Gas-phase synthesis of the benzyl radical (C₆H ₅CH₂)

Dicarbon (C₂), 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 (C₆H₅CH₂) by the crossed molecular beam reaction of dicarbon, C₂(X ¹Σ_g⁺, a³Π_u), with 2-methyl-1,3-butadiene (isoprene; C₅H₈; X ¹A′) accessing the triplet and singlet C₇H 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 C₇H₇ radical species through atomic hydrogen elimination. The benzyl radical (C₆H₅CH₂), the thermodynamically most stable C₇H₇ isomer, is determined as the major product. Do cross the streams! Ab initio electronic structure calculations and crossed molecular beam experiments on the reaction of dicarbon with isoprene are presented. The picture shows a flux contour map of the reaction of dicarbon with isoprene that forms the benzyl radical and atomic hydrogen at a collision energy of 43 kJ mol^-1.