Theoretical studies of fine-structure effects and long-range forces : potential-energy surfaces and reactivity of O(3P)+OH(2Π)

The role of fine structure in reactions without barriers in the potential-energy surface is examined in general, and calculations are carried out for the specific case of O+OH→H+O2. The long-range Hamiltonian, including electrostatic (dipole–quadrupole and quadrupole–quadrupole) and spin-oribt interactions, is expressed in the asymptotic (separated species) basis for the 18 doubly degenerate states correlating to ground-state reactants O(3P2,1,0)+OH(2Π3/2,1/2). Adiabatic potential-energy surfaces are determined by diagonalization of the long-range Hamiltonian. The adiabaticity of the reaction has been analyzed using general considerations about nonadiabatic processes and confirmed by direct integration of the coupled equations. The half collision through the coupling region is found to be predominantly adiabatic for the state correlating to reaction. Single-surface reaction cross sections and rate constants have been obtained using the adiabatic capture, infinite-order sudden approximation method. Our results indicate that the reaction is probably fast even at very low temperatures. The effect of reagent rotation on the reaction cross section is also discussed.