Effect of an S-1/S-0 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

摘要

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 S-1/S-0 conical intersection in nitramide which is absent in nitromethane.