Alternative mechanisms of thermal decomposition of o -nitrotoluene in the gas phase

The density functional theory methods were used to demonstrate that during the thermal decomposition of o -nitrotoluene, with the formation of 5-methylene-6- aci -nitrocyclohexa- 1,3-diene ( aci -form) being the primary event, the rotation of the =N(O)OH group around the CN double bond in the aci -form is of key importance. The activation enthalpy is lower for this step than for the alternative process of H atom transfer between the O atoms in this group. This accounts for the competitive formation of the experimentally observed products of о -nitrotoluene thermal decomposition, namely, the hydroxyl radical and water. The activation barriers of the reactions were estimated over a broad temperature range, which indicated the possible contribution of о -nitrotoluene thermal decomposition and other alternative primary event mechanisms (nitro—nitrite rearrangement, bicyclization) to the efficient rate constant. The results account for the differences between the activation parameters experimentally determined at various temperatures by different authors.