Deuterium isotope effects in condensed-phase thermochemical decomposition reactions of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine

Abstract

The deuterium isotope effect was applied to condensed-phase thermochemical reactions of HMX and HMX-</8 by using isothermal techniques. Dissimilar deuterium isotope effects revealed a mechanistic dependence of HMX upon different physical states which may singularly predominate in a specific type of thermal event. Solid-state HMX thermochemical decomposition produces a primary deuterium isotope effect (DIE), indicating that covalent C-H bond rupture is the rate-controlling step in this phase. An apparent inverse DIE is displayed by the mixed melt phase and can be attributed to C-H bond contraction during a weakening of molecular lattice forces as the solid HMX liquefies. The liquid-state decomposition rate appears to be controlled by ring C-N bond cleavage as evidenced by a secondary DIE and higher molecular weight products. These results reveal a dependence of the HMX decomposition process on physical state and lead to a broader mechanistic interpretation which explains the seemingly contradictory data found in current literature reviews.

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

“…In every case, the same mechanistic rate-controlling bond rupture found in the ambient pressure decomposition process for each compound, proved to be a significant ratecontrolling mechanistic feature observed in rapid deflagration [8,14], high-pressure combustion [14][15][16], and thermal explosion events [3,14]. Furthermore, it is detected as being a significant contributing mechanistic feature in impact or shock explosive initiation sensitivity [12,[17][18][19][20][21][22][23].…”

“…The seminal KDIE-based thermochemical decomposition investigation of a heterocyclic nitramine was accomplished with HMX and HMX-d 8 and revealed a different KDIE and ratecontrolling mechanistic step dependence on the HMX physical state [8]. This IDSC study showed a rate-controlling C H bond rupture occurred during solid state decomposition along with other rate-controlling features in the mixed-melt and liquid state decompositions.…”

“…This critical rate-controlling bond rupture has been determined for the ambient pressure thermochemical decomposition processes of TNT [10], TATB [3], HMX [8,12], and RDX [11,12]. In every case, the same mechanistic rate-controlling bond rupture found in the ambient pressure decomposition process for each compound, proved to be a significant ratecontrolling mechanistic feature observed in rapid deflagration [8,14], high-pressure combustion [14][15][16], and thermal explosion events [3,14].…”

“…It is during this slowest reaction step that the second critical homolytic bond rupture occurs which significantly regulates the overall rate at which this slowest reaction channel and the entire decomposition process proceeds. This subsequent KDIE-determined rate-controlling chemical bond rupture, accompanied by a concomitant atom transfer [8], occurs early in the thermochemical decomposition process [2,[8][9][10] and usually is not synonymous with the trigger linkage, but closely follows it. This rate-controlling bond rupture can be determined in situ using the kinetic deuterium isotope effect (KDIE) approach in conjunction with ambient pressure isothermal DSC [2,3,[8][9][10][11], reaction quenching [11], or TGA [12] analyses.…”

“…This IDSC study showed a rate-controlling C H bond rupture occurred during solid state decomposition along with other rate-controlling features in the mixed-melt and liquid state decompositions. Proposed was an initial N NO 2 bond rupture to form the NO 2 radical which then, either, unimolecularly from an HMX radical intermediate existing in a proposed cage effect [24], or bimolecularly from a neutral HMX molecule, transfers a hydrogen atom via NO 2 radical abstraction to form the unstable HONO species [8]. This unstable species undergoes further decomposition to form other oxidizing and catalytic species [8], such as NO, NO 2 , and H 2 O, or in some cases, possibly OH and NO [25], while resultant HMX radicals continue along various decomposition reaction channels and condensed phases [8].…”