An Experimental and Theoretical Study on the Formation of 2-Methylnaphthalene (C₁₁H₁₀/C₁₁H₃D₇) in the Reactions of the Para-Tolyl (C₇H₇) and Para-Tolyl-d7 (C₇D₇) with Vinylacetylene (C₄H₄)

Abstract

We present for the very first time single collision experimental evidence that a methyl-substituted polycyclic aromatic hydrocarbon (PAH)-2-methylnaphthalene-can be formed without an entrance barrier via indirect scattering dynamics through a bimolecular collision of two non-PAH reactants: the para-tolyl radical and vinylacetylene. Theory shows that this reaction is initiated by the addition of the para-tolyl radical to either the terminal acetylene carbon (C(4)) or a vinyl carbon (C(1)) leading eventually to two distinct radical intermediates. Importantly, addition at C(1) was found to be barrierless via a van der Waals complex implying this mechanism can play a key role in forming methyl substituted PAHs in low temperature extreme environments such as the interstellar medium and hydrocarbon-rich atmospheres of planets and their moons in the outer Solar System. Both reaction pathways involve a sequence of isomerizations via hydrogen transfer, ring closure, ring-opening and final hydrogen dissociation through tight exit transition states to form 2-methylnaphthalene in an overall exoergic process. Less favorable pathways leading to monocyclic products are also found. Our studies predict that reactions of substituted aromatic radicals can mechanistically deliver odd-numbered PAHs which are formed in significant quantities in the combustion of fossil fuels.

A crossed molecular beam and ab initio study on the formation of 5- and 6-methyl-1,4-dihydronaphthalene (C11H12) via the reaction of meta-tolyl (C7H7) with 1,3-butadiene (C4H6)

“…The reaction leading to indene has an entrance barrier of about 14 kJ mol À1 , while the reactions of phenyl radicals with vinylacetylene and 1,3-butadiene involve submerged barriers implying naphthalene (C 10 H 8 ) and 1,4-dihydro-naphthalene (C 10 H 10 ) can be formed in low temperature interstellar environments, as low as 10 K, such as in molecular clouds. 32 Here, we expand these studies and apply crossed molecular beams and ab initio calculations to investigate the formation routes of methylsubstituted PAHs through the use of methyl-substituted aromatic radicals. [21][22][23] Recent studies in our group provide compelling evidence of the formation of dimethyldihydronaphthalenes via the reaction of the para-tolyl radicals (C 6 H 4 CH 3 ) with isoprene (C 5 H 8 ); 15 subsequent studies also proposed a novel route to form methylsubstituted naphthalene via the reaction of the para-tolyl radicals (C 6 H 4 CH 3 ) with vinylacetylene (CH 2 CHCCH).…”

Reaction Dynamics of the 4-Methylphenyl Radical (p-Tolyl) with 1,2-Butadiene (1-Methylallene): Are Methyl Groups Purely Spectators?

“…Note that these pathways involve moderate barriers to addition of magnitude 8−9 kJ mol −1 , which are slightly above the energies of the separated reactants. However, within the accuracy of our calculations of ±10 kJ mol −1 , this barrier might drop below the energies of the separated reactants, possibly resulting in a submerged barrier as seen for the phenyl−vinylacetylene, 21 4-tolyl−vinylacetylene, 26 and phenyl −1,3-butadiene 44 systems. These intermediates can either undergo unimolecular decomposition by eliminating an atomic hydrogen and/or a methyl group or isomerize prior to further decomposition.…”

“…23,24 Experiments carried out under bulk conditions showed that the reaction of the C 7 H 7 radical with the methyl radical produces styrene (C 8 H 8 ) and molecular hydrogen. 25 However, with the exception of the reaction of 4-tolyl (p-tolyl) radicals with vinylacetylene (C 4 H 4 ), which leads to the formation of 2-methylnapthalene, 26 reactions of any C 7 H 7 isomer with unsaturated hydrocarbons under single collision conditions have been elusive. In this paper, we report on the reaction of the 4-methylphenyl (4-tolyl) radical with 1,2-butadiene [H 2 CCCH(CH 3 )] and elucidate to what extent substituted PAHs can be formed under single collision conditions.…”

Reaction Dynamics in Astrochemistry: Low-Temperature Pathways to Polycyclic Aromatic Hydrocarbons in the Interstellar Medium

“…Bicyclic aromatic molecules and their methyl-substituted counterparts formed in exoergic and de facto barrierless bimolecular reactions of phenyl-type radicals with vinylacetylene (C 4 H 4 ), 1,3-butadiene (C 4 H 6 ), and 1-/2-methyl-1,3-butadiene (C 5 H 8 ): (6) naphthalene (C 10 H 8 ; phenyl-vinylacetylene), (7) 1-methylnaphthalene (C 10 H 7 CH 3 ; meta-tolyl-vinylacetylene), (8) 2-methylnaphthalene (C 10 H 7 CH 3 ; meta/para-tolyl-vinylacetylene), (9) 1,4-dihydronaphthalene (C 10 H 10 ; phenyl-1,3-butadiene), (10) 9-methyl-1,4-dihydronaphthalene (C 10 H 9 CH 3 ; meta-tolyl-1,3-butadiene), (11) 8-methyl-1,4-dihydronaphthalene (C 10 H 9 CH 3 ; meta/para-tolyl-1,3-butadiene), (12) 1-methyl-1,4-dihydronaphthalene (C 10 H 9 CH 3 ; phenyl-1-methyl-1,3-butadiene), and (13) 2-methyl-1,4-dihydronaphthalene (C 10 H 9 CH 3 ; phenyl-2-methyl-1,3-butadiene) (83,(90)(91)(92).…”