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)

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.

LOW TEMPERATURE FORMATION OF NITROGEN-SUBSTITUTED POLYCYCLIC AROMATIC HYDROCARBONS (PANHs)—BARRIERLESS ROUTES TO DIHYDRO(iso)QUINOLINES

“…The formation of these intermediates via van der Waals complexes and through barriers located below the energy of the reactants (submerged barriers) imply that these reactions are de-facto barrierless and can occur at very low temperatures, such as 10 K, prevailing in cold molecular clouds. Previous studies of aromatic radicals (phenyl, tolyl) reacting with 1,3-butadiene have found the channel leading to addition of the radical carbon to the central carbon atom not to be competitive relative to the addition to the CH 2 moiety even at elevated collision energies (Kaiser et al 2012;Parker et al 2014a;Muzangwa et al 2015). The central addition channel pathways were, therefore, omitted from present consideration as a competing channel in low temperature interstellar environments.…”

“…This mechanism adds to the increasing catalog of reactions involving phenyl-type aromatic radicals with the radical located on the aromatic ring, such as phenyl (Kaiser et al 2012), paratolyl (Parker et al 2014a), and meta-tolyl (Muzangwa et al 2015) with 1,3-butadiene and vinylacetylene that lead to facile PAH formation at low temperatures, through the formation of a van der Waals complex. PANH formation at low temperatures has implications for the astrochemistry of biologically relevant molecules.…”

“…Therefore, at very low temperatures of cold molecular clouds, such as in TMC-1 with average temperatures of about 10 K, the reaction will still be able to proceed. These reactions form part of a growing group of de facto barrierlesstype reactions, leading to aromatization to PAH-type species at ultralow temperatures, involving aromatic radicals with the radical located on the aromatic ring that undergo addition to a C4 double conjugated unsaturated hydrocarbon such as 1,3butadiene (C 4 H 6 ) or vinylacetylene (C 4 H 4 ; Kaiser et al 2012;Parker et al 2012aParker et al , 2014aParker et al , 2014bParker et al , 2015Dangi et al 2014;Kaiser et al 2015;Muzangwa et al 2015;Yang et al 2015). Previously, aromatization reactions had been calculated to proceed over an energy barrier implying that they could only be formed in high temperature environments such as in the inner regions of circumstellar envelopes of dying carbon stars like IRC+10216 to supply the necessary kinetic transitional energy to overcome the barriers (Tielens et al 2000;Kislov et al 2005;Tielens 2008;Cherchneff 2011).…”

Facile One-Pot Synthesis of Benzo[h]isoquinolines via Strong Acid Triggered [1,2]-Sigmatropic Rearrangement: A Theoretical and Experimental Studies

“…Computational methodology: A number of minima and saddle points were located on the potential energy surface to obtain equilibrium structures and transition states. The doubleended anharmonic downward distortion following (D-ADDF) methodology was used for search of the transition states [32][33][34][35]. The types of all the stationary points found were confirmed by harmonic frequency calculations.…”

On the formation and the isomer specific detection of methylacetylene (CH3CCH), propene (CH3CHCH2), cyclopropane (c-C3H6), vinylacetylene (CH2CHCCH), and 1,3-butadiene (CH2CHCHCH2) from interstellar methane ice analogues

“…11,15,16 Similarly, 1,3-butadiene can react with the ethynyl radical (CCH) to form benzene (C 6 H 6 ), 23 with the phenyl radical (C 6 H 5 ) to form 1,4-dihydronaphthalene (C 10 H 10 ), 41 or with the tolyl radical (C 7 H 7 ) to form 5-and 6-methyl-1,4-dihydronaphthalene (C 11 H 12 ). 42 Recently, it was shown that the methylacetylene molecule cannot be synthesized via gas-phase reactions alone. 27,43 An extensive review of the chemical pathways leading to methylacetylene in the gas phase concluded that there are no efficient gas-phase synthetic pathways available to reproduce the observed abundances of this molecule in the cold ISM.…”