Second order configuration interaction wave functions based on molecular orbitals determined from a state-averaged multiconfigurational self-consistent field procedure are used to investigate the intermediate complex driven model for the spin-forbidden reaction CH3(X 2A2)+N(4S)→HCN(X 1Σ+)+H2(X 1Σ+). The minimum energy crossing point (MECP), the minimum energy point on the surface of intersection connecting the reactant channel, A′′3 potential energy surface, and product channel A1 potential energy surface, is determined directly, i.e., without a priori characterization of the individual potential energy surfaces. The MECP is found to be 8.2 kcal/mol below of the reactants. The structure at the MECP clearly evinces the incipient formation of a H2 bond. Barrierless paths from the reactants to the intermediate complex-methylnitrene, from the intermediate complex to the MECP, and from the MECP to the products are established. The absence of a barrier on these paths supports the intermediate complex mechanism. Spin–orbit interactions are determined to be ∼30 cm−1 for points on the surface of intersection in the vicinity of the MECP. Spin–orbit interactions and the local potential surface topology at the MECP are used to probe the efficiency of the intersystem crossing using the Landau–Zener model. A reduced dimensionality model is proposed.

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