Dynamics of the reaction of C3(a³Πu) radicals with C2H2: A new source for the formation of C5H.

The reaction C₃(a³Π_u ) + C₂H₂ → C₅H + H was investigated at collision energy 10.9 kcal mol^-1 that is less than the enthalpy of ground-state reaction C₃(X¹Σ_g⁺) + C₂H₂ → C₅H + H. C₃(a³Π_u ) radicals were synthesized from 1% C4F6/He by pulsed high-voltage discharge. The title reaction was conducted in a crossed molecular-beam apparatus equipped with a quadrupole-mass filter. Product C₅H was interrogated with time-of-flight spectroscopy and synchrotron vacuum-ultraviolet ionization. Reactant C₃(a³Π_u ) and product C₅H were identified using photoionization spectroscopy. The ionization thresholds of C₃(X¹Σ_g+) and C₃(a³Π_u ) are determined as 11.6 ± 0.2 eV and 10.0 ± 0.2 eV, respectively. The C₅H product is identified as linear pentynylidyne that has an ionization energy 8.4 ± 0.2 eV. The title reaction releases translational energy 10.6 kcal mol^-1 in average and has an isotropic product angular distribution. The quantum-chemical calculation indicates that the C₃(a³Π_u ) radical attacks one of the carbon atoms of C₂H₂ and subsequently a hydrogen atom is ejected to form C₅H + H, in good agreement with the experimental observation. As far as we are aware, the C₃(a³Π_u)+ C₂H₂ reaction is investigated for the first time. This work gives an implication for the formation of C₅H from the C₃(a³ nu ) + C₂H₂ reaction occurring in a combustion or discharge process ofC₂H₂. [ABSTRACT FROM AUTHOR]