Mechanisms of Nitramine Thermolysis

Abstract

The thermal decomposition of a number of nitramines was studied in dilute solution and in the melt. The nitramines included acyclic mononitramines [dimethylnitramine (DMN), diethylnitramine (DEN), dipropylnitramine (DPN), and diisopropylnitramine (DIPN)], cyclic mononitramines [N-nitro-piperidine (NPIP) and N-nitropyrrolidine (NPyr)], cyclic dinitramines [N-dinitropiperazine (pDNP), 1,3-dinitro-1,3-diazacyclopentane (DNI), and 1,3-dinitro-1,3diazacyclohexane (mDNP)], and 1,3,5-trinitro-1,3,5-triazocyclohexane (RDX), octahydro-1,3,5,7tetranitro-1,3,5,7-tetrazocine(HMX), hexanitro-hexaazaisowurtzitane (HNIW), and 1,3,3trinitroazetidine (TNAZ). For the acyclic and cyclic monoand di-nitramines, the corresponding nitrosamines were the only or major condensed-phase product. Kinetics and activation parameters were determined for the thermolysis of dilute solutions (0.01 to 1.0 wt%) over range 200 to 300C. The thermolyses were found to be first-order with the rate constants unaffected by use of deuterated solvent. As the nitramines became more complex than dimethylnitramine (DMN), the rate of decomposition increased and the product distribution became more complex. As the length of the aliphatic chain increased (DMN < DEN < DPN), the rate of thermolysis increased, yet nitrosamine remained the only observed condensed-phase product. When a secondary carbon was attached to the N-nitramine (DIPN) rather than primary (DPN), the rate of decomposition increased and a new condensed-phase product was observed. Among the cyclic nitramines, the rate of decomposition increased as the number ofNNO2 groups increased (NPIP < pDNP; NPyr < DNI; mDMP < RDX). The position of the nitramine groups affected the decomposition; meta NNO2 groups (mDNP) decomposed faster than para (pDNP). Ring strain decreased stability: mDNP < DNI; HMX < RDX. In complex nitramines, the increase in decomposition rate, the appearance of new products, and the change in the relative importance of nitrosamine and of N2 and N2O is attributed to new decomposition routes available to them. However, since complex nitramines (e.g. RDX) maintain first-order kinetics and since most have activation energies in the range of 40 to 50 kcal/mol, it is believed that the triggering mechanism remains N-NO2 homolysis. Intramolecular hydrogen transfer is also considered an important mode of nitramine decomposition. Introduction Before attempting to understand the mechanisms operating in species containing multiple nitramine functionalities, we examined the simple nitramine, dimethylnitramine (DMN). To avoid complications resulting from multiphase decomposition and autocatalysis, we studied the thermolysis in dilute solution. Two different N-labeling studies were performed. In both, complete label scrambling was observed in the N2 and N2O gases and in the dimethylnitrosamine. Only partial scrambling of the N-label was observed in the reactant, DMN. Kinetic studies of