Isomer-Specific Product Detection of Gas-Phase Xylyl Radical Rearrangement and Decomposition Using VUV Synchrotron Photoionization

Xylyl radicals are intermediates in combustion processes since their parent molecules, xylenes, are present as fuel additives. In this study we report on the photoelectron spectra of the three isomeric xylyl radicals and the subsequent decomposition reactions of the o-xylyl radical, generated in a tubular reactor and probed by mass selected threshold photoelectron spectroscopy and VUV synchrotron radiation. Franck-Condon simulations are applied to augment the assignment of elusive species. Below 1000 K, o-xylyl radicals decompose by hydrogen atom loss to form closed-shell o-xylylene, which equilibrates with benzocyclobutene. At higher temperatures relevant to combustion engines, o-xylylene generates styrene in a multistep rearrangement, whereas the p-xylylene isomer is thermally stable, a key point of difference in the combustion of these two isomeric fuels. Another striking result is that all three xylyl isomers can generate p-xylylene upon decomposition. In addition to C8H8 isomers, phenylacetylene and traces of benzocyclobutadiene are observed and identified as further reaction products of o-xylylene, while there is also some preliminary evidence for benzene and benzyne formation. The experimental results reported here are complemented by a comprehensive theoretical C8H8 potential energy surface, which together with the spectroscopic assignments can explain the complex high-temperature chemistry of o-xylyl radicals.