Abstract
Extended multireference configuration interaction with singles and doubles (MR-CISD) calculations of nitroethylene (H2C=CHNO2) were carried out to investigate the photodynamical deactivation paths to the ground state. The ground (S0) and the first five valence excited electronic states (S1–S5) were investigated. In the first step, vertical excitations and potential energy curves for CH2 and NO2 torsions and CH2 out-of-plane bending starting from the ground state geometry were computed. Afterward, five conical intersections, one between each pair of adjacent states, were located. The vertical calculations mostly confirm the previous assignment of experimental spectrum and theoretical results using lower-level calculations. The conical intersections have as main features the torsion of the CH2 moiety, different distortions of the NO2 group and CC, CN, and NO bond stretchings. In these conical intersections, the NO2 group plays an important role, also seen in excited state investigations of other nitro molecules. Based on the conical intersections found, a photochemical nonradiative deactivation process after a π–π* excitation to the bright S5 state is proposed. In particular, the possibility of NO2 release in the ground state, an important property in nitro explosives, was found to be possible.
Highlights
The explosophore NO2 group is an essential component in many propellants and explosives of practical relevance.[1]
In the detonation initiation of these materials, photochemical processes are very important.[6−12] In particular, as has been increasingly recognized in the field of photochemical reactions,[13,14] nonradiative decomposition processes of energetic molecules through conical intersections play an important and even dominant role.[15−17] investigations of nonadiabatic processes occurring in energetic molecules are especially interesting
We recently investigated the electronic excitation spectra of energetic molecules containing nitro groups, namely, nitramide (H2NNO2),[18] N,N-dimethylnitramine, ((CH3)2NNO2),19 1,1diamino-2,2-dinitroethylene (FOX-7 or DADNE),[20] and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX).[21]
Summary
The explosophore NO2 group is an essential component in many propellants and explosives of practical relevance.[1] These nitro compounds are usually thermally unstable, composed of broken N−N, O−C and C−N covalent bonds, have significant biological activity, and may contribute to atmospheric pollution.[2] decomposition processes of nitro compounds are especially important because they occur at relatively low temperatures and are involved in the detonation of explosives.[3,4]. Most of the energetic molecules have diffuse electronic spectra and propensity to dissociate and rearrange.[5] In the detonation initiation of these materials, photochemical processes are very important.[6−12] In particular, as has been increasingly recognized in the field of photochemical reactions,[13,14] nonradiative decomposition processes of energetic molecules through conical intersections play an important and even dominant role.[15−17] investigations of nonadiabatic processes occurring in energetic molecules are especially interesting. In spite of the importance of excited states in energetic materials research, only few studies have been carried out in this field.[6,17]
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