title: Formation of 6-methyl-1,4-dihydronaphthalene in the reaction of the p-tolyl radical with 1,3-butadiene under single-collision conditions.
abstract: Crossed molecular beam reactions of p-tolyl (C7H7) plus 1,3-butadiene (C4H6), p-tolyl (C7H7) plus 1,3-butadiene-d6 (C4D6), and p-tolyl-d7 (C7D7) plus 1,3-butadiene (C4H6) were carried out under single-collision conditions at collision energies of about 55 kJ mol(-1). 6-Methyl-1,4-dihydronaphthalene was identified as the major reaction product formed at fractions of about 94% with the monocyclic isomer (trans-1-p-tolyl-1,3-butadiene) contributing only about 6%. The reaction is initiated by barrierless addition of the p-tolyl radical to the terminal carbon atom of the 1,3-butadiene via a van der Waals complex. The collision complex isomerizes via cyclization to a bicyclic intermediate, which then ejects a hydrogen atom from the bridging carbon to form 6-methyl-1,4-dihydronaphthalene through a tight exit transition state located about 27 kJ mol(-1) above the separated products. This is the dominant channel under the present experimental conditions. Alternatively, the collision complex can also undergo hydrogen ejection to form trans-1-p-tolyl-1,3-butadiene; this is a minor contributor to the present experiment. The de facto barrierless formation of a methyl-substituted aromatic hydrocarbons by dehydrogenation via a single event represents an important step in the formation of polycyclic aromatic hydrocarbons (PAHs) and their partially hydrogenated analogues in combustion flames and the interstellar medium.
authors.affiliation: Department of Chemistry, University of Hawaii at Manoa , Honolulu, Hawaii 96822, United States.
authors.forename: Dorian S N
authors.initials: DS
authors.lastname: Parker
authors.forename: Beni B
authors.initials: BB
authors.lastname: Dangi
authors.forename: Ralf I
authors.initials: RI
authors.lastname: Kaiser
authors.forename: Adeel
authors.initials: A
authors.lastname: Jamal
authors.forename: Mikhail
authors.initials: M
authors.lastname: Ryazantsev
authors.forename: Keiji
authors.initials: K
authors.lastname: Morokuma
mesh_terms: D002070:Butadienes / Q000737:chemistry*; D003500:Cyclization; D005609:Free Radicals / Q000737:chemistry; D007536:Isomerism; D007700:Kinetics; D009281:Naphthalenes / Q000737:chemistry*; D011789:Quantum Theory; D014050:Toluene / Q000737:chemistry*
chemical_list: D002070:Butadienes; D005609:Free Radicals; D009281:Naphthalenes; C031721:naphthalene; D014050:Toluene; C031763:1,3-butadiene