AB-INITIO STUDY OF NO2 .5. NONADIABATIC VIBRONIC STATES AND LEVELS OFTHE (X)OVER-TILDA (2)A(1) (A)OVER-TILDA B-2(2) CONICAL INTERSECTION/

We have computed 1500 nonadiabatic levels of the (X) over tilde (2)A(1 )/(A) over tilde B-2(2) conical intersection of NO2, up to 18 700 cm(- 1). By using a bond lengths-bond angle Hamiltonian, the molecular stat es 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 u p to 10 000 cm(-1), with a root mean square deviation (RMSD) of 17.8 c m(-1), and 691 nonadiabatic bands up to 15 000 cm(-1) and 1060 up to 1 7 000 cm(-1) have been converged within 1.9 and 4.4 cm(-1), respective ly, by using 6117 basis functions per symmetry, and several states hav e been assigned. Up to 9500 cm(-1) we have essentially found (2)A(1) v ibrational states, some of them mixed by the Delon-Jost resonances. Th e nonadiabatic coupling then begins near the B-2(2) (0,0,0) origin, wh ich we assign to an electronically mixed band at 9747 cm(-1), and grad ually increases via the interaction between bending states of (2)A(1) and B-2(2). The vibronic mixing is more important above 12 000 cm(-1), where both electronic species contribute to several nonadiabatic stat es, but the B-2(2) 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 electro nic states, and giving a fully chaotic spectrum which follows a Wigner -type distribution. Our results thus explain the beginning and the dev elopment of the (2)A(1)/B-2(2) nonadiabatic interaction, from the regu lar far-infrared region up to the chaotic yellow portion of the spectr um. They are in good agreement with the available experimental data, a llowing the assignment of several observed bands up to 16 000 cm(-1), and increase remarkably the number of known NO2 vibronic levels. (C) 1 996 American Institute of Physics.