MECHANISM OF THE C2H5+O-2 REACTION

The geometries, energies, and vibrational frequencies of the reactants , transition states, intermediates, and products of the reaction of et hyl radical with the oxygen molecule have been examined using density functional theory (DFT). Rather different theoretical predictions are obtained from the BLYP, B3LYP, and BHLYP methods. Comparisons with exp erimental deductions and high-level coupled cluster results suggest th at the B3LYP method is superior for the C2H5+O-2 problem. Using the B3 LYP method with a triple-zeta plus double-polarization plus f function (TZ2Pf) basis set, a transition state between the ethylperoxy radical and products is discovered which lies 3.3 kcal mol(-1) below reactant s. This transition-state energy is consistent with the observed high y ields of ethylene in the high-temperature reaction and is in good agre ement with the height of the barrier estimated via modeling of the exp erimental kinetic data. However, this transition state (TS1) correspon ds not to the internal proton transfer leading to the hydroperoxyethyl radical C2H4OOH but to the concerted elimination of ethylene. For the reverse reaction C2H4+HO2-->C2H4OOH, the TZ2Pf UB3LYP classical barri er is 11.2 kcal mol(-1). (C) 1997 American Institute of Physics.