Electronic structure aspects of the spin-forbidden reaction CH₃(X ²A₂″) + N(⁴S)→HCN(X 1∑⁺) + H₂(X ¹∑_g⁺)

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 ¹∑⁺) + H₂(X ¹∑⁺). The minimum energy crossing point (MECP), the minimum energy point on the surface of intersection connecting the reactant channel, ³A″ potential energy surface, and product channel ¹A′ 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 ∼30cm^-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.