Crossed beam reactions of the phenyl (C₆H₅; X ²A₁) and phenyl- d ₅ radical (C ₆D₅; X²A₁) with 1,2-butadiene (H₂CCCHCH₃; X¹A')

We explored the reactions on the phenyl (C₆H₅; X ²A₁) and phenyl-d₅ (C₆D₅; X²A₁) radical with 1,2-butadiene (C₄H ₆; X¹A') at a collision energy of about 52 ± 3 kJ mol^-1 in a crossed molecular beam apparatus. The reaction of phenyl with 1,2-butadiene is initiated by adding the phenyl radical with its radical center to the π electron density at the C1/C3 carbon atom of 1,2-butadiene. Later, the initial collision complexes isomerize via phenyl group migration from the C1/C3 carbon atoms to the C2 carbon atom of the allene moiety of 1,2-butadiene. The resulting intermediate undergoes unimolecular decomposition through hydrogen atom emission from the methyl group of the 1,2-butadiene moiety via a rather loose exit transition state leading to 2-phenyl-1,3-butadiene in an overall exoergic reaction (δ_RG = -72 ± 10 kJ mol ^-1). This finding reveals the strong collision-energy dependence of this system when the data are compared with those of the phenyl radical with 1,2-butadiene previously recorded at collision energies up to 160 kJ mol ^-1, with the previous study exhibiting the thermodynamically less stable 1-phenyl-3-methylallene (δ_RG = -33 ± 10 kJ mol^-1) and 1-phenyl-2-butyne (δ_RG = -24 ± 10 kJ mol^-1) to be the dominant products.