Synchrotron Photoionization Mass Spectrometry Measurements of Kinetics and Product Formation in the Allyl Radical (H2CCHCH2) Self-Reaction
journal contributionposted on 02.10.2008, 00:00 by Talitha M. Selby, Giovanni Meloni, Fabien Goulay, Stephen R. Leone, Askar Fahr, Craig A. Taatjes, David L. Osborn
Product channels for the self-reaction of the resonance-stabilized allyl radical, C3H5 + C3H5, have been studied with isomeric specificity at temperatures from 300−600 K and pressures from 1−6 Torr using time-resolved multiplexed photoionization mass spectrometry. Under these conditions 1,5-hexadiene was the only C6H10 product isomer detected. The lack of isomerization of the C6H10 product is in marked contrast to the C6H6 product in the related C3H3 + C3H3 reaction, and is due to the more saturated electronic structure of the C6H10 system. The disproportionation product channel, yielding allene + propene, was also detected, with an upper limit on the branching fraction relative to recombination of 0.03. Analysis of the allyl radical decay at 298 K yielded a total rate coefficient of (2.7 ± 0.8) × 10−11 cm3 molecule−1 s−1, in good agreement with previous experimental measurements using ultraviolet kinetic absorption spectroscopy and a recent theoretical determination using variable reaction coordinate transition state theory. This result provides independent indirect support for the literature value of the allyl radical ultraviolet absorption cross-section near 223 nm.