Abstract and subjects
The electronic excitation energies and excited-state potential energy surfaces of nitrobenzene, 2,4,6-trinitroaniline (TNA), and 2,4,6-trinitrotoluene (TNT) are calculated using time-dependent density functional theory and multiconfigurational ab initio methods. We describe the geometrical and energetic character of excited-state minima, reaction coordinates, and nonadiabatic regions in these systems. In addition, the potential energy surfaces for the lowest two singlet (S-0 and S-1) and lowest two triplet (T-1 and T-2) electronic states are investigated, with particular emphasis on the SI relaxation pathway and the nonadiabatic region leading to radiationless decay of SI population. In nitrobenzene, relaxation on SI occurs by out-of-plane rotation and pyramidalization of the nitro group. Radiation less decay can take place through a nonadiabatic region, which, at the TD-DFT level, is characterized by near-degeneracy of three electronic states, namely, SI, So, and T-2. Moreover, spin orbit coupling constants for the S-0/T-2 and S-1/T-2 electronic state pairs were calculated to be as high as 60 cm(-1) in this region. Our results suggest that the Si population should quench primarily to the T-2 state. This finding is in support of recent experimental results and sheds light on the photochemistry of heavier nitroarenes. In TNT and TNA, the dominant pathway for relaxation on SI is through geometric distortions, similar to that found for nitrobenzene, of a single ortho-substituted NO2. The two singlet and lowest two triplet electronic states are qualitatively similar to those of nitrobenzene along a minimal SI energy pathway.