Phenylpropargyl Radicals and Their Dimerization Products: An IR/UV Double Resonance Study

Abstract

Two C(9)H(7) isomers, 1-phenylpropargyl and 3-phenylpropargyl, have been studied by IR/UV double resonance spectroscopy in a free jet. The species are possible intermediates in the formation of soot and polycyclic aromatic hydrocarbons (PAH). The radicals are generated by flash pyrolysis from the corresponding bromides and ionized at 255-297 nm in a one-color, two-photon process. Mid-infrared radiation between 500 and 1800 cm(-1) is provided by a free electron laser (FEL). It is shown that the two radicals can be distinguished by their infrared spectra. In addition, we studied the dimerization products originating from the phenylpropargyl self-reaction. We utilize the fact that the pyrolysis tube can be considered to be a flow reactor permitting us to investigate the chemistry in such a thermal reactor. Dimerization of phenylpropargyl produces predominately species with m/z = 228 and 230. A surprisingly high selectivity has been found: The species with m/z = 230 is identified to be para-terphenyl, whereas m/z = 228 can be assigned to 1-phenylethynyl-naphthalene. The results allow to derive a mechanism for the dimerization of phenylpropargyl and suggest hitherto unexplored pathways to the formation of soot and PAH.

Formation of polycyclic aromatic hydrocarbons from bimolecular reactions of phenyl radicals at high temperatures

“…Interestingly, the best match to the experimental spectrum is provided by para-terphenyl. A conventional IR spectrum of para-terphenyl recorded in a heated gas cell for comparison 28 is depicted in the second trace from top and matches the present spectrum in terms of both band positions and intensities. The computed spectra for the three isomers para-(p-), meta-(m-) and ortho-(o-) terphenyl are given in the lower traces of Fig.…”

“…The condensation of phenyl radicals leads predominately to p-terphenyl, and thus to the same product as the dimerization of the previously investigated 1-and 3-phenylpropargyl radicals. 28 Shukla and Koshi assumed o-terphenyl to be the dominant product of phenyl condensation, but the assignment was based on chemical arguments rather than structural information. 12 Comandini et al reported on the other hand the presence of all isomers.…”

Dimerization of the Benzyl Radical in a High‐Temperature Pyrolysis Reactor Investigated by IR/UV Ion Dip Spectroscopy

“…This methodology has previously been successfully employed to study av ariety of different system, for example, biomolecules, [33] hydrated aromatic molecules, [34] and radicals. [16,17] Af requency-doubled Nd:YAG pumped dye laser ionized molecules in the jet via [1+ +1] REMPI. The UV wavelength was fixed at 265 nm by using frequency-doubled Coumarin 153 with 0.65 mJ of power and ar epetition rate of 20 Hz.…”

“…In previouse xperiments, we investigated bimolecular reactions of propargyl, [15] phenyl, [16] and phenylpropargyl radicals [17] and observedf ormation of PAHs with up to three aromatic rings.…”

On the formation of cyclopentadiene in the C3H5˙ + C2H2 reaction

“…It is now widely believed that the main reaction path leading to the first aromatic ring species proceeds via the condensation of two resonantly stabilized propargyl (C 3 H 3 ) radicals. 20 Reactions of the resonantly stabilized cyclopentadienyl (c-C 5 H 5 , CPDyl) radical with a variety of unsaturated hydrocarbons have gained increased attention. [12][13][14][15][16] After the first ring has formed, PAH growth can proceed via subsequent addition of C 2 H 2 groups in the hydrogen abstraction carbon addition (HACA) mechanism, or via the addition of larger hydrocarbon species, such as phenyl radicals.…”