Formation of 2- and 1-methyl-1,4-dihydronaphthalene isomers via the crossed beam reactions of phenyl radicals (C6H5) with isoprene (CH2C(CH3)CHCH2) and 1,3-pentadiene (CH2CHCHCHCH3)

Abstract

Crossed molecular beam reactions were exploited to elucidate the chemical dynamics of the reactions of phenyl radicals with isoprene and with 1,3-pentadiene at a collision energy of 55 ± 4 kJ mol⁻¹. Both reactions were found to proceed via indirect scattering dynamics and involve the formation of a van-der-Waals complex in the entrance channel. The latter isomerized via the addition of the phenyl radical to the terminal C1/C4 carbon atoms through submerged barriers forming resonantly stabilized free radicals C₁₁H₁₃, which then underwent cis–trans isomerization followed by ring closure. The resulting bicyclic intermediates fragmented via unimolecular decomposition though the atomic hydrogen loss via tight exit transition states located 30 kJ mol⁻¹ above the separated reactants in overall exoergic reactions forming 2- and 1-methyl-1,4-dihydronaphthalene isomers. The hydrogen atoms are emitted almost perpendicularly to the plane of the decomposing complex and almost parallel to the total angular momentum vector (‘sideways scattering’) which is in strong analogy to the phenyl–1,3-butadiene system studied earlier. RRKM calculations confirm that 2- and 1-methyl-1,4-dihydronaphthalene are the dominating reaction products formed at levels of 97% and 80% in the reactions of the phenyl radical with isoprene and 1,3-pentadiene, respectively. This barrier-less formation of methyl-substituted, hydrogenated PAH molecules further supports our understanding of the formation of aromatic molecules in extreme environments holding temperatures as low as 10 K.