Crossed beam reactions of the phenyl (C6H5; X(2)A1) and phenyl-d5 radical (C6D5; X(2)A1) with 1,2-butadiene (H2CCCHCH3; X(1)A').

We explored the reactions on the phenyl (C6H5; X(2)A1) and phenyl-d5 (C6D5; X(2)A1) radical with 1,2-butadiene (C4H6; X(1)A') 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 (ΔRG = -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 (ΔRG = -33 ± 10 kJ mol(-1)) and 1-phenyl-2-butyne (ΔRG = -24 ± 10 kJ mol(-1)) to be the dominant products.