Non - Born – Oppenheimer molecular dynamics of the spin - forbidden reaction O ( 3 P ) + CO ( X

The lowest-energy singlet (1 A') and two lowest-energy triplet (1 A' and 1 A") electronic states of CO2 are characterized using dynamically-weighted multireference configuration interaction (dw-MRCI+Q) electronic structure theory calculations extrapolated to the complete basis set (CBS) limit. Global analytic representations of the dw-MRCI+Q/CBS singlet and triplet surfaces and of their CASSCF/aug-cc-pVQZ spin-orbit coupling surfaces are obtained via the interpolated moving least squares (IMLS) semiautomated surface fitting method. The spinforbidden kinetics of the title reaction is calculated using the coupled IMLS surfaces and coherent switches with decay of mixing (CSDM) non-Born–Oppenheimer molecular dynamics. The calculated spin-forbidden association rate coefficient (corresponding to the high pressure limit of the rate coefficient) is 7–35x larger at 1000–5000 K than the rate coefficient used in many detailed chemical models of combustion. A dynamical analysis of the multistate trajectories is presented. The trajectory calculations reveal direct (nonstatistical) and indirect (statistical) spin-forbidden reaction mechanisms and may be used to test the suitability of transition-state-theory-like statistical methods for spin-forbidden kinetics. Specifically, we consider the appropriateness of the “double passage” approximation, of assuming statistical distributions of seam crossings, and of applications of the unified statistical model for spinforbidden reactions. Electronic mail: [email protected] Electronic mail: [email protected]