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 (NH(2)NO(2) --> NH(2) + NO(2)) and nitromethane (CH(3)NO(2) --> CH(3) + NO(2)) and the nitro-nitrite rearrangement of these nitrocompounds (RNO(2) --> 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 NH(2)ONO occurs in a two-step mechanism: (i) NH(2)NO(2) --> NH(2) + NO(2) and (ii) NH(2) + NO(2) --> NH(2)ONO, 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 CH(3)NO(2) 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.