Multireference generalized Van Vleck perturbation theory (GVVPT2) study of the NCO + HCNO reaction: insight into intermediates.

Possible mechanisms for the NCO + HCNO reaction, previously studied using single reference methods, have been revisited using the second-order generalized Van Vleck perturbation theory (GVVPT2) variant of multireference perturbation theory. Large scale GVVPT2 calculations, with 25 active orbitals, demonstrate that the electronic structures of the ground states of all eight previously identified plausible doublet spin intermediates of this reaction are substantially multiconfigurational with maximum configuration amplitudes not larger than 0.87. Moreover, comparison of GVVPT2 results with underlying MCSCF calculations demonstrates a pronounced effect of dynamic correlation on the configurational structure. GVVPT2 calculations of vertical excitation energies of the intermediates showed that all the intermediates, except one, have significant excitation energies to both the same and other spatial symmetries as the ground states. The relative GVVPT2 energies of the intermediates were found to be consistent with earlier calculations, including the prediction of a negative temperature dependence of the rate constant, and support available experimental observations.