Effect of an S1/S0 Conical Intersection on the Chemistry of Nitramide in Its Ground State. A Comparative CASPT2 Study of the Nitro-Nitrite Isomerization Reactions in Nitramide and Nitromethane

The potential energy surfaces for the dissociation of nitramide (NH2NO2 → NH2 + NO2) and nitromethane (CH3NO2 → CH3 + NO2) and the nitro-nitrite rearrangement of these nitrocompounds (RNO2 → RONO) as well as the dissociations of the nitrite isomers (RONO → RO + NO) have been studied with the second-order multiconfigurational perturbation theory (CASPT2) by computation of numerical energy gradients for stationary points. It is found that multiconfigurational methods [CASPT2 and complete active space SCF (CAS−SCF)] predict that the isomerization of nitramide to NH2ONO occurs in a two-step mechanism: (i) NH2NO2 → NH2 + NO2 and (ii) NH2 + NO2 → NH2ONO, the second step involving surmounting an activation barrier. Contrastingly, Hartree−Fock based approaches give isomerization as a one-step reaction. Additionally, both mono- and multiconfigurational methods predict that nitro-nitrite rearrangement of CH3NO2 is a one-step process. The difference in the reaction mechanisms of these two isoelectronic molecules arises from the presence of an S1/S0 conical intersection in nitramide which is absent in nitromethane.