GEOMETRIES AND ENERGIES OF NITROBENZENE STUDIED BY CAS-SCF CALCULATIONS

Optimized geometries and energies in the ground (S-0) and excited stat es of nitrobenzene were calculated using the CAS-SCF method. The optim ized geometries in the S-1, S-2, T-1, T-2, and T-3 states are very dif ferent from that in the S-0 state. Most significantly, the nitro group is largely bent out of the phenyl plane in the excited states. The po tential curves along the nitro rotation coordinate around the C-N bond and the out-of-plane bending mode of the nitro group in the S-0, S-1, T-1, and T-2 states were calculated in relation to the excited states dynamics. It is found that the potential curves along the bending mod e in the excited states are very flat compared with that in the S-0 st ate. The mechanisms for the fast relaxation from the S-1 and T-1 state s are discussed based on the ab initio results.