CASSCF, MP2, and CASMP2 studies on addition reaction of singlet molecular oxygen to ethylene molecule

Abstract

The reaction mechanism and relative stabilities of the intermediates and transition states in the reaction of 1O2 plus ethylene molecule using ab initio molecular orbital (MO) theories at several levels of theory with the correction of the nondynamic and dynamic electron correlation effects were systematically investigated. Full geometry optimizations of the corresponding biradical (BR) intermediates, perepoxide (PE) and 1,2-dioxetane (DO) were performed by complete-active-space CASSCF{2,2}/6-31G* method with nondynamic electron correlation effect and Moller-Plesset MP2/6-31G* method with dynamic electron correlation effect. For the 1,4-biradical intermediates, new gauche-type 1BR state was found at both CASSCF and MP2 levels, corresponding to the transition state of the rotation motion of the O2 moiety. It was found from the intrinsic-reaction-coordinate (IRC) study that another gauche-type 1BR transition state connects smoothly to the reactant system 1O2+H2C(DOUBLE BOND)CH2 and the gauche minimum 1BR state, showing that the reaction through the 1,4-biradical intermediates initially proceeds through the gauche transition state to form the gauche minimum 1BR state, following that the free rotation of O2 moiety occurs due to the energy barrier less than 4.0 kcal/mol. The stability of the perepoxide is surprisingly sensitive to the levels of the 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 as a reaction product. From the results of the CCSD and CCSD(T) methods, the reaction of 1O2+H2C(DOUBLE BOND)CH2 proceeds by a two-step mechanism through the 1,4-biradical intermediates rather than through the perepoxide. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 787–801, 1997