Intersystem crossings in model energetic materials

We consider the role of the lowest singlet-tripler intersystem crossing in molecular nitromethane, nitramine, and nitric acid using ab initio complete active space self-consistent field (CAS SCF) wave functions. These systems represent the simplest models of C-(NO2), N-(NO2), and O-(NO2) bonds in energetic materials. The lowest triplet state of these molecules exhibits a minimum equilibrium structure where the nitro group is no longer coplanar with the X (C, N, O) atom, in contrast to the equilibrium geometry of the ground-state singlets. CAS SCF and density functional theory (DFT) fully optimized triplet potential energy curves confirm that the triplets are adiabatically bound with respect to X-(NO2) bond dissociation pathway, with energy barriers at the CAS SCF level of 33, 25, and 15 kcal/mol, respectively. DFT optimizations produced barriers 9-15 kcal/mol lower than the CAS SCF. Singlet-triplet minimum energy crossing points have been located at 13, 8, and 4 kcal/mol above the respective triplet minima. The reported calculations should predict fast nonradiative transitions due to the crossings with the ground surfaces. This prediction is discussed in connection with the energetic properties of these systems.