Abstract
The crossed molecular beams reactions of the 1-propynyl radical (CH3CC; X2A1) with benzene (C6H6; X1A1g) and D6-benzene (C6D6; X1A1g) were conducted to explore the formation of C9H8 isomers under single-collision conditions. The underlying reaction mechanisms were unravelled through the combination of the experimental data with electronic structure and statistical RRKM calculations. These data suggest the formation of 1-phenyl-1-propyne (C6H5CCCH3) via the barrierless addition of 1-propynyl to benzene forming a low-lying doublet C9H9 intermediate that dissociates by hydrogen atom emission via a tight transition state. In accordance with our experiments, RRKM calculations predict that the thermodynamically most stable isomer – the polycyclic aromatic hydrocarbon (PAH) indene – is not formed via this reaction. With all barriers lying below the energy of the reactants, this reaction is viable in the cold interstellar medium where several methyl-substituted molecules have been detected. Its underlying mechanism therefore advances our understanding of how methyl-substituted hydrocarbons can be formed under extreme conditions such as those found in the molecular cloud TMC-1. Implications for the chemistry of the 1-propynyl radical in astrophysical environments are also discussed.
Highlights
During the last decades, the C9H9 and C9H8 potential energy surfaces (PES) have received considerable attention from the astrochemistry, combustion, and chemical reaction dynamics communities in exploring the formation of indene (C9H8; [1]) along with its 1-phenyl-1-propyne (C6H5CCCH3; [2]), 1-phenyl-1,2-propadiene (C6H5CHCCH2; [3]), and 3-phenyl-1-propyne (C6H5CH2CCH; [4]) isomers in the interstellar medium (ISM) and in combustion systems (Fig. 1)[1,2,3,4,5,6,7,8,9,10,11]
We monitored potential products formed from the reactive scattering of the 1-propynyl radical (CH3CC; 39 amu) with benzene (C6H6; 78 amu) along ΘCM at m/z = 117 (C9H9+) and 116 (C9H8+) to assess the formation of a persistent reaction intermediate and/or hydrogen loss reaction product, respectively
The TOF spectra were normalized with respect to ΘCM and integrated to yield the laboratory angular distribution (Fig. 3), which is nearly symmetric around ΘCM thereby suggesting the presence of indirect reaction dynamics via one or more relatively long-lived C9H9 intermediate(s) preceding dissociation to C9H8 plus atomic hydrogen
Summary
The C9H9 and C9H8 potential energy surfaces (PES) have received considerable attention from the astrochemistry, combustion, and chemical reaction dynamics communities in exploring the formation of indene (C9H8; [1]) along with its 1-phenyl-1-propyne (C6H5CCCH3; [2]), 1-phenyl-1,2-propadiene (C6H5CHCCH2; [3]), and 3-phenyl-1-propyne (C6H5CH2CCH; [4]) isomers in the interstellar medium (ISM) and in combustion systems (Fig. 1)[1,2,3,4,5,6,7,8,9,10,11]. We present the results of crossed molecular beams experiments of the 1-propynyl radical (CH3CC; X2A1) with benzene (C6H6; X1A1g) and D6-benzene (C6D6; X1A1g) and combine these data with novel electronic structure calculations on the C9H9 PES These studies reveal an overall exoergic, but in contrast to previous systems, entrance-barrierless reaction accessing the C9H9 surface via addition of the 1-propynyl radical to the benzene ring leading eventually to the formation of the 1-phenyl-1-propyne product (C6H5CCCH3) along with atomic hydrogen. If formed, the 1-propynyl radical could react rapidly with benzene[27] upon collision to yield 1-phenyl-1-propyne which may be susceptible to observation due to its large dipole moment of 0.48 Debye
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
