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)?

Physical Chemistry Chemical Physics Pub Date: 2015-02-11 DOI: 10.1039/C5CP00311C

Abstract

The crossed molecular beam reactions of the meta-tolyl radical with 1,3-butadiene and D6-1,3-butadiene were conducted at collision energies of 48.5 kJ mol?1 and 51.7 kJ mol?1. The reaction dynamics propose a complex-forming reaction mechanism via addition of the meta-tolyl radical with its radical center either to the C1 or C2 carbon atom of the 1,3-butadiene reactant forming two distinct intermediates, which are connected via migration of the meta-tolyl group. Considering addition to C1 proceeds by formation of a van-der-Waals complex below the energy of the separated reactants, we propose that in cold molecular clouds holding temperatures as low as 10 K, the reaction of the meta-tolyl radical with 1,3-butadiene is de-facto barrier less. At elevated temperatures such as in combustion processes, the reaction can also proceed via addition to C2 by overcoming the entrance barrier to addition (11 kJ mol?1). Eventually, the resonantly stabilized free radical intermediate C11H13 undergoes isomerization to a cis form, followed by rearrangement through two distinct ring closures at the para- and ortho-position of tolyl radical to yield cyclic intermediates. These intermediates then emit a hydrogen atom forming 6- and 5-methyl-1,4-dihydronaphthalene via tight exit transition states. The steady state branching ratio, 70.0% and 29.2%, at the collision energy of 51.7 kJ mol?1, of 6- and 5-methyl-1,4-dihydronaphthalene, respectively, is determined mainly by the rates of reverse ring opening of cyclic intermediates. The formation of the thermodynamically less stable 1-meta-tolyl-trans-1,3-butadiene was found to be a less important pathway (0.8%). The reaction of the meta-tolyl radical with 1,3-butadiene leads without entrance barrier to two methyl substituted PAH derivatives holding 1,4-dihydronapthalene cores: 5- and 6-methyl-1,4-dihydronaphthalene thus providing a barrierless route to odd-numbered PAH derivatives under single collision conditions.

Graphical abstract: 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)
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