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

Abstract

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.

Theoretical study of the absorption and nonradiative deactivation of 1-nitronaphthalene in the low-lying singlet and triplet excited states including methanol and ethanol solvent effects

“…A recent theoretical study, 58 also for the nitrobenzene and other nitroaromatic compounds, arrived to the same description of the deactivation mechanism, but some differences were obtained in comparison with our results for the 1-nitronahtalene. In that theoretical study, the S 1 electronic state is a dark n→π * excited state, which they assume is populated by some mechanism from the bright S 4 electronic state.…”

“…55 The spin-orbit coupling between some singlet and triplet electronic states in nitroaromatic compounds is expected to be large. 13,56,57 An example is a recent theoretical study of three different nitroarenes, 58 where the spin-orbit couplings between the S 1 and T 2 electronic states for nitrobenzene, 2,4,6trinitrotoluene and 2,4,6-trimitroaniline are equal to 53.0, 60.1, and 50.4 cm −1 , respectively. We attribute the large spinorbit coupling in the 1-nitronaphthalene to the perpendicularity between the nodal planes of the HOMO-4 and HOMO-2 nonbonding orbitals in the NO 2 group, since by the El-Sayed rule, the spin flip has to be accompanied by a change in the spatial symmetry.…”

Quenching of Charge Transfer in Nitrobenzene Induced by Vibrational Motion

“…[8][9][10] In order to unravel the underlying mechanisms behind this rich photochemistry, it is of fundamental importance to properly characterize its molecular electronic excited states. Therefore, the electronic structure of nitrobenzene was widely investigated in the past by both experimental [11][12][13][14][15][16][17][18] and theoretical [18][19][20][21] techniques. Furthermore, nitrobenzene can be considered the smallest prototype of nitrated polycyclic aromatic hydrocarbons, a class of compounds that constitute an important type of widespread environmental pollutants with phototoxic effects, e.g., stimulating the formation of skin cancer.…”

Quantum mechanical and experimental analyses of TNT metabolite 2-hydroxylamino-4,6-dinitrotoluene

“…Regarding the employed research tools, the DFT methods have been widely used in theoretical simulations of TNT and its derivatives [10,16,[21][22][23][24][25][26][27][28][29][30][31], whereas the application of MP2 methods are relatively limited due to its high computational cost [24,29,31]. However, the most popular DFT method B3LYP [32][33][34][35] was found being inaccurate in simulating dispersion-dominated non-covalent interactions [36].…”