Formation of C 9 H 2 and C 10 H 2 from Reactions C 3 H + C 6 H 2 and C 4 H + C 6 H 2 .

The reactions of C3 H and C4 H radicals with C6 H2 were investigated for the first time. Reactants C3 H, C4 H, and C6 H2 were synthesized in two beams of C2 H2 diluted with helium by pulsed high-voltage discharge. We measured translational-energy distributions, angular distributions, and photoionization-efficiency spectra of C9 H2 and C10 H2 produced from the title reactions in a crossed molecular-beam apparatus using synchrotron vacuum-ultraviolet photoionization. The C3 H (C4 H) + C6 H2 reaction releases 42% (33%) of available energy into the translational degrees of freedom of product C9 H2 (C10 H2 ) + H and scatters products into a nearly isotropic angular distribution. The photoionization-efficiency spectrum of C9 H2 (C10 H2 ) is in good agreement with that of C9 H2 (C10 H2 ) produced from the C7 H (C8 H) + C2 H2 reaction. The ionization threshold, after deconvolution, was determined as 8.0 ± 0.1 eV for C9 H2 and 8.8 ± 0.1 eV for C10 H2 . The combination of measurements of product translational-energy release and photoionization-efficiency spectra indicates productions of3 HC9 H/c-1 HC3 (C)C5 H/c-1 HC7 (C)CH + H and1 HC10 H + H in the two title reactions, which are supported also by quantum-chemical calculations. Ratios branching to the three isomers of C9 H2 remain unknown. This work demonstrates that long carbon-chain molecules (e.g., C9 H2 and C10 H2 ) can be synthesized from reactions of Cm H (e.g., m = 3 and 4) radicals with polyynes (e.g., HC6 H) and gives some valuable implications to planetary, interstellar, and combustion chemistry.