Crossed Molecular Beam Study on the Formation of Phenylacetylene and Its Relevance to Titan’s Atmosphere

Abstract

The crossed molecular beam experiment of the deuterated ethynyl radical (C 2 D; X 2 Σ + ) with benzene [C 6 H 6 (X 1 A 1g )] and its fully deuterated analog [C 6 D 6 (X 1 A 1g )] was conducted at a collision energy of 58.1 kJ mol -1 . Our experimental data suggest the formation of the phenylacetylene-d 6 via indirect reactive scattering dynamics through a long-lived reaction intermediate; the reaction is initiated by a barrierless addition of the ethynyl-d 1 radical to benzene-d 6 . This initial collision complex was found to decompose via a tight exit transition state located about 42 kJ mol -1 above the separated products; here, the deuterium atom is ejected almost perpendicularly to the rotational plane of the decomposing intermediate and almost parallel to the total angular momentum vector. The overall experimental exoergicity of the reaction is shown to be 121 ( 10 kJ mol -1 ; this compares nicely with the computed reaction energy of -111 kJ mol -1 . Even though the experiment was conducted at a collisional energy higher than equivalent temperatures typically found in the atmosphere of Titan (94 K and higher), the reaction may proceed in Titan's atmosphere as it involves no entrance barrier, all transition states involved are below the energy of the separated reactants, and the reaction is exoergic. Further, the phenylacetylene was found to be the sole reaction product.

Formation of benzene in the interstellar medium

“…Because the hydrogen atoms in 1,3-butadiene can be replaced by organic side groups, the reaction of ethynyl with 1,3-butadiene presents the simplest representative of a reaction class in which aromatic molecules with a benzene core can be formed from acyclic precursors via barrierless reactions of the ethynyl radicals with substituted 1,3-butadiene molecules. Electronic structure calculations predicted further that the phenylacetylene molecule (C 6 H 5 CCH), synthesized from exoergic, barrierless reactions of benzene with the ethynyl radical (41,42), can even react with a second ethynyl radical to form 1,2-diethynylbenzene [C 6 H 4 ðC 2 HÞ 2 ] plus a hydrogen atom. The reaction of 1,2-diethynylbenzene with a third ethynyl radical in turn produces an intermediate, which isomerizes via ring closure and emits atomic hydrogen to yield a dehydrogenated, aromatic, and bicyclic naphthalene core.…”

The millimetre-wave rotational spectrum of phenylacetylene

“…Phenylacetylene is a fundamental hydrocarbon derivative of benzene and is implicated in chemical reaction networks associated with formation of polyaromatic hydrocarbons in combustion and in the interstellar medium (ISM) [1]. Many different studies on the formation of phenylacetylene at astrophysically relevant conditions have been carried out.…”

“…Many different studies on the formation of phenylacetylene at astrophysically relevant conditions have been carried out. It can be formed from benzene by impact shock [2] or collisions with the ethynyl radical [1]. It can also be formed by photochemical reactions from acetylene [3], vinylacetylene [4], or 1,3-butadiene [5].…”

“…Phenylacetylene has been formed in a mixture of gases simulating the atmosphere of Saturn's moon Titan [6], and studies in Refs. [1,4,5] were, in fact, also motivated by the desire to explore the rich hydrocarbon chemistry expected to be present on Titan. Phenylacetylene is a polar molecule and is thus attractive as a potential radioastronomy marker of unsaturated, or even polyaromatic hydrocarbon chemistry, which is postulated to be relevant to the ISM on the basis of infrared spectroscopic signatures [7].…”

Tuning the Ground State Symmetry of Acetylenyl Radicals

“…1,2 Homolytic CH cleavage leads to the acetylenyl radical HCC, a molecule that is a key intermediate in acetylene combustion 3,4 and has also been observed in a cold matrix, 5−7 in molecular beams, 8−10 and in the interstellar medium. 11−14 In astrochemistry and astrophysics, HCC serves as a detective species for carbon-rich environments.…”