Thermal Decomposition of Energetic Materials. 4. Deuterium Isotope Effects and Isotopic Scrambling Condensed-Phased Decomposition of 1,3,5- Trinitrohexahydro-S-Triazine.

The inter- versus intramolecular origin of the products formed in the thermal decomposition of 1,3,5-trinitrohexahydro-s-triazine RDX has been traced by isotopic crossover experiments using mixtures of differently labeled analogues of RDX. The isotopic analogues of RDX used in the experiments include 2H, 13C, 15N, and 180. The fraction of isotopic scrambling and the extent of the deuterium kinetic isotope effect DKIE are reported for the different thermal decomposition products. Isotopic scrambling is not observed for the N-N bond in N20 and only in small amounts 7 in the C-H bonds in CH20, consistent with a mechanism of their formation through methylene nitramine precursors. A product, oxy-s-triazine OST, C3H3N30, does not undergo isotopic scrambling in HD, 14N 15N, or 13C180 experiments, and its rate of formation exhibits a DKIE of 1.5. These results are consistent with the formation of OST via unimolecular decomposition of RDX. Another product, 1-nitroso-3,5-dinitrohexahydro-s-triazine ONDNTA, C3H6N605, is found to be formed with complete scrambling of the N-NO bond, suggesting an N-N bond cleavage and a radical recombination process in its formation. One of the hydrogen containing products, H20, exhibits a DKIE of 1.5 or - 0.1. In contrast, CH20 and ONDNTA have DKIEs of 1.05 or - 0.1 and 1.05 or - 0.2, respectively, indicating that hydrogen transfer is not involved in the rate-limiting step of the reaction pathway leading to the formation of these products.