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≡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 Δ) + 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) × 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 VUV 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 -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 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, are discussed.

Product Branching Fractions of the CH + Propene Reaction from Synchrotron Photoionization Mass Spectrometry

“…11,17,18 The experiment comprises a reaction flow tube coupled to a photoionization mass spectrometer.…”

Gas phase reactive collisions, experimental approach

“…A more appropriate tool for this kind of study is light produced from a synchrotron. Recent studies have been undertaken at room temperature coupling a slow flow cell reactor to the ALS synchrotron in Berkeley [127][128][129][130][131][132]. By scanning the VUV light it is possible to detect products according to their ionisation potential which allows one to identify isomers.…”

Communication: On the first ionization threshold of the C2H radical

“…Being a very reactive radical with a C≡ ≡C triple bond, it is involved in the primary steps of the long carbon chain and polycyclic aromatic hydrocarbon (PAH) production. 2, 8 Several experimental and theoretical investigations have been carried out to study the C 2 H radical reactivity, [9][10][11] its absorption spectroscopy, [12][13][14][15][16][17] and its production from acetylene photolysis. [18][19][20][21] Although a few theoretical works have been performed on the C 2 H + cation, [22][23][24][25] the experimental studies of this cation are scarcer.…”