Journal of Physical Chemistry, Vol.98, No.28, 7004-7008, 1994
Mechanisms of Nitramine Thermolysis
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-nitropiperidine (NPIP) and N-nitropyrrolidine (NPyr)], cyclic dinitramines [N-dinitropiperazine (pDNP), 1,3-dinitro-1,3-diazacyclopentane (DNI), and 1,3-dinitro-1,3-diazacyclohexane (mDNP)], and 1,3,5-trinitro-1,3,5-triazocyclohexane (RDX), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), hexanitrohexaazaisowurtzitane (HNIW), and 1,3,3-trinitroazetidine (TNAZ). For the acyclic and cyclic mono- and dinitramines, the corresponding nitrosamines were the only or major condensed-phase product. Kinetics and activation parameters were determined for the thermolysis of dilute solutions (0.01-1.0 wt %) over the range 200-300 degrees C. The thermolyses were found to be first-order with the rate constants unaffected by the 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 the 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 of NNO2 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 N-2 and N2O are 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 40-50 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.
Keywords:CONDENSED-PHASE DECOMPOSITION;GASEOUS PYROLYSIS PRODUCTS;1;3;5 TRIAZINE RDX;THERMAL-DECOMPOSITION;ENERGETIC MATERIALS;TEMPORAL BEHAVIORS;OCTAHYDRO-1;3;5;7-TETRANITRO-1;3;5;7-TETRAZOCINE;1;3;5-TRINITRO-1;3;5-TRIAZACYCLOHEXANE;1;3;5-TRINITROHEXAHYDRO-S-TRIAZINE;RATES