Gas-Phase Synthesis of the Benzyl Radical (C 6 H 5 CH 2 )
Dicarbon ($C_2$), the simplest bare carbon molecule, is ubiquitous in the interstellar medium and in combustion flames. A gas-phase synthesis is presented of the benzyl radical ($C_6H_5CH_2$) by the crossed molecular beam reaction of dicarbon, $C_2(X^1Σ_g^+, a^3Π_u$), with 2-methyl-1,3-butadiene (isoprene; $C_5H_8$; X 1 A') accessing the triplet and singlet $C_7H_8$ potential energy surfaces (PESs) under single collision conditions. The experimental data combined with ab initio and statistical calculations reveal the underlying reaction mechanism and chemical dynamics. On the singlet and triplet surfaces, the reactions involve indirect scattering dynamics and are initiated by the barrierless addition of dicarbon to the carbon–carbon double bond of the 2-methyl-1,3-butadiene molecule. These initial addition complexes rearrange via multiple isomerization steps, leading eventually to the formation of $C_7H_7$ radical species through atomic hydrogen elimination. The benzyl radical ($C_6H_5CH_2$), the thermodynamically most stable $C_7H_7$ isomer, is determined as the major product.