The reaction mechanism and relative stabilities of the intermediates and transition states in the reaction of 1 O plus ethylene molecule using ab initio molecular 2 Ε½ . orbital MO theories at several levels of theory with the correction of the nondynamic and dynamic electron correlation effects were systematically investigated. Full geometry Ε½ . Ε½ . optimizations of the corresponding biradical BR intermediates, perepoxide PE and Ε½ . Γ 4 U 1,2-dioxetane DO were performed by complete-active-space CASSCF 2,2 r6-31G method with nondynamic electron correlation effect and MΓΈller-Plesset MP2r6-31G U method with dynamic electron correlation effect. For the 1,4-biradical intermediates, new gauche-type 1 BR state was found at both CASSCF and MP2 levels, corresponding to the transition state of the rotation motion of the O moiety. It was found from the intrinsic-2 Ε½ . 1 reaction-coordinate IRC study that another gauche-type BR transition state connects 1 1 smoothly to the reactant system O q H C CH and the gauche minimum BR state, 2 2 2
showing that the reaction through the 1,4-biradical intermediates initially proceeds through the gauche transition state to form the gauche minimum 1 BR state, following that the free rotation of O moiety occurs due to the energy barrier less than 4.0 2 kcalrmol. The stability of the perepoxide is surprisingly sensitive to the levels of the Ε½ . theory and the basis sets employed. The coupled-cluster methods, CCSD and CCSD T , gave the reasonable stabilities of 1,4-biradical intermediates, perepoxide, and dioxetane Ε½ . as a reaction product. From the results of the CCSD and CCSD T methods, the reaction 1 of O q H C CH proceeds by a two-step mechanism through the 1,4-biradical 2 2 2 intermediates rather than through the perepoxide.