Theoretical studies of reactions of fulminic acid and other molecules related to nitric oxide

Possible mechanisms for the NCO + HCNO and OH + HCNO reactions were studied using the second-order generalized Van Vleck perturbation theory (GVVPT2). Equilibrium geometries were obtained at the B3LYP variant of density functional theory (DFT) and at the multiconfigurational self consistent field (MCSCF) level of theory.;The ground states electronic structures of all eight doublet spin intermediates of the NCO + HCNO reaction are substantially multiconfigurational with maximum amplitudes not larger than 0.87. 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 consistent with earlier calculations, including the prediction of a negative temperature dependence of the rate constant, and support available experimental observations.;The electronic structures of all six plausible doublet spin intermediates for the OH + HCNO reaction are substantially multiconfigurational. A new pathway through an intermediate referred to in the literature as d1 has also been predicted to be a low energy channel that could lead to products, CO + H2NO. Vertical excitation energies of the intermediates to different symmetries other than the ground state showed that two intermediates have very small excitation energies and, hence, nonadiabatic effects can be expected to be significant especially in the surrounding C1 symmetry parts of the entire potential energy surfaces. Vertical excitations to states of the same symmetry as the ground state prove to be very large. GVVPT2 results support experimental observations, including a negative temperature dependence on the rate constant.;The low-lying excited electronic states of nitric oxide dimer (NO) 2 were investigated using the GVVPT2 method. An active space that could accurately describe the excitation energies of the bound dimer and the dissociation limit was realized in this work. This active space was used to calculate C 2v-symmetry potential energy curves for all the singlet and triplet electronic states that correlate with the first two dissociation channels of nitric oxide dimer (NO)2. From these curves, the location of minima and conical intersections were identified.