The ethane + oxygen(,2) reaction mechanism: High-level ab initio characterizations

The C2H˙5+O2 reaction, central to ethane oxidation and thus of fundamental importance to hydrocarbon combustion chemistry, has been examined in detail via highly sophisticated electronic structure methods. The geometries, energies, and harmonic vibrational frequencies of the reactants, transition states, intermediates, and products for the reaction of the ethyl radical (X˜ 2A ') with O2 (X S-g3 , a 1Deltag) have been investigated using the CCSD and CCSD(T) ab initio methods with basis sets ranging in quality from double-zeta plus polarization (DZP) to triple-zeta plus double polarization with f functions (TZ2Pf). Five mechanisms (M1--M5) involving the ground-state reactants have been systematically explored: (M1) Direct hydrogen abstraction from the ethyl radical by O2 to give ethylene + HO˙2 with an overall 0 K activation energy, Ea(0 K) = +15.1 kcal mol--1 with CCSD(T)/TZ2Pf//CCSD(T)/TZ2P. (M2) Ethylperoxy beta-hydrogen transfer with O-O bond rupture to yield oxirane + ·OH; Ea(0 K) = +5.3 kcal mol--1 with CCSD(T)/TZ2Pf//CCSD(T)/TZ2P. (M3) Ethylperoxy alpha-hydrogen transfer with O-O bond rupture to yield acetaldehyde + ·OH; Ea(0 K) = +11.5 kcal mol--1 with CCSD(T)/TZ2P//CCSD(T)/DZP. (M4) Ethylperoxy beta-hydrogen transfer with C-O bond rupture to yield ethylene + HO˙2 ; Ea(0 K) = +5.3 kcal mol--1 with CCSD(T)/TZ2Pf//CCSD(T)/TZ2P, the C-O bond rupture barrier lying 1.2 kcal mol--1 above the O-O bond rupture barrier of M2 at the CCSD(T)/TZ2P//CCSD(T)/DZP level. (M5) Concerted elimination of HO˙2 from the ethylperoxy radical to give ethylene + HO˙2 ; Ea(0 K) = --0.9 kcal mol --1 with CCSD(T)/TZPf//CCSD(T)/TZ2P. We show that M5 is energetically preferred and is also the only mechanism consistent with experimental observations of a negative temperature coefficient. The reverse reaction (C2H 4 + HO˙2 → ·C2H4OOH) has a zero-point corrected barrier of 14.4 kcal mol--1 with CCSD(T)/TZ2P//CCSD(T)/DZP.