Formation of 6‑Methyl-1,4-dihydronaphthalene in the Reaction of the <i>p</i>‑Tolyl Radical with 1,3-Butadiene under Single-Collision Conditions

Crossed molecular beam reactions of <i>p</i>-tolyl (C₇H₇) plus 1,3-butadiene (C₄H₆), <i>p</i>-tolyl (C₇H₇) plus 1,3-butadiene-<i>d</i>₆ (C₄D₆), and <i>p</i>-tolyl-<i>d</i>₇ (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 (<i>trans</i>-1-<i>p</i>-tolyl-1,3-butadiene) contributing only about 6%. The reaction is initiated by <i>barrierless</i> addition of the <i>p</i>-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 <i>trans</i>-1-<i>p</i>-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

Combined Crossed Molecular Beam and Ab Initio Investigation of the Reaction of Boron Monoxide (BO; X²Σ⁺) with 1,3-Butadiene (CH₂CHCHCH₂; X¹A_g) and Its Deuterated Counterparts

The reactions of the boron monoxide (¹¹BO; X²Σ⁺) radical with 1,3-butadiene (CH₂CHCHCH₂; X¹A_g) and its partially deuterated counterparts, 1,3-butadiene-<i>d</i>₂ (CH₂CDCDCH₂; X¹A_g) and 1,3-butadiene-<i>d</i>₄ (CD₂CHCHCD₂; X¹A_g), were investigated under single collision conditions exploiting a crossed molecular beams machine. The experimental data were combined with the state-of-the-art ab initio electronic structure calculations and statistical RRKM calculations to investigate the underlying chemical reaction dynamics and reaction mechanisms computationally. Our investigations revealed that the reaction followed indirect scattering dynamics through the formation of ¹¹BOC₄H₆ doublet radical intermediates via the barrierless addition of the ¹¹BO radical to the terminal carbon atom (C1/C4) and/or the central carbon atom (C2/C3) of 1,3-butadiene. The resulting long-lived ¹¹BOC₄H₆ intermediate(s) underwent isomerization and/or unimolecular decomposition involving eventually at least two distinct atomic hydrogen loss pathways to 1,3-butadienyl-1-oxoboranes (CH₂CHCHCH¹¹BO) and 1,3-butadienyl-2-oxoboranes (CH₂C (¹¹BO)­CHCH₂) in overall exoergic reactions via tight exit transition states. Utilizing partially deuterated 1,3-butadiene-<i>d</i>₂ and -<i>d</i>₄, we revealed that the hydrogen loss from the methylene moiety (CH₂) dominated with 70 ± 10% compared to an atomic hydrogen loss from the methylidyne group (CH) of only 30 ± 10%; these data agree nicely with the theoretically predicted branching ratio of 80% versus 19%