Dynamics of the reaction of C2 with C6H2: An implication for the formation of interstellar C8H.

The reaction C₂ + C₆H₂ → C8H + H was investigated for the first time. Reactant C₂ (C₆H₂) was synthesized from 1% C3F6/He (5% C₂H₂/He) by pulsed high-voltage discharge. We measured the translational-energy distribution, the angular distribution, and the photoionization spectrum of product C₈H in a crossed molecular-beam apparatus using synchrotron vacuum-ultraviolet ionization. This reaction released average translational energy of 8.5 kcal mol^-1 corresponding to a fraction of 0.37 in translation. C₈H was identified as octatetranyl based on the maximal translational-energy release 23 ± 2 kcal mol^-1 and the ionization threshold 8.9 ± 0.2 eV. Kinematic constraints can qualitatively account for the nearly isotropic angular distribution. The quantum-chemical calculations indicate that the exothermic reactions C₂ (X 1¹∑_g⁺/a ³∏_u) + HC₆H → C₈H + H can proceed without entrance and exit barriers, implying the importance in the cold interstellar medium. This work verifies that interstellar C₈H can be formed through the C₂ + C₆H₂ reaction. [ABSTRACT FROM AUTHOR]