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 kC2H+C2H6(T) = 1.19 × 10−12T0.54 ± 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.
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