Asymptotic potentials and rate constants in the adiabatic capture centrifugal sudden approximation for X + OH(X-2 Pi) -> OX + H(S-2) reactions where X = O(P-3), S(P-3) or N(S-4)
Résumé
New long-range multipolar coefficients for the X + OH(X-2 Pi) interactions, where X = O(P-3), S(P-3) and N(S-4), are given here. They have been evaluated on the basis or monomer properties of the atoms and OH such as the dipole and quadrupole moments, and the static and dynamic polarizabilities. Each matrix element of the 18 x 18 (8 x 8 for N + OH) quasi-degenerate asymptotic potentials has been built up by means of the perturbation theory up to second order including or not the fine-structure of O, S and OH. The adiabatic potentials, obtained after diagonalization of the full matrix, show many crossings and complex behaviors near the asymptotes. Using the entrance channel ground state potential, the "adi-abatic capture in the centrifugal sudden approximation" (ACCSA) approach has been found very convenient to get upper limit rate constants for the X + OH --> XO + H reactions together with their temperature dependence. It is particularly sensitive to the long-range part of the potential in the low temperature regime. Three types of ground state potentials have been used to evaluate the influence of the potential on the magnitude and temperature dependence of the rate constants: one is obtained by supermolecular ab initio calculations and the other two by perturbation theory including or not the spin-orbit splittings. According to the potential which is employed, different behaviors of the rate constants are observed in the low and high temperature ranges while their absolute values are close in the middle temperature range. The ACCSA rate constants overestimate, by a factor of 2, more accurate kinetic values derived from quasi-classical and quantum dynamical calculations performed with global potential energy surfaces. This is attributed to a significant lower reaction probability than the one accounted for in the capture approximation employed here.