Influence of atomic fine structure on bimolecular rate constants. The Cl(²P) + HCl reaction

In this paper we use quantum reactive scattering calculations to study the influence of atomic fine structure on the rate constants for atom-diatom reactions, using the Cl(²P) + HCl reaction as an example. We find that the biggest effect of the fine structure is a static one which arises because the spin-orbit term in the Hamiltonian stabilizes the reagents and products differently than the ²Σ _1/2 saddle point. This causes the activation energy to increase by about 29% of the atomic ²P _1/2 - ²P_3/2 energy difference. When this effect is factored out, the single and multiple surface rate constants are found to be in very good agreement (provided that the usual electronic statistical factors are applied to the single surface rate constant), with differences of less than 20% over the temperature range that we considered. We also study how the size of the spin-orbit constant influences this result, and we find that the overall rate is relatively insensitive to spin-orbit splitting (except for the shift in activation energy). Fine structure resolved rate constants for Cl(²P _1/2 ) are found to vary by many orders of magnitude as the spin-orbit splitting is varied between zero and the experimental value, corresponding to a change in the dynamics from statistical to adiabatic limits.