Product Detection of the CH(X²Π) Radical Reaction with Cyclopentadiene: A Novel Route to Benzene

The reaction of the methylidyne radical (CH(X²Π)) with cyclopentadiene (c-C₅H₆) is studied in the gas phase at 4 Torr and 373 K using a multiplexed photoionization mass spectrometer. Under multiple collision conditions, the dominant product channel observed is the formation of C₆H₆ + H. Fitting the photoionization spectrum using reference spectra allows for isomeric resolution of C₆H₆ isomers, where benzene is the largest contributor with a relative branching fraction of 90 (±5)%. Several other C₆H₆ isomers are found to have smaller contributions, including fulvene with a branching fraction of 8 (±5)%. Master Equation calculations for four different entrance channels on the C₆H₇ potential energy surface are performed to explore the competition between CH cycloaddition to a C═C bond vs CH insertion into C-H bonds of cyclopentadiene. Previous studies on CH addition to unsaturated hydrocarbons show little evidence for the C-H insertion pathway. The present computed branching fractions support benzene as the sole cyclic product from CH cycloaddition, whereas fulvene is the dominant product from two of the three pathways for CH insertion into the C-H bonds of cyclopentadiene. The combination of experiment with Master Equation calculations implies that insertion must account for ∼10 (±5)% of the overall CH + cyclopentadiene mechanism.