Kinetics Study of Reaction CH_3N·NH_2+OH by Multireference Second-Order Perturbation Theory and Multireference Coupled-Cluster Theory

摘要

The H-abstraction reactions of monomethylhydrazine (MMH) generate four radical intermediates: CH_3N·NH_2, cis-CH_3NHN·H, trans-CH_3NHN·H, and C·H_2NHNH_2, which undergo further decomposition to smaller radicals and stable products. The β-scission of H atom from the CH_3N·NH_2 radical is not as feasible as the β-scission of NH_2 and CH_3 radicals from the other three radicals due to its higher energy barrier (Sun et al., J. Phys. Chem. A, 2012, 116(33), 8419-8430). Consequently, the CH_3N·NH_2 radical undergoes further oxidation reactions. Due to the strong quasi-degeneracy effect, the kinetics of CH_3N·NH_2 + OH was studied in detail with application of several ab initio quantum chemistry theories. Twenty-five geometries of the stationary point of potential energy surface were investigated by using multi-reference second-order perturbation theory. The single point energies were refined by quadratic configuration interaction theory, coupled-cluster theory, locally renormalized coupled-cluster theory, and state selective multi-reference coupled-cluster theory including Brillouin-Wigner and Mukherjee's approaches, with single and double excitations and correction for triple excitations. For the OH addition to the central and terminal nitrogen of the CH_3N·NH_2 radical, the state-averaged active space (4e,3o) consists of two degenerated p orbitals of the OH and the p orbitals of N atoms in the transition states and H-bonded complexes. For the dissociation of the CH_3N(OH)NH_2 and CH_3NNH_2(OH) adducts, a larger active space was applied. Specifically, for the OH intramolecular abstraction of methyl H, the active space consists of 10 electrons distributed in 8 orbitals: σ and π bonding and antibonding pairs of N-N bond, σ bonding and antibonding pair of O-H bond, the π bonding of N-O bond, and the s orbital of central nitrogen. The predominant channel was found to be the OH addition to terminal N atom of CH_3N·NH_2 with submerged energy barriers via two optical isomeric H-bonded complexes with energy of -5.73 kcal/mol; the forming CH_3NNH_2OH adduct decomposes via small energy barrier to trans- and cis-methyldiazene and water with exothermicity up to 75 kcal/mol. The large exothermicity and negative energy barrier of the reaction implicate its significant importance in MMH earlier ignition.