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

Product detection studies of C(3 P) atom reactions with butene (C4 H8 ) 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(3 P) + trans-but-2-ene and C(3 P) + cis-but-2-ene reactions, various isomers of C4 H5 and C5 H7 are identified as reaction products formed via CH3 and H elimination. Assuming equal ionization cross sections for all C4 H5 and C5 H7 isomers, C4 H5 :C5 H7 branching ratios of 0.63:1 and 0.60:1 are derived for the C(3 P) + trans-but-2-ene and the C(3 P) + cis-but-2-ene reactions, respectively. For the C(3 P) + but-1-ene reaction, two reaction channels are observed: the H-elimination channel, leading to the formation of the ethylpropargyl isomer, and the C3 H3 + C2 H5 channel. Assuming equal ionization cross sections for ethylpropargyl and C3 H3 radicals, a branching ratio of 1:0.95 for the C3 H3 + C2 H5 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.