The reaction of C2H radicals with C2H6: Absolute rate coefficient measurements for T = 295–800 K, and quantum chemical study of the molecular mechanism

Abstract

The absolute rate coefficient of the title reaction was measured for the first time at elevated temperatures, using a pulsed laser photolysis/chemiluminescence (PLP/CL) technique. C₂H radicals were generated by excimer laser photodissociation of acetylene at 193 nm, and pseudo-first-order decays of thermalised C₂H were monitored in real-time using the CH(A²Δ → X²Π) chemiluminescence from the reaction of C₂H with excess O₂ as an ethynyl probe. The rate coefficients, measured over the temperature range 295–779 K, exhibit a slight non-Arrhenius behaviour and can be represented by k_C2H+C2H6(T) = 1.19 × 10⁻¹²T^{0.54 ± 0.20} exp[(180 ± 70) K/T] cm³ molecule⁻¹ s⁻¹. Ab initio calculations were performed at the MP2 and CCSD(T) levels of theory to elucidate the molecular mechanism of the C₂H + alkane reactions. It is concluded that direct H-abstraction, leading to C₂H₂ + alkyl radical, has by far the lowest energy barrier, ≈3 kJ mol⁻¹, whereas substitution and insertion reactions face barriers in excess of 100 kJ mol⁻¹ and are therefore not competitive.