On the mechanism of the reaction CH(X 2Π)+N2(X 1Σ+g)→HCN(X 1Σ+) +N(4S ). I. A theoretical treatment of the electronic structure aspects of the intersystem crossing

Abstract

The reaction CH(X 2Π)+N2(X 1Σ+g)→HCN(X 1Σ+) +N(4S) has been suggested as the initial step in the formation of ‘‘prompt’’ NO in flame fronts. Since the reaction is spin-forbidden an intersystem crossing must occur in the vicinity of the allowed crossing hypersurface of the lowest doublet and quartet potential energy surfaces. In this work the electronic structure aspects to this intersystem crossing are considered using multireference configuration interaction wave functions. The key to this treatment is a new implementation of a constrained analytic gradient search algorithm which is used to locate regions of the doublet–quartet crossing hypersurface. In those regions of nuclear coordinate space the spin–orbit coupling (matrix elements of HSO) is determined using the full microscopic Breit–Pauli spin–orbit interaction (that is both the spin–orbit and spin–other–orbit contributions are included). Included in this treatment are the largest configuration state function expansions, 700 000–900 000 terms, used to date to evaluate matrix elements HSO within the full Breit–Pauli approximation. Our conclusions are as follows. The lowest energy point on the doublet–quartet crossing hypersurface corresponds, approximately, to a C2v nuclear configuration in which the HC moiety has been inserted into a highly stretched N2 bond. The spin–orbit coupling in this region is approximately 12 cm−1. This region is estimated to be endoergic with respect to the reactant channel asymptote by approximately 7.5 kcal/mol. A second region of the crossing hypersurface corresponds to a perturbed nitrogen atom collinearly adjacent to the nitrogen side of the HCN moiety which is in its ground electronic state. The spin–orbit coupling in this region is considerably larger, approximately 44 cm−1. However, this region is estimated to be endoergic with respect to the reactant channel asymptote by over 50 kcal/mol. A qualitative Landau–Zener analysis of the electronic structure data provides the first computational evidence supporting the role of this reaction in the production of prompt NO in flame fronts.