Reactions of Atomic Carbon with Butene Isomers: Implications for Molecular Growth in Carbon-Rich Environments

Product detection studies of C(³P) atom reactions with butene (C₄H₈) isomers (but-1-ene, cis-but-2-ene, trans-but-2-ene) are carried out in a flow tube reactor at 353 K and 4 Torr under multiple collision conditions. Ground state carbon atoms are generated by 248 nm laser photolysis of tetrabromomethane, CBr₄, in a buffer of helium. Thermalized reaction products are detected using synchrotron tunable VUV photoionization and time-of-flight mass spectrometry. The temporal profiles of the detected ions are used to discriminate products from side or secondary reactions. For the C(³P) + trans-but-2-ene and C(³P) + cis-but-2-ene reactions, various isomers of C₄H₅ and C₅H₇ are identified as reaction products formed via CH₃ and H elimination. Assuming equal ionization cross sections for all C₄H₅ and C₅H₇ isomers, C₄H₅:C₅H₇ branching ratios of 0.63:1 and 0.60:1 are derived for the C(³P) + trans-but-2-ene and the C(³P) + cis-but-2-ene reactions, respectively. For the C(³P) + but-1-ene reaction, two reaction channels are observed: the H-elimination channel, leading to the formation of the ethylpropargyl isomer, and the C₃H₃ + C₂H₅ channel. Assuming equal ionization cross sections for ethylpropargyl and C₃H₃ radicals, a branching ratio of 1:0.95 for the C₃H₃ + C₂H₅ and H + ethylpropargyl channels is derived. The experimental results are compared to previous H atom branching ratios and used to propose the most likely mechanisms for the reaction of ground state carbon atoms with butene isomers.