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.

Accurate standard enthalpies of formation for allene, propyne, and four C3H3 isomers involved in soot formation mechanisms have been determined through systematic focal point extrapolations of ab initio energies. Auxiliary corrections have been applied for anharmonic zero-point vibrational energy, core electron correlation, the diagonal Born-Oppenheimer correction (DBOC), and scalar relativistic effects. Electron correlation has been accounted for via second-order Z-averaged perturbation theory (ZAPT2) and primarily through coupled-cluster theory, including single, double, and triple excitations, as well as a perturbative treatment of connected quadruple excitations [ROCCSD, ROCCSD(T), ROCCSDT, and UCCSDT(Q)]. The correlation-consistent hierarchy of basis sets, cc-pVXZ (X = D, T, Q, 5, 6), was employed. The CCSDT(Q) corrections do not exceed 0.12 kcal mol(-)1 for the relative energies of the systems considered here, indicating a high degree of electron correlation convergence in the present results. Our recommended values for the enthalpies of formation are as follows: Delta(f)H(o)(0)(propargyl) = 84.76, Delta(f)H(o)(0) (1-propynyl) = 126.60, Delta(f)H(o)(0) (cycloprop-1-enyl) = 126.28, Delta(f)H(o)(0)(cycloprop-2-enyl) = 117.36, Delta(f)H(o)(0)(allene) = 47.41, and Delta(f)H(o)(0)(propyne) = 46.33 kcal mol(-1), with estimated errors no larger than 0.3 kcal mol(-1). The corresponding C3H3 isomerization energies are about 1 kcal mol(-1) larger than previous coupled-cluster results and several kcal mol(-1) below those previously obtained using density functional theory.