Reaction Dynamics of Phenyl Radicals in Extreme Environments: A Crossed Molecular Beam Study

Abstract

Polycyclic aromatic hydrocarbons (PAHs)sorganic compounds that consist of fused benzene ringssand their hydrogendeficient precursors have attracted extensive interest from combustion scientists, organic chemists, astronomers, and planetary scientists. On Earth, PAHs are toxic combustion products and a source of air pollution. In the interstellar medium, research suggests that PAHs play a role in unidentified infrared emission bands, diffuse interstellar bands, and the synthesis of precursor molecules to life. To build clean combustion devices and to understand the astrochemical evolution of the interstellar medium, it will be critical to understand the elementary reaction mechanisms under single collision conditions by which these molecules form in the gas phase.Until recently, this work had been hampered by the difficulty in preparing a large concentration of phenyl radicals, but the phenyl radical represents one of the most important radical species to trigger PAH formation in high-temperature environments. However, we have developed a method for producing these radical species and have undertaken a systematic experimental investigation. In this Account, we report on the chemical dynamics of the phenyl radical (C 6 H 5 ) reactions with the unsaturated hydrocarbons acetylene (C 2 H 2 ), ethylene (C 2 H 4 ), methylacetylene (CH 3 CCH), allene (H 2 CCCH 2 ), propylene (CH 3 CHCH 2 ), and benzene (C 6 H 6 ) utilizing the crossed molecular beams approach. For nonsymmetric reactants such as methylacetylene and propylene, steric effects and the larger cones of acceptance drive the addition of the phenyl radical to the nonsubstituted carbon atom of the hydrocarbon reactant. Reaction intermediates decomposed via atomic hydrogen loss pathways. In the phenyl-propylene system, the longer lifetime of the reaction intermediate yielded a more efficient energy randomization compared with the phenyl-methylacetylene system. Therefore, two reaction channels were open: hydrogen losses from the vinyl and from the methyl groups. All fragmentation pathways involved tight exit transition states. In the range of collision energies investigated, the reactions are dictated by phenyl radical addition-hydrogen atom elimination pathways. We did not observe ring closure processes with the benzene ring.Our investigations present an important step toward a systematic investigation of phenyl radical reactions under single collision conditions similar to those found in combustion flames and in high-temperature interstellar environments. Future experiments at lower collision energies may enhance the lifetimes of the reaction intermediates, which could open up competing ring closure channels to form bicyclic reaction products.

Chemical dynamics of the formation of the ethynylsilylidyne radical (SiCCH(X Π2)) in the crossed beam reaction of ground state silicon atoms (Si(P3)) with acetylene (C2H2(X ∑g+1))

“…16 Pulsed ground state silicon atoms were produced in the primary source by laser ablation of a silicon rod at 266 nm by focusing a few millijoules per pulse on the rotating silicon rod. The ablated species were seeded in neat hydrogen carrier gas ͑Matheson; 99.9999%͒ at 3040 torr backing pressure released by a Proch-Trickl pulsed valve operated at pulse widths of 80 s and Ϫ400 V. After passing a skimmer, a four-slot chopper wheel mounted after the ablation zone selected a part out of the seeded ground state silicon atom beam with a peak velocity of 2329Ϯ 38 m s −1 and a speed ratio of 5.5Ϯ 0.9.…”

A VUV Photoionization Study of the Combustion-Relevant Reaction of the Phenyl Radical (C6H5) with Propylene (C3H6) in a High Temperature Chemical Reactor

“…[11][12][13] In detail, PAHs have been suggested to be formed via hydrogen abstractionacetylene addition (HACA) sequences 14 or via phenyl addition -cyclization pathways (PAC). 15 Alternative pathways have been investigated via kinetic studies [16][17][18][19] and crossed beam experiments [20][21][22][23] of phenyl radicals with unsaturated hydrocarbons. Very recently, indane has been identified as one of the products in a low-pressure premixed toluene/oxygen/ argon fuel rich flames by .…”

Reaction Dynamics of Carbon‐Centered Radicals in Extreme Environments Studied by the Crossed Molecular Beam Technique

“…116 The apparatus consists of two source chambers at a crossing angle of 90 , a stainless steel scattering chamber, and an ultrahigh-vacuum tight, rotatable, differentially pumped quadrupole mass spectrometric (QMS) detector that can be pumped down to a vacuum in the high 10 À13 Torr range ( Fig. 11.1).…”

“…116 Specific details and the methodology of the crossed beam approach are given for the reactions of phenyl radicals with acetylene, ethylene, and benzene as simplest representatives of unsaturated molecules containing triplet, double, and "aromatic" bonds, respectively. Then, the conclusions are transferred to the methylacetylene, allene, and propylene reactants to elucidate generalized concepts on the chemical dynamics, reactivity, and reaction mechanisms of the reactions of phenyl radicals with unsaturated hydrocarbons in extreme environments.…”