Nitrocellulose-based propellants: elucidation of the mechanisms of the diphenylamine stabilizer employing density functional theory

The decomposition of energetic materials, especially propellants, produces nitrous gas and free acid nitrates which accelerates its rate velocity. To inhibit this inconvenient autocatalytic reaction, molecules known as stabilizers are usually included in their composition. One of them, diphenylamine (DPA), is primarily used in single-base compositions. In this work, we used density functional theory, with the RI-B2PLYP-D3BJ exchange-correlation functional, to investigate the N-nitrosation mechanism corresponding to a reaction between DPA and a nitro compound producing N-nitroso-diphenylamine (NNDPA). For this purpose, geometry optimizations (equilibrium and transition states), minimum energy paths, Gibbs free activation energies, and rate constants of four reaction mechanisms were computed both in the gas phase and simulating the nitrocellulose environment, which produced similar results. We found that the N-nitrosation mechanism with HNO2 is the most favored, with a rate constant of 5.3 x 10(-4) g center dot mu mol(-1)center dot s(-1). In contrast, the formation of NNDPA is less likely to depend on NO+ in agreement with the experimental results. Cationic reaction mechanisms are hardly viable, whereas interactions with NO or N2O3 are possible. Nonetheless, the former is too slow (1.02 x 10(-35) g center dot mu mol(-1)center dot s(-1)) and the latter depends on the scarcely available nitro compound as a reactant, despite being considerably fast (1.61 x 10(4) g center dot mu mol(-1)center dot s(-1)).