Autocatalytic Mechanism Induced by NO₂^- Ion with Effect of Proton-Coupled Electron Transfer in Early Pyrolysis of Azacyclo-Nitramine Compounds RDX: a New Path of Denitration Reaction

We show an autocatalytic mechanism involving a novel denitration reaction pathway derived from the pyrolysis of a typical azacyclo-nitramine compound, 1,3,5-trinitro-1,3,5-triazine RDX. In the current mechanism understanding, the denitration reaction of azacyclo-nitramine compounds can almost only occur through N-N bond homolysis and the HONO elimination reaction. Our first-principles molecular dynamics study identified an autocatalytic denitration reaction pathway induced by the NO₂^- ion with the effect of proton-coupled electron transfer (PCET). That is, C₃H_xN_xO_y transfers a proton from −CH₂- groups to the NO₂^- ion; synchronously, electrons are transferred from the N-heterocycle of C₃H_xN_xO_y to the −NO₂ group and results in the formation of HONO and a new NO₂^- ion (C₃H_xN_xO_y + NO₂^- → C₃H_x-1N_x-1O_y-2 + NO₂^- + HONO). It is also found that the autocatalytic denitration reaction caused by PCET has characteristics of both a low energy barrier and high energy release. For the NO₂ radical, it also has an autocatalytic effect to promote dehydrogenation and denitration of C₃H_xN_xO_y, but dehydrogenation and denitration induced by a NO₂ radical are two asynchronous processes. Compared with the NO₂^- ion with NO₂ radicals, the denitration reaction induced by the NO₂^- ion with the effect of PCET is more favorable kinetically. This work theoretically confirmed the hypothesis that azacyclo-nitramine compounds have autocatalytic effects. In general, the mechanism of the autocatalytic reaction induced by the NO₂^- ion with the effect of PCET at the early stage of RDX pyrolysis is revealed for the first time. It is of great significance to the application of PCET in the NO₂^- ionic fluid catalyzed molecular denitration reaction and promotes an understanding of the chemical kinetics of the C-H-N-O system under extreme conditions.