Crossed-molecular-beam study on the formation of phenylacetylene from phenyl radicals and acetylene.

During the last decades, polycyclic aromatic hydrocarbons (PAHs) and other related aromatic compounds, such as ionized PAHs, have received considerable attention from astronomers, astrobiologists, environmentalists, and the combustion community. In the interstellar medium, PAHlike species account for up to 10% of the cosmic carbon budget, have been suggested as carriers of both the unidentified infrared (UIR) emission bands and the diffuse interstellar bands (DIBs), and are also associated with the origin of life. On Earth, however, PAHs are severe pollutants and are considered as airborne toxic chemicals because of their mutagenic and carcinogenic character. Considering a yearly emission rate of 1.6 million tons in combustion processes, PAHs and soot also contribute significantly to global warming. A quantitative understanding of the formation of PAHs is therefore essential to develop clean combustion devices and understand the astrochemical and astrobiological evolution of the interstellar medium. Despite the crucial importance of PAHs, even the formation route of the very first building block of this type of compounds (namely, the phenylacetylene molecule, C6H5CCH) is unknown. Chemical reaction networks that model the formation of PAH in combustion flames and in the interstellar medium imply that the phenyl radical (that is, C6H5) represents the most important transient species to initiate the formation of the very first PAH. The crucial steps of these reaction models are hydrogen abstraction– acetylene addition sequences. The chemical networks concur that the reaction of the phenyl radical with acetylene initiates the synthesis of PAH through the addition of phenyl to the acetylene molecule. Because of the central role of the phenyl– acetylene reaction, the kinetics of this system have been wellestablished (covering the temperature range up to 1500 K). Theoretical studies of this benchmark system predicted the initial formation of a [C8H7]* adduct which either decomposed back to the reactants or fragmented into the phenylacetylene molecule (C6H5CCH) plus atomic hydrogen [Eq. (1)]. Nevertheless, despite the central role of the C6H5ðXA1Þ þ C2H2ðX X g þÞ ! 1⁄2C8H7 * ! C6H5CCHðXA1Þ þ HðS1=2Þ ð1Þ phenyl–acetylene reaction as the trigger to PAH formation, the theoretical investigations have never been verified experimentally under single-collision conditions. Consequently, the nature of the true reaction products of this elementary reaction has remained elusive to date. To shed light on this fundamental question, we conducted a crossedmolecular-beam study of the gas-phase reaction of the phenyl radical with acetylene to synthesize phenylacetylene plus atomic hydrogen via [C8H7]* intermediate(s). This system presents the prototype reaction of an aromatic radical with ubiquitous acetylene molecules to form phenylacetylene by means of a single radical–neutral collision. Benzene, a likely precursor of the phenyl radical, has been observed together with acetylene, for example, toward the carbon-rich planetary nebula CRL 618, where it might be photodissociated to form the phenyl radical. On the other hand, the phenyl radical might result from benzene in combustion flames through hydrogen abstraction processes. The reaction is also interesting from the viewpoint of a physical–organic chemist as it represents a benchmark system to unravel the chemistry of radical-substitution reactions initiated by polyatomic organic radicals. The reactive-scattering signal was recorded at mass-tocharge ratios (m/z) between m/z 102 (C8H6 ) and m/z 98 (C8H2 ) (at distinct laboratory angles). The time-of-flight (TOF) spectra monitored at m/z 101–98 were superimposable—at all angles (and after scaling)—to those taken at m/z 98 (Figure 1). This represents a crucial finding and suggests that the phenyl-versus-atomic-hydrogen exchange pathway to form the C8H6 isomer(s) is open and that the signal at lower m/z values originates from the dissociative ionization of the reaction product in the electron-impact ionizer. We also detected a signal at m/z 103. Since the patterns of the TOF spectra were identical to those recorded at m/z 102 and the intensity of the ion counts at m/z 103 was only about 10% of that observed at m/z 102, we concluded that the signal at m/z 103 originates from the ionized C isotopomer of C8H6, that is, from CC7H6, but not from the C8H7 adduct. The TOF spectra and laboratory angular distributions of the reactive-scattering signal at m/z 102 (namely, of C8H6) are shown in Figure 1. To pin down whether the hydrogen atom is lost from the phenyl group or from the acetylenic unit, we conducted the crossed-beam reaction of phenyl radicals with fully deuterated acetylene. In this case, the loss of a hydrogen atom from the phenyl group should result in a signal at m/z 104 (C8H4D2 ), while the release of atomic deuterium is expected to be monitored at m/z 103 (C8H5D ). We observed signals at m/z 103 and 104. However, both TOF spectra were superimposable, and the ion counts at m/z 104 were only about 10% of those at m/z 103. This result indicates that [*] Dr. X. Gu, Dr. F. Zhang, Dr. Y. Guo, Prof. R. I. Kaiser Department of Chemistry University of Hawaii at Manoa Honolulu, HI 96822 (USA) Fax: (+1)808-956-5908 E-mail: ralfk@hawaii.edu Homepage: http://www.chem.hawaii.edu/Bil301/welcome.html