The C₂H₅ + O₂ Reaction Mechanism:  High-Level ab Initio Characterizations

Abstract

The C2H5 • + 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 (X̃ 2A‘) with O2 (X 3 , a 1Δg) 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 are introduced within the context of previous experimental and theoretical studies. In this work, each mechanism is systematically explored, giving the following overall 0 K activation energies with respect to ground-state reactants, E a(0 K), at our best level of theory: (M1) direct hydrogen abstraction from the ethyl radical by O2 to give ethylene + HO2 •, E a(0 K) = +15.1 kcal mol-1; (M2) ethylperoxy β-hydrogen transfer with O−O bond rupture to yield oxirane + •OH, E a(0 K) = +5.3 kcal mol-l; (M3) ethylperoxy α-hydrogen transfer with O−O bond rupture to yield acetaldehyde + •OH, E a(0 K) = +11.5 kcal mol-1; (M4) ethylperoxy β-hydrogen transfer with C−O bond rupture to yield ethylene + HO2 •, E a(0 K) = +5.3 kcal mol-1, the C−O bond rupture barrier lying 1.2 kcal mol-1 above the O−O bond rupture barrier of M2; (M5) concerted elimination of HO2 • from the ethylperoxy radical to give ethylene + HO2 •, E a(0 K) = −0.9 kcal mol-1. 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 (C2H4 + HO2 • → •C2H4OOH) has a zero-point-corrected barrier of 14.4 kcal mol-1.

A comprehensive modeling study of iso-octane oxidation

“…However, for type 3 reactions above the activation energy increases with the type of olefinic carbon atom. Recent studies have shown that, for the case of ethyl radical adding to O 2 , the reaction does not proceed through the Q OOH path to olefin plus HȮ 2 radical, but reacts through a HȮ 2 radical elimination channel [78]. It is reasonable to expect that other larger alkyl radicals react with O 2 and proceed through a HȮ 2 radical elimination channel as well.…”

Mechanisms of catalyst action in the TiO2-mediated photocatalytic degradation and cis–trans isomerization of maleic and fumaric acid

“…Step B is thermodynamically downhill and related reactions are well known in the organic literature [72][73][74][75][76][77][78]. In aqueous solution, 13 is an obvious precursor to 9 by either acid or base catalyzed ring opening.…”

Oxidation and combustion of the n-hexene isomers: A wide range kinetic modeling study

“…An alkylperoxy radicals (ROO·) can form alkene and HO 2 radical through an elimination reaction [32]. In the case of alkenylperoxy radicals, this direct elimination will yield diolefin rather than an alkene (Fig.…”