Rate constants and the H atom branching ratio of the reactions of the methylidyne CH(X2Π) radical with C2H2, C2H4, C3H4 (methylacetylene and allene), C3H6 (propene) and C4H8 (trans-butene)

The reactions of the CH radical with several unsaturated hydrocarbons C2H2 (acetylene), C2H4 (ethylene), C3H4 (methyl-acetylene and allene), C3H6 (propene) and C4H8 (trans-butene) were studied at room temperature, in a low-pressure fast-flow reactor. CH(X2Π, v = 0) radicals were obtained from the reaction of CHBr3 with potassium atoms. The overall rate constants at 300 K are CH + C2H2: (3.6 ± 0.6) × 10−10, CH + C2H4: (3.1 ± 0.6) × 10−10, CH + C3H4 (methyl-acetylene): (3.4 ± 0.6) × 10−10, CH + C3H4 (allene): (3.6 ± 0.6) × 10−10, CH + C3H6 (propene): (4.2 ± 0.8) × 10−10 and CH + C4H8 (trans-butene): (4.0 ± 0.80) × 10−10 cm3 molecule−1 s−1 (errors are cited at the level of ±1σ). Absolute atomic hydrogen production was determined by vacuum ultra-violet (VUV) resonance fluorescence, H production from the CH + CH4 reaction being used as a reference. Observed H branching ratios for these CH reactions were: C2H2: 0.90 ± 0.08, C2H4: 0.94 ± 0.08, C3H4 (methyl-acetylene): 0.98 ± 0.08, C3H4 (allene): 0.97 ± 0.08, C3H6 (propene): 0.78 ± 0.10, C4H8 (trans-butene): 0.69 ± 0.12 (errors are cited at the level of ±1σ). A compilation of the available kinetic data on these reactions has been made in order to propose rate coefficients for each possible channel of the different reactions for astrochemical models.