A combined crossed molecular beam and theoretical investigation of the reaction of the meta-tolyl radical with vinylacetylene – toward the formation of methylnaphthalenes

Abstract

Crossed molecular beam experiments and electronic structure calculations on the reaction of the meta-tolyl radical with vinylacetylene were conducted to probe the formation of methyl-substituted naphthalene isomers. We present the compelling evidence that under single collision conditions 1- and 2-methylnaphthalene can be formed without an entrance barrier via indirect scattering dynamics through a bimolecular collision of two non-PAH reactants: the meta-tolyl radical and vinylacetylene. The electronic structure calculations, conducted at the UCCSD(T)-F12b/cc-pVDZ//UM06-2x/cc-pVTZ + ZPE(UM06-2x/cc-pVTZ) level of theory, reveal that this reaction is initiated by the barrierless addition of the meta-tolyl radical to the terminal vinyl carbon (C1) of vinylacetylene, via a van-der-Waals complex implying that this mechanism can play a key role in forming methyl-substituted PAHs in low temperature extreme environments such as the low temperature interstellar medium and hydrocarbon-rich atmospheres of planets and their moons in the outer solar system. The reaction mechanism, proposed from the C11H11 potential energy surface, involves a sequence of isomerizations involving hydrogen transfer and ring closure, followed by hydrogen dissociation, which eventually leads to 1- and 2-methylnaphthalene in an overall exoergic process.

LOW TEMPERATURE FORMATION OF NITROGEN-SUBSTITUTED POLYCYCLIC AROMATIC HYDROCARBONS (PANHs)—BARRIERLESS ROUTES TO DIHYDRO(iso)QUINOLINES

“…Therefore, at very low temperatures of cold molecular clouds, such as in TMC-1 with average temperatures of about 10 K, the reaction will still be able to proceed. These reactions form part of a growing group of de facto barrierlesstype reactions, leading to aromatization to PAH-type species at ultralow temperatures, involving aromatic radicals with the radical located on the aromatic ring that undergo addition to a C4 double conjugated unsaturated hydrocarbon such as 1,3butadiene (C 4 H 6 ) or vinylacetylene (C 4 H 4 ; Kaiser et al 2012;Parker et al 2012aParker et al , 2014aParker et al , 2014bParker et al , 2015Dangi et al 2014;Kaiser et al 2015;Muzangwa et al 2015;Yang et al 2015). Previously, aromatization reactions had been calculated to proceed over an energy barrier implying that they could only be formed in high temperature environments such as in the inner regions of circumstellar envelopes of dying carbon stars like IRC+10216 to supply the necessary kinetic transitional energy to overcome the barriers (Tielens et al 2000;Kislov et al 2005;Tielens 2008;Cherchneff 2011).…”

On the formation and the isomer specific detection of methylacetylene (CH3CCH), propene (CH3CHCH2), cyclopropane (c-C3H6), vinylacetylene (CH2CHCCH), and 1,3-butadiene (CH2CHCHCH2) from interstellar methane ice analogues

“…Vinylacetylene can react with the phenyl radical producing the prototypical PAH known as naphthalene (C 10 H 8 ), 18,135 as well as with the para-tolyl radical (C 6 H 4 CH 3 ) forming 2-methylnaphthalene (C 11 H 10 ), 19 and finally with the meta-tolyl radical (C 6 H 4 CH 3 ) creating 1-methylnaphthalene (C 11 H 10 ). 20 Similarly, 1,3-butadiene forms multiple PAH precursors as well as PAH molecules. Intriguingly, the reaction of 1,3-butadiene with the ethynyl radical produces benzene, and the injection of 1,3-butadiene from dust grains greatly improves model abundances of benzene again showing a link between the solid and the gas phase.…”

“…[12][13][14] Also, vinylacetylene (CH 2 CHCCH) has been suggested as a hydrocarbon important for Titan's chemistry, 11,15,16 and forms prototype PAHs such as naphthalene (C 10 H 8 ), 1-and 2-methylnaphthalene (C 11 H 10 ), and ortho-benzyne (o-C 6 H 4 ) in the gas phase. [17][18][19][20] Finally, 1,3-butadiene (CH 2 CHCHCH 2 ) is also fundamental as an essential building block in the gas-phase growth of prototypical PAHs through reactions with dicarbon (C 2 ) to form the phenyl radical (C 6 H 5 ), with the tolyl radical (C 6 H 4 CH 3 ) forming 6-methyl-1,4-dihydronaphthalene (C 11 H 12 ), and with the ethynyl radical (CCH) producing benzene (C 6 H 6 ). [21][22][23] The C 3 H 4 isomer methylacetylene (CH 3 CCH) was first observed towards Sagittarius B2 and Orion A via the 5 0 -4 0 transitions at 85 GHz.…”

Formation of bicyclic polycyclic aromatic hydrocarbons (PAHs) from the reaction of a phenyl radical with cis-3-penten-1-yne

“…The nal single-point energies of all species were gained using the 23,25 which is expected to generate relative energies of various species within the accuracy of 1-2 kcal mol À1 , 51 and extensively used to study PAH growth mechanism. 8,9,18,[21][22][23] The majority of the intermediate species and transition structures found in this study were either closed shell singlets or open shell doubles. The diradical species were found by using triplet spin multiplicity, and the calculations of diradical species contain negligible spin contamination.…”

“…Structurally, CH^C-CH]CH-CH 3 is the methylsubstituted product of vinylacetylene (CHCCHCH 2 ), which can form naphthalene via the bimolecular reactions with para-tolyl and phenyl radicals without entrance barriers under single collision conditions. [21][22][23] Thus, it is meaningful to investigate the reaction of phenyl radical with CHCCHCHCH 3 .…”