Quantum chemistry studies of electronically excited nitrobenzene, TNA, and TNT

摘要

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 S_1 relaxation pathway and the nonadiabatic region leading to radiationless decay of S_1 population. In nitrobenzene, relaxation on S_1 occurs by out-of-plane rotation and pyramidalization of the nitro group. Radiationless decay can take place through a nonadiabatic region, which, at the TD-DFT level, is characterized by near-degeneracy of three electronic states, namely, S _1, S_0, 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 S_1 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 S _1 is through geometric distortions, similar to that found for nitrobenzene, of a single ortho-substituted NO_2. The two singlet and lowest two triplet electronic states are qualitatively similar to those of nitrobenzene along a minimal S_1 energy pathway.