The effect of spin-orbit coupling on fast neutral chemical reaction O(3P)+CH3→CH3O

The effect of nonadiabatic transitions through the spin-orbit couplings has been investigated on the fast neutral reaction, O(P-3)+CH3-->CH3O. Adiabatic potential energies and the spin-orbit coupling terms have been evaluated for the four electronic states of CH3O (E-2, (2)A(2), E-4, and (4)A(2)) that correlate with the O(P-3)+CH3 asymptote, as a function of CO distance and OCH angle under the C-3v symmetry, by ab initio electronic structure calculations using multireference internally contracted single and double excitation configuration interaction method with the 6-311G(2df,2pd) basis sets. Multistate quantum reactive scattering calculations have been carried out with the use of thus obtained potential energies and spin-orbit coupling matrices, based on the generalized R-matrix propagation method. The calculated thermal rate constants show a slight positive dependence on temperature in a range between 50 and 2000 K, supporting the previous experimental results. It is shown that the spin-orbit coupled excited states give rise to reflections over the centrifugal barrier due to the quantum interference. Classical capture calculations yield larger rate constants due to the neglect of quantum reflections. It is concluded that the effect of nonadiabatic transitions is of minor importance on the overall reactivity in this reaction. (C) 2004 American Institute of Physics.