Liquid state thermochemical decomposition of neat 1,3,5,5-tetranitrohexahydropyrimidine (DNNC) and its DNNC-d2, DNNC-d4, DNNC-d6 structural isotopomers: Mechanistic entrance into the DNNC molecule

Global kinetics for the liquid state thermochemical decomposition of neat 1,3,5,5-tetranitrohexahydropyrimidine (DNNC), perdeuterio-labeled DNNC-d6, and partially deuterium-labeled DNNC-d2 and DNNC-d4 isotopomers were obtained by isothermal differential scanning calorimetry (IDSC). Molecular kinetic deuterium isotope effect (KDIE) values obtained with DNNC and DNNC-d6 from 174 to 194 deg. C revealed that C-H bond rupture regulates both an endothermic catalytic initiation and the exothermic propagation of the liquid thermochemical decomposition process. Using IDSC-based KDIE comparisons with the DNNC-d2, DNNC-d4, and DNNC-d6 isotopomers, a more detailed chemical structure/mechanistic relationship emerged by entering the interior of the DNNC molecule. Here structural kinetic KDIE results showed the rate-controlling C-H bond rupture has its origin at the non-equivalent C-2 methylene group sandwiched between the two nitrated DNNC nitrogen ring atoms, versus at the chemically equivalent C-4 and C-6 methylene ring positions located elsewhere in the DNNC molecule. Elucidation of such mechanistic features should aid in the structural design of new high-energy compounds with improved thermochemical properties. A 170.0 kJ/mol activation energy appeared for the endothermic induction period, and a lower 104.2 kJ/mol activation energy was determined for the exothermic acceleratory portion of the DNNC decomposition process. The global liquid and solid state thermochemical decomposition processes for DNNC are compared