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

Abstract

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.

Insights in m-xylene decomposition under fuel-rich conditions by imaging photoelectron photoion coincidence spectroscopy

“…Finally, the mechanism of Gaïl and Dagaut [10] was developed from a previous reaction mechanism of the oxidation of p -xylene [9] and was also tested against low-pressure measurements. A rearrangement of the m -xylyl radical, which was observed in [14,15] and will be discussed later, is not included in any of the models. In [8,10,26] the isomerization is implied by considering the direct formation of p -xylylene from the m -xylyl radical ( m -CH 3 C 6 H 4 CH 2 = p -CH 2 C 6 H 4 CH 2 + H).…”

“…This radical can be clearly identified from the measured TPES in the reaction zone. Figure 3 shows the TPE signal of m/z = 105, measured at HAB = 6.5 mm, in comparison to the Franck-Condon simulation from [15] . isomers obtained from [15] are indicated.…”

“…Figure 3 shows the TPE signal of m/z = 105, measured at HAB = 6.5 mm, in comparison to the Franck-Condon simulation from [15] . isomers obtained from [15] are indicated. The sharp peak in our measured TPES fits perfectly to the adiabatic ionization energy of m -xylyl (7.11 eV) and also matches the calculated TPES very well.…”

“…Mouis et al [13] investigated the effect of m -xylene on soot in laminar ethylene co-flow diffusion flames from 1 to 5 atm. The pyrolysis of the xylyl radicals, as the first decomposition products of xylenes in combustion processes, was studied in detail experimentally by iPEPICO from the pyrolysis of xylyl bromides in a SiC reactor by Hemberger et al [14,15] and theoretically in [16,17] .…”

Insights into gas-phase reaction mechanisms of small carbon radicals using isomer-resolved product detection

“…A good example of this was the recent study of xylyl radical pyrolysis. 46 The article by Bodi et al discusses the virtues of the iPEPICO method for isomer-specific detection of gas-phase mixtures. 45 Laboratory-based tabletop techniques continue to develop for the isomer-specific product detection of gas-phase species.…”