Quantum Chemistry Studies of Electronically Excited Nitrobenzene, TNA, and TNT

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 (S0 and S1) and lowest two triplet (T1 and T2) electronic states are investigated, with particular emphasis on the S1 relaxation pathway and the nonadiabatic region leading to radiationless decay of S1 population. In nitrobenzene, relaxation on S1 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, S1, S0, and T2. Moreover, spin–orbit coupling constants for the S0/T2 and S1/T2 electronic state pairs were calculated to be as high as 60 cm–1 in this region. Our results suggest that the S1 population should quench primarily to the T2 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 S1 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 S1 energy pathway.