Reaction force analyses of nitro-aci tautomerizations of trinitromethane, the elusive trinitromethanol, picric acid and 2,4-dinitro-1H-imidazole

We have analyzed computationally, in terms of the reaction force, the nitro -> aci tautomerizations of trinitromethane, trinitromethanol, picric acid and 2,4-dinitro-1H-imidazole. These processes involve intramolecular transfer of a hydrogen to an NO2 oxygen, forming the aci tautomer (a nitronic acid). The reaction force naturally and unambiguously divides an activation barrier into two components: (1) the energy required for initial structural changes in the reactant(s), and (2) the energy associated with the first portion of the transition to product(s). In each of these tautomerizations, the first component is dominant. For trinitromethane, it is so large that the resulting total activation barrier makes C-NO2 bond scission energetically preferable. On the other hand, trinitromethanol-which appears to be unknown-readily undergoes fragmentation in conjunction with hydrogen transfer. Picric acid has the interesting feature that the reaction is almost complete after the first portion of the activation process, marked by the minimum of the reaction force. In all four reactions, the properties of the systems at the force minimum, transition state and force maximum are consistent with the concept of a transition'' region in a chemical reaction versus simply a transition state.