Gas-Phase Formation of C₅H₆ Isomers via the Crossed Molecular Beam Reaction of the Methylidyne Radical (CH; X²Π) with 1,2-Butadiene (CH₃CHCCH₂; X¹A′)

Abstract

The bimolecular gas-phase reaction of the methylidyne radical (CH; X 2 Π) with 1,2-butadiene (CH 2 CCHCH 3 ; X 1 A′) was investigated at a collision energy of 20.6 kJ mol −1 under single collision conditions. Combining our laboratory data with high-level electronic structure calculations, we reveal that this bimolecular reaction proceeds through the barrierless addition of the methylidyne radical to the carbon−carbon double bonds of 1,2-butadiene leading to doublet C 5 H 7 intermediates. These collision adducts undergo a nonstatistical unimolecular decomposition through atomic hydrogen elimination to at least the cyclic 1-vinyl-cyclopropene (p5/p26), 1-methyl-3-methylenecyclopropene (p28), and 1,2-bis(methylene)cyclopropane (p29) in overall exoergic reactions. The barrierless nature of this bimolecular reaction suggests that these cyclic C 5 H 6 isomers might be viable targets to be searched for in cold molecular clouds like TMC-1.

Theoretical Study of the Reaction of the Methylidyne Radical (CH; X2Π) with 1-Butyne (CH3CH2CCH; X1A′)

“…The initial interaction between the reactants may occur via methylidyne addition to acetylenic carbon atoms or to the triple CC bond of 1-butyne and methylidyne insertion into various C–H bonds of the C 4 H 6 molecule. The doublet C 5 H 7 PES was investigated gradually and systematically, in continuation of our previous CH + 1,3-/1,2-butadiene studies , and keeping the notation of intermediates and bimolecular products consistent with the previous works. Here, our calculations showed that the methylidyne radical plus 1-butyne system accesses 21 C 5 H 7 intermediates ( i98–i117 and complex2 ) and 92 transition states, leading to the hydrogen atom emission C 5 H 6 products ( p1 , p4 , p7 , p9 , p10 , p15 , p16 , p20 , p30 , p36 , p45 , p46 , and p50 ), methyl radical elimination C 4 H 4 products ( p23 , p32 , p35 , and p51 ), ethyl radical loss C 3 H 2 products ( p43 , p44 , and p47 ), linear C 3 H radical emission C 2 H 6 product ( p48 ), propargyl radical loss C 2 H 4 product ( p24 ), and allyl CH 2 CHCH 2 and 3-propenyl CH 3 CHCH radical loss C 2 H 2 products ( p37 and p49 , respectively) (Figures , , , , ; Table ).…”

“…Here, our calculations showed that the methylidyne radical plus 1-butyne system accesses 21 C 5 H 7 intermediates ( i98–i117 and complex2 ) and 92 transition states, leading to the hydrogen atom emission C 5 H 6 products ( p1 , p4 , p7 , p9 , p10 , p15 , p16 , p20 , p30 , p36 , p45 , p46 , and p50 ), methyl radical elimination C 4 H 4 products ( p23 , p32 , p35 , and p51 ), ethyl radical loss C 3 H 2 products ( p43 , p44 , and p47 ), linear C 3 H radical emission C 2 H 6 product ( p48 ), propargyl radical loss C 2 H 4 product ( p24 ), and allyl CH 2 CHCH 2 and 3-propenyl CH 3 CHCH radical loss C 2 H 2 products ( p37 and p49 , respectively) (Figures , , , , ; Table ). Because some pathways of the CH + 1-butyne reaction eventually access the intermediates considered earlier, in the studies of CH reactions with the other C 4 H 6 isomers, , we occasionally stop the PES presentation and our discussion at the common intermediates. Moreover, as will be seen ( vide infra ) from the calculations of product branching ratios, the prevalent reaction products would be formed through relatively short pathways, which do not involve the common intermediates.…”

“… a A value for the collision energy was chosen to be similar to that in the recent crossed molecular beam studies of the CH reactions with 1,3- and 1,2-butadienes (refs and ). …”

“…In order to reproduce the crossed molecular beam conditions emulating those in outer space, the calculations were performed at the zero-pressure limit. Note that a detailed description of the conditions of crossed molecular beam experiments is provided in numerous previous studies, in particular, in the recent publications on the CH + 1,3-/1,2-butadiene reactions. , Finally, RRKM rate constants were used to evaluate reaction product branching ratios within steady-state approximation. , …”

“…In addition to its complexity, the C 5 H 7 PES is important both for combustion and astrochemistry because it can be accessed by a number of chemical reactions between hydrocarbon molecules and radicals present in flames and/or the interstellar medium including CH + C 4 H 6 , CH 2 + C 4 H 5 , CH 3 + C 4 H 4 , CH 4 + C 4 H 3 , C 2 H + C 3 H 6 , C 2 H 2 + C 3 H 5 , C 2 H 3 + C 3 H 4 , C 2 H 4 + C 3 H 3 , and others. In recent studies, we began our detailed investigation of the C 5 H 7 surface by carrying out combined experimental crossed molecular beam and theoretical electronic structure/kinetic studies of the reactions of the methylidyne radical CH­(X 2 Π) with two C 4 H 6 isomers, 1,3- and 1,2-butadienes. , For instance, we found that the methylidyne radical may add barrierlessly to the terminal carbon atom and/or carbon–carbon double bond of 1,3-butadiene, leading to doublet C 5 H 7 collision complexes, which, according to the experimental data, undergo non-statistical unimolecular decomposition via H atom emission, yielding the cyclic cis - and trans -3-vinyl-cyclopropene products, whereas the statistically favored product is cyclopentadiene. The CH + 1,2-butadiene reaction has been shown to proceed also through the barrierless addition of the methylidyne radical to the carbon–carbon double bonds of 1,2-butadiene, leading to C 5 H 7 collision adducts, which lose atomic hydrogen to produce at least the cyclic 1-vinyl-cyclopropene, 1-methyl-3-methylenecyclopropene, and 1,2- bis (methylene) cyclopropane C 5 H 6 isomers.…”