Energy Disposal in Unimolecular Reactions

Department of Chemistry, North Dakota State University, Fargo, North Dakota 58105, U.S.A. 

A reaction-path based method is described to obtain information from ab initio quantum chemistry calculations about the dynamics of energy disposal in exothermic unimolecular reactions important in the initiation of detonation in energetic materials. Such detailed information at the microscopic level may be used directly or as input for molecular dynamics simulations to gain insight relevant for the macroscopic processes. The semiclassical method, whieh uses potential energy surface information in the broad vicinity of the steepest descent reaction path, treats a reaction coordinate classically and the vibrational motions perpendicular to the reaction path quantum mechanically. Solution of the time-dependent Schroedinger equation leads to detailed predictions about the energy disposal in exothermic chemical reactions. The method is described and applied to the unimolecular decomposition of methylene nitramine. 

This reaction proceeds via a five-center cyclic transition state and releases substantial energy to the product fragments. 

Thus a first step —a major step—in understanding initiation at a molecular level is to determine what unimolecular step(s) lead to substantial energy release. Furthermore, the detailed dynamics of that energy release determines whether the energy is available to drive subsequent chemistry or instead is dissipated from the site to the surrounding lattice. While the focus of the calculations presented here is isolated molecules, these calculations are designed to be relevant to dynamics of decomposition of molecules embedded in a crystal lattice. 

---