Direct Observation of <i>para</i>-Xylylene as the Decomposition Product of the <i>meta</i>-Xylyl Radical Using VUV Synchrotron Radiation

Abstract

Xylyl (methylbenzyl) radicals are important combustion intermediates, formed in the pyrolysis and oxidation of xylenes and other substituted aromatic fuel additives. We have used VUV synchrotron radiation and imaging photoelectron photoion coincidence (iPEPICO) spectroscopy techniques to identify <em>para</em>-xylylene as the dominant stable C₈H₈ product arising from thermal decomposition of the <em>meta</em>-xylyl radical. A complex rearrangement from a <em>meta</em>- to a <em>para</em>-substituted aromatic, supported by quantum chemical calculations, can rationalize the observed reaction products. This work provides the first experimental evidence for the pyrolysis products of the <em>meta</em>-xylyl radical and can explain why the decomposition of this radical is considerably slower than that of the <em>ortho </em>and <em>para </em>isomers. This study emphasizes the utility of VUV synchrotron radiation and iPEPICO spectroscopy to tackle the reaction mechanism of combustion-relevant processes.