CLASSICAL DYNAMICS SIMULATIONS OF UNIMOLECULAR DECOMPOSITION OF CH2NNO2 - HONO ELIMINATION VS N-N BOND SCISSION

Classical dynamics simulations of the-unimolecular decomposition of CH 2NNO2 have been performed. A potential energy function was developed b ased on MCSCF and MRCI calculations of Mowrey, Page, Adams, and Lengsf ield (J. Chem. Phys. 1990, 93, 1857). Rates and mechanisms for the pri mary decomposition channels of CH2NNO2 are presented. The two primary decomposition pathways are (I) N-N bond scission to form H2CN and NO2 and (II) concerted dissociation via a five-center transition state to eliminate HONO + HCN. The classical barrier heights differ by 2 kcal/m ol. Reactions I and II are first-order decay processes and are well-be haved with increasing energy. At low energies I is the major decomposi tion pathway, but at high energies II becomes equally probable. Produc t energy distributions for I are unremarkable, with the relative trans lational and rotational distributions peaked near zero; however, distr ibutions for II show interesting behavior. The trajectories resulting in II that do not experience secondary HONO decomposition have a trans lational energy distribution that is shifted significantly away from z ero, as expected for reactions with large back reaction barriers. The trajectories resulting in II that undergo secondary HONO decomposition , however, have a translational energy distribution that is similar to the distributions in I, indicating very little translational energy e xcitation upon formation. Rotational energy distributions for II are p eaked near zero, regardless of whether: HONO decomposes. Most of the a vailable product energy for II goes into vibration. Our results, calcu lated under microcanonical conditions in which energy is partitioned i n a statistical manner among the internal modes, are not consistent wi th the molecular beam measurements of RDX, in which CH2NNO2 is a prima ry decomposition product that subsequently decomposes only through con certed molecular eliminations.