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 (S<sub>0</sub> and S<sub>1</sub>) and lowest two triplet (T<sub>1</sub> and T<sub>2</sub>) electronic states are investigated, with particular emphasis on the S<sub>1</sub> relaxation pathway and the nonadiabatic region leading to radiationless decay of S<sub>1</sub> population. In nitrobenzene, relaxation on S<sub>1</sub> 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<sub>1</sub>, S<sub>0</sub>, and T<sub>2</sub>. Moreover, spin–orbit coupling constants for the S<sub>0</sub>/T<sub>2</sub> and S<sub>1</sub>/T<sub>2</sub> electronic state pairs were calculated to be as high as 60 cm<sup>–1</sup> in this region. Our results suggest that the S<sub>1</sub> population should quench primarily to the T<sub>2</sub> 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<sub>1</sub> is through geometric distortions, similar to that found for nitrobenzene, of a single ortho-substituted NO<sub>2</sub>. The two singlet and lowest two triplet electronic states are qualitatively similar to those of nitrobenzene along a minimal S<sub>1</sub> energy pathway.