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

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• 作者：Satchin Soorkia ; Adam J. Trevitt ; Talitha M. Selby ; David L. Osborn ; Craig A. Taatjes ; Kevin R. Wilson ; Stephen R. Leone
• 刊名：Journal of Physical Chemistry A
• 出版年：2010
• 出版时间：March 11, 2010
• 年：2010
• 卷：114
• 期：9
• 页码：3340-3354
• 全文大小：542K
• 年卷期：v.114,no.9(March 11, 2010)
• ISSN：1520-5215

文摘

The rate coefficient for the reaction of the ethynyl radical (C₂H) with 1-butyne (H—C≡C—CH₂—CH₃) is measured in a pulsed Laval nozzle apparatus. Ethynyl radicals are formed by laser photolysis of acetylene (C₂H₂) at 193 nm and detected via chemiluminescence (C₂H + O₂ → CH (A²Δ) + CO₂). The rate coefficients are measured over the temperature range of 74−295 K. The C₂H + 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) × 10⁻¹⁰(T/295 K)^{−(0.04±0.03)} cm³ 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 (C₅H₄) and m/z = 78 (C₆H₆) corresponding to the CH₃-loss and H-loss channels, respectively. Photoionization efficiency (PIE) curves are used to analyze the isomeric composition of both product channels. The C₅H₄ products are found to be exclusively linear isomers composed of ethynylallene and methyldiacetylene in a 4:1 ratio. In contrast, the C₆H₆ 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 (C₄H₂) formation via the C₂H₅-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.