Mechanism of Thermal Unimolecular Decomposition of TNT (2,4,6-Trinitrotoluene):  A DFT Study

The widespread and long-term use of TNT has led to extensive study of its thermal and explosive properties. Although much research on the thermolysis of TNT and polynitro organic compounds has been undertaken, the kinetics and mechanism of the initiation and propagation reactions and their dependence on the temperature and pressure are unclear. Here, we report a comprehensive computational DFT investigation of the unimolecular adiabatic (thermal) decomposition of TNT. On the basis of previous experimental observations, we have postulated three possible pathways for TNT decomposition, keeping the aromatic ring intact, and calculated them at room temperature (298 K), 800, 900, 1500, 1700, and 2000 K and at the detonation temperature of 3500 K. Our calculations suggest that at relatively low temperatures, reaction of the methyl substituent on the ring (C-H alpha attack), leading to the formation of 2,4-dinitro-anthranil, is both kinetically and thermodynamically the most favorable pathway, while homolysis of the C-NO(2) bond is endergonic and kinetically less favorable. At approximately 1250-1500 K, the situation changes, and the C-NO(2) homolysis pathway dominates TNT decomposition. Rearrangement of the NO(2) moiety to ONO followed by O-NO homolysis is a thermodynamically more favorable pathway than the C-NO(2) homolysis pathway at room temperature and is the most exergonic pathway at high temperatures; however, at all temperatures, the C-NO(2) --> C-ONO rearrangement-homolysis pathway is kinetically unfavorable as compared to the other two pathways. The computational temperature analysis we have performed sheds light on the pathway that might lead to a TNT explosion and on the temperature in which it becomes exergonic. The results appear to correlate closely with the experimentally derived shock wave detonation time (100-200 fs) for which only the C-NO(2) homolysis pathway is kinetically accessible.