Formation of C₉H₂ and C₁₀H₂ from Reactions C₃H + C₆H₂ and C₄H + C₆H₂

The reactions of C₃H and C₄H radicals with C₆H₂ were investigated for the first time. Reactants C₃H, C₄H, and C₆H₂ were synthesized in two beams of C₂H₂ diluted with helium by pulsed high-voltage discharge. We measured translational-energy distributions, angular distributions, and photoionization-efficiency spectra of C₉H₂ and C₁₀H₂ produced from the title reactions in a crossed molecular-beam apparatus using synchrotron vacuum-ultraviolet photoionization. The C₃H (C₄H) + C₆H₂ reaction releases 42% (33%) of available energy into the translational degrees of freedom of product C₉H₂ (C₁₀H₂) + H and scatters products into a nearly isotropic angular distribution. The photoionization-efficiency spectrum of C₉H₂ (C₁₀H₂) is in good agreement with that of C₉H₂ (C₁₀H₂) produced from the C₇H (C₈H) + C₂H₂ reaction. The ionization threshold, after deconvolution, was determined as 8.0 ± 0.1 eV for C₉H₂ and 8.8 ± 0.1 eV for C₁₀H₂. The combination of measurements of product translational-energy release and photoionization-efficiency spectra indicates productions of ³HC₉H/c-¹HC₃(C)C₅H/c-¹HC₇(C)CH + H and ¹HC₁₀H + H in the two title reactions, which are supported also by quantum-chemical calculations. Ratios branching to the three isomers of C₉H₂ remain unknown. This work demonstrates that long carbon-chain molecules (e.g., C₉H₂ and C₁₀H₂) can be synthesized from reactions of C_mH (e.g., m = 3 and 4) radicals with polyynes (e.g., HC₆H) and gives some valuable implications to planetary, interstellar, and combustion chemistry.