Experimental and theoretical study of the reaction of the ethynyl radical with acetylene (HCC+HCCH) Original Research Article

The absolute rate coefficient of the reaction of the ethynyl radical with acetylene was measured using a pulsed laser photolysis/chemiluminescence (PLP/CL) technique in which HCC radicals were generated by excimer laser photodissociation of acetylene at 193 nm, and pseudo-first-order exponential decays of thermalized HCC were monitored in real time by the CH(A 2Δ→X 2Π) CL resulting from their reaction with O2. The rate coefficient as determined using this single, absolute technique over the extended temperature range of 295⩽T (K)<800 was found to exhibit no temperature dependence, the result being kacetylene=(1.3±0.1)×10−10 cm3 molecule−1 s−1. B3LYP and CCSD(T)/6-311++G(d,p) calculations revealed that while the direct H-abstraction has a barrier of ≈40 kJ mol−1, the terminal C-addition is barrier-free giving the 2-ethynyl-vinyl radical (HCC–CHCH) which either dissociates directly into diacetylene (HCC–CCH+H) or first rearranges to 1-ethynyl-vinyl (HCC–CCH2) before undergoing a H-loss. In both these pathways, all intermediates and transition structures lie energetically well below the reactants and therefore both fragmentation pathways are open to thermal reactants. An additional C4H2+H forming pathway through the vinylidene HCCH–CHC was identified; it is expected to contribute at higher flame temperatures.