Theoretical predictions of the initial decomposition steps of dimethylnitramine.

The structures and energies of the reactants, products, and transition states of the initial steps in the gas-phase decomposition of dimethylnitramine (DMNA) have been determined by quantum chemical calculations at the B3LYP density-functional theory, MP2, and G2 levels. The pathways considered are NO2 elimination, HONO elimination, and nitro-nitrite rearrangement. The NO2 elimination is predicted to be the main channel of the gas-phase decomposition of DMNA in accord with experiment. The values of the Arrhenius parameters, log A=16.6+/-0.5 and Ea=40.0+/-0.6 kcal/mol, for the N-NO2 bond-fission reaction were obtained using a canonical variational theory with B3LYP energies and frequencies. The HONO-elimination channel has the next lowest activation energy of 44.7+/-0.5 kcal/mol (log A=13.6+/-0.5) and is characterized by a five-member transition-state configuration in which a hydrogen atom from one of the methyl groups is transferred to an oxygen atom of NO2. Tunneling contributions to the rate of this reaction have been estimated. The nitro-nitrite rearrangement reaction occurs via a transition state in which both oxygen atoms of NO2 are loosely bound to the central nitrogen atom, for which Rice-Ramsperger-Kassel-Marcus theory predicts log A=14.4+/-0.6 and Ea=54.1+/-0.8 kcal/mol.