Theoretical study of the reaction of ethane with oxygen molecules in the ground triplet and singlet delta states.

Quantum chemical calculations are carried out to study the reaction of ethane with molecular oxygen in the ground triplet and singlet delta states. Transition states, intermediates, and possible products of the reaction on the triplet and singlet potential energy surfaces are identified on the basis of the coupled-cluster method. The basis set dependence of coupled-cluster energy values is estimated by the second-order perturbation theory. The values of energy barriers are also refined by using the compound CBS-Q and G3 techniques. It was found that the C(2)H(6) + O(2)(X(3)Σ(g)(-)) reaction leads to the formation of C(2)H(5) and HO(2) products, whereas the C(2)H(6) + O(2)(a(1)Δ(g)) process produces C(2)H(4) and H(2)O(2) molecules. The appropriate rate constants of these reaction paths are estimated on the basis of variational and nonvariational transition-state theories assuming tunneling and possible nonadiabatic transitions in the temperature range 500-4000 K. The calculations showed that the rate constant of the C(2)H(6) + O(2)(a(1)Δ(g)) reaction path is much greater than that of the C(2)H(6) + O(2)(X(3)Σ(g)(-)) one. At the same time, the singlet and triplet potential surface intersection is detected that leads to the appearance of the nonadiabatic quenching channel O(2)(a(1)Δ(g)) + C(2)H(6) → O(2)(X (3)Σ(g)(-)) + C(2)H(6). The rate constant of this process is estimated with the use of the Landau-Zener model. It is demonstrated that, in the case of the existence of thermal equilibrium in the distribution of molecules over the electronic states, at low temperatures (T < 1200 K) the main products of the reaction of C(2)H(6) with O(2) are C(2)H(4) and H(2)O(2), rather than C(2)H(5) and HO(2). At higher temperature (T > 1200 K) the situation is inverted.