High-energy materials (HEMs) comonly feature relatively weak C–NO 2 bonds that facilitate rapid decomposition, making the nitro group a key explosophore. Multiple electronic states are suggested to play a significant role in the decomposition of HEMs. Therefore, exploring both ground and excited-state potential energy surfaces (PESs) describing the dissociation processes is essential for gaining mechanistic insight. 1- nitropene (NP) can be considered as a minimal model for studying the photochemistry of nitro compounds such as nitrobenzene (NB) and ortho-nitrotoluene (oNT). The photochemical pathways of trans- and cis-NP starting from the lowest bright state, have been investigated using the complete active space self-consistent field (CASSCF) method combined with second-order perturbative energy corrections (CASPT2). This involves the optimization of various stationary points and minimum energy crossing points on the PESs of relevant singlet and triplet electronic states. Our results sug- gest that both the trans and cis isomers have similar photo-decay channels except in the case of cis-NP, additional excited-state intramolecular hydrogen transfer (ESIHT) path competes with dissociation and reduces the yield of photoproducts. A total of four distinct energy transfer channels have been identified on the S 1 PES for both the trans and cis isomers of NP: (1) adiabatic pathways leading to NO 2 ( 2 B 2 ), (2) inter- system crossing to the lowest triplet state (3) internal conversion to the ground-state reactant in trans-NP or a hydrogen-transferred product in cis-NP, and (4) internal con- version resulting in dissociated photoproducts. Among the four processes, intersystem crossing has been found to be dominant, suggesting a high triplet quantum yield. The molecule on the ground state can undergo nitro-nitrite isomerization followed by the dissociation into NO and CH 3 –CH–CHO. Two different mechanisms are identified for the nitro-nitrite isomerization: one involves a roaming transition state characterized by the partially dissociated NO 2 moiety, and the other proceeds through a conventional transition state. The proposed dominant pathways and the corresponding photoproducts qualitatively explain the experimental observations in the photochemistry of NB and oNT.
Gudem et al. (Mon,) studied this question.