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 en ergetic materials. The lowest triplet state of these molecules exhibits a m inimum equilibrium structure where the nitro group is no longer coplanar wi th the X (C, N, O) atom, in contrast to the equilibrium geometry of the gro und-state singlets. CAS SCF and density functional theory (DFT) fully optim ized triplet potential energy curves confirm that the triplets are adiabati cally 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. Sing let-triplet minimum energy crossing points have been located at 13, 8, and 4 kcal/mol above the respective triplet minima. The reported calculations s hould predict fast nonradiative transitions due to the crossings with the g round surfaces. This prediction is discussed in connection with the energet ic properties of these systems.