A crossed molecular beam study on the formation of hexenediynyl radicals (H2CCCCCCH; C6H3 (X2A′)) via reactions of tricarbon molecules, C3(X1Σg+), with allene (H2CCCH2; X1A1) and methylacetylene (CH3CCH; X1A1)

Abstract

Crossed molecular beams experiments have been utilized to investigate the reaction dynamics between two closed shell species, i.e. the reactions of tricarbon molecules, C₃(X¹Σ_g⁺), with allene (H₂CCCH₂; X¹A₁), and with methylacetylene (CH₃CCH; X¹A₁). Our investigations indicated that both these reactions featured characteristic threshold energies of 40–50 kJ mol⁻¹. The reaction dynamics are indirect and suggested the reactions proceeded via an initial addition of the tricarbon molecule to the unsaturated hydrocarbon molecules forming initially cyclic reaction intermediates of the generic formula C₆H₄. The cyclic intermediates isomerize to yield eventually the acyclic isomers CH₃CCCCCH (methylacetylene reaction) and H₂CCCCCCH₂ (allene reaction). Both structures decompose via atomic hydrogen elimination to form the 1-hexene-3,4-diynyl-2 radical (C₆H₃; H₂CCCCCCH). Future flame studies utilizing the Advanced Light Source should therefore investigate the existence of 1-hexene-3,4-diynyl-2 radicals in high temperature methylacetylene and allene flames. Since the corresponding C₃H₃, C₄H₃, and C₅H₃ radicals have been identified via their ionization potentials in combustion flames, the existence of the C₆H₃ isomer 1-hexene-3,4-diynyl-2 can be predicted as well.