Formation of C3H2, C5H2, C7H2, and C9H2 from reactions of CH, C3H, C5H, and C7H radicals with C2H2

Abstract

The C_m+2H₂ family can be classified into two categories – C_2n+1H₂ and C_2n+2H₂. C_m+2H₂ are important intermediates in the syntheses of large carbonaceous molecules. An understanding of the formation mechanisms of both odd and even carbon-numbered C_m+2H₂ is beneficial to atmospheric, astronomical, and combustion chemistry. HC_2n+2H (polyynes) are believed to be producible from C_2nH + C₂H₂ and C₂H + C_2nH₂ reactions but C_2n+1H₂ (n ≥ 2) attract less attention to their formation mechanisms. In the present study, we make up for the lack of knowledge on C_2n+1H₂ formation mechanisms by investigating the reactions C_2n−1H + C₂H₂ → C_2n+1H₂ + H with n = 1–4. The dynamics of reactions of C_2n−1H radicals with C₂H₂ are explored in crossed molecular beams using products C_2n+1H₂. The translational-energies and angular distributions of the hydrogen-loss channels of products are unraveled by measuring time-of-flight spectra and photoionization-efficiency spectra of C_2n+1H₂ with tunable synchrotron vacuum-ultraviolet ionization. The C_2n+1H₂ product includes two isomers, c-¹HC_2n−1(C)CH and ³HC_2n+1H, which are identified by the maximal translational-energy release and the photoionization threshold. Furthermore, quantum-chemical calculations indicate that the title reactions incur a small or negligible entrance barrier and are nearly isoergic except for the barrierless exothermic reaction CH + C₂H₂ → C₃H₂ + H. We demonstrate for the first time that C₅H₂, C₇H₂, and C₉H₂ are producible from the title reactions. In conjunction with studies on the C_2nH + C₂H₂ reactions, a brief picture for the C_mH (m = 1–8) + C₂H₂ → C_m+2H₂ + H reactions can be outlined.