Ab Initio Study of Photodissociation and Isomerization Pathways on the Singlet and Triplet Manifolds of 1-Nitropropene

Abstract

High-energy materials (HEMs) commonly feature relatively weak C–NO2 bonds that facilitate rapid decomposition, making the nitro group a key explosophore. Multiple electronic states 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 insights. The trans and cis isomers of 1-nitropropene (NP) can be considered as minimal models for studying the photochemistry of nitro compounds such as nitrobenzene (NB) and ortho-nitrotoluene (oNT), respectively. These two isomers correspond to two different regions of the NP PES and can interconvert through torsional motion around the C═C double bond. However, their photochemistry other than along the torsional mode needs to be considered separately, as they are representative analogues of NB and oNT, where this torsional motion is restricted by the aromatic ring. 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 suggest that both the trans and cis isomers have similar photodecay channels, except that in the case of cis-NP, an additional excited-state intramolecular hydrogen transfer (ESIHT) path competes with dissociation and reduces the yield of photofragments. A total of four distinct energy transfer channels have been identified from the S1 PES regions of both trans and cis isomers of NP: (1) adiabatic pathways leading to NO2 (2B2) formation, (2) intersystem crossing (ISC) to the lowest triplet state, (3) internal conversion (IC) to the ground-state reactant in trans-NP or to a hydrogen-transferred product in cis-NP, and (4) IC resulting in the formation of dissociated photoproducts. Among the four processes, ISC has been found to be dominant, suggesting a high triplet quantum yield. The molecule on the lowest energy triplet state favorably relaxes to the ground-state reactant via ISC. Alternatively, the molecule can form either the epoxide or the NO + CH3–CH–CHO on the triplet state. In the case of cis-NP, a competing hydrogen transfer process also exists as a deactivation route on the T1 surface. NP generated through IC or ISC retains excess internal energy, which drives further transformations on the ground state. This includes nitro-nitrite isomerization followed by the dissociation into NO and CH3–CH–CHO. Two different mechanisms are identified for the nitro-nitrite isomerization: one involves a roaming transition state characterized by the partially dissociated NO2 moiety, and the other proceeds through a conventional transition state. The proposed dominant pathways and the corresponding photoproducts qualitatively explain the experimental observations of the photochemistry of NB and oNT.