Quantum chemical calculation of the primary thermolysis reactions of 1,1-diamino-2,2-dinitroethylene (FOX-7)

Abstract

The primary thermolysis reactions of 1,1-diamino-2,2-dinitroethylene FOX-7 (1), one of the promising energetic materials, were studied. A number of competing reactions of the primary thermolysis step was calculated: nitro group elimination, rearrangement of the nitro group to the nitrite group, transfer of oxygen and hydrogen atoms to the neighboring carbon atom, and transfer of hydrogen atom to oxygen of the nitro group. The reactions were modeled by the DFT PBE0 method with the cc-pVDZ basis set and by the CCSD(T)/aug-cc-pVDZ method. The activation enthalpies (ΔHa) of several reaction channels range from 200 to 260 kJ mol−1, and they should be taken into account when estimating the thermal stability of the compounds analogous to compound 1. For compound 1, the highest contribution to the overall thermolysis rate is made by the transfer of the hydrogen atom from the amino group to the carbon atom at the nitro groups. This reaction can proceed via four different channels with ΔHa = 200, 206, 235, and 242 kJ mol−1. For the oxygen atom transfer to carbon ΔHa = 244 kJ mol−1, and for the nitro-nitrite isomerization reaction ΔHa = 263 kJ mol−1. The hydrogen atom transfer to oxygen of the nitro group with the formation of nitronic acid (intermediate product) is characterized by the lowest activation energy (130 kJ mol−1), but further transformations via this route occur with higher (more than 300 kJ mol−1) ΔHa values. The checking of the DFT results at the CCSD(T)/aug-cc-pVDZ level showed that the first method only slightly overestimated the energies of various stated over the initial compound 1 and correctly described the ratio of rates of various thermolysis routes.