The C_2H_5 + O_2 Reaction Mechanism: High-Level ab Initio Characterizations

摘要

The C_2H_5· + O_2 reaction, central to ethane oxidation and thus of fundamental importance of 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 the ethyl radical (X-tilde ~2A') with O_2(X ~3Σ_G~-, a ~1Δ_g) 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 O_2 to give ethylene + HO_2·, 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~(-1); (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 + HO_2·, 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 H_2O· from the ethylperoxy radical radical to give ethylene + HO_2·, E_a(0 K) = + 5.3 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 (C_2H_4 + H_2O· → ·C_2H_4OOH) has a zero-point-corrected barrier of 14.4 kcal mol~(-1).