Reaction of the C2H Radical with 1-Butyne (C4H6): Low-Temperature Kinetics and Isomer-Specific Product Detection

Abstract

The rate coefficient for the reaction of the ethynyl radical (C(2)H) with 1-butyne (H-C[triple bond]C-CH(2)-CH(3)) is measured in a pulsed Laval nozzle apparatus. Ethynyl radicals are formed by laser photolysis of acetylene (C(2)H(2)) at 193 nm and detected via chemiluminescence (C(2)H + O(2) --> CH (A(2)Delta) + CO(2)). The rate coefficients are measured over the temperature range of 74-295 K. The C(2)H + 1-butyne reaction exhibits no barrier and occurs with rate constants close to the collision limit. The temperature-dependent rate coefficients can be fit within experimental uncertainties by the expression k = (2.4 +/- 0.5) x 10(-10)(T/295 K)(-(0.04+/-0.03)) cm(3) molecule(-1) s(-1). Reaction products are detected at room temperature (295 K) and 533 Pa using a multiplexed photoionization mass spectrometer (MPIMS) coupled to the tunable vacuum ultraviolet synchrotron radiation from the Advanced Light Source at the Lawrence Berkeley National Laboratory. Two product channels are identified for this reaction: m/z = 64 (C(5)H(4)) and m/z = 78 (C(6)H(6)) corresponding to the CH(3)-loss and H-loss channels, respectively. Photoionization efficiency (PIE) curves are used to analyze the isomeric composition of both product channels. The C(5)H(4) products are found to be exclusively linear isomers composed of ethynylallene and methyldiacetylene in a 4:1 ratio. In contrast, the C(6)H(6) product channel includes two cyclic isomers, fulvene 18(+/-5)% and 3,4-dimethylenecyclobut-1-ene (DMCB) 32(+/-8)%, as well as three linear isomers, 2-ethynyl-1,3-butadiene 8(+/-5)%, 3,4-hexadiene-1-yne 28(+/-8)%, and 1,3-hexadiyne 14(+/-5)%. Within experimental uncertainties, we do not see appreciable amounts of benzene and an upper limit of 10% is estimated. Diacetylene (C(4)H(2)) formation via the C(2)H(5)-loss channel is also thermodynamically possible but cannot be observed due to experimental limitations. The implications of these results for modeling of planetary atmospheres, especially of Saturn's largest moon Titan and the relationships to combustion reactions, are discussed.