0,0) origin, which we assign to an electronically mixed band at 9747 cm−1, and gradually increases via the interaction between bending states of 2A1 and 2B2. The vibronic mixing is more important above 12 000 cm−1, where both electronic species contribute to several nonadiabatic states, but the 2B2 bending progressions can be followed up to about 16 000 cm−1, since they give rise to clumps of strongly mixed vibronic bands. Above 16 000 cm−1 finally, the nonadiabatic interactions are very strong, masking all the vibrational progressions of both electronic states, and giving a fully chaotic spectrum which follows a Wigner-type distribution. Our results thus explain the beginning and the development of the 2A1/2B2 nonadiabatic interaction, from the regular far-infrared region up to the chaotic yellow portion of the spectrum. They are in good agreement with the available experimental data, allowing the assignment of several observed bands up to 16 000 cm−1, and increase remarkably the number of known NO2 vibronic levels.