The effect of spin-orbit splitting on the association kinetics of barrierless halogen atom-hydrocarbon radical reactions.
The effect of the geometry dependence of spin-orbit splitting on transition state theory (TST) predictions for radical-radical recombination rate coefficients is examined. The effects are illustrated with direct ab initio variable-reaction-coordinate (VRC)-TST calculations for the reactions of two types of hydrocarbon radicals (R = CH(3) and CH(2)CHCH(2)) with three halogen atoms (X = F, Cl, and Br). These halogen atoms exhibit a range of spin-orbit interaction strengths, while their interactions with the two hydrocarbon radicals exhibit a range of attractiveness. The transition state dividing surfaces for these barrierless reactions occur over a range of R-X fragment separations ( approximately 3-7 A) where the magnitude of the spin-orbit splitting is strongly geometry dependent. Perturbative models for incorporating the energetic effect of spin-orbit splitting into barrierless kinetics are presented and tested. Simply neglecting the variation in the spin-orbit splitting is demonstrated to contribute an error of less than 15% to the predicted rate coefficients for all but the CH(2)CHCH(2) + Br reaction, where its neglect increases the rate by up to a factor of 2. For the CH(2)CHCH(2) + Br reaction, the effect of spin-orbit splitting is not perturbative and instead qualitatively changes the long-range interaction potential and association dynamics. The present theoretical predictions are compared with available experimental measurements and previous theoretical work. For the CH(3) + F association reaction, the errors associated with limitations in the basis set and in the active space are studied, and a detailed comparison is made between VRC-TST and rigid rotor-harmonic oscillator variational TST.
Jasper
Ahren W
AW
Combustion Research Facility, Sandia National Laboratories, P.O. Box 969, Livermore, California 94551-0969, USA.
Klippenstein
Stephen J
SJ
Harding
Lawrence B
LB