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

Abstract

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 x N x O y transfers a proton from −CH2– groups to the NO2 – ion; synchronously, electrons are transferred from the N-heterocycle of C3H x N x O y to the −NO2 group and results in the formation of HONO and a new NO2 – ion (C3H x N x O 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 x N x O 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.