Infrared identification of the σ-complex of Cl-C₆H ₆ in the reaction of chlorine atom and benzene in solid para-hydrogen

The reaction of a chlorine atom with benzene (C₆H₆) is important in organic chemistry, especially in site-selective chlorination reactions, but its product has been a subject of debate for five decades. Previous experimental and theoretical studies provide no concrete conclusion on whether the product is a π- or σ-form of the Cl-C₆H ₆ complex. We took advantage of the diminished cage effect of para-hydrogen (p-H₂) to produce Cl in situ to react with C ₆H₆ (or C₆D₆) upon photolysis of a Cl₂/C₆H₆ (or C₆D₆)/p- H₂ matrix at 3.2 K. The infrared spectrum, showing intense lines at 1430.5, 833.6, 719.8, 617.0, and 577.4 cm^-1, and several weaker ones for Cl-C₆H₆, and the deuterium shifts of observed new lines unambiguously indicate that the product is a 6-chlorocyclohexadienyl radical, i.e., the σ-complex of Cl-C₆H₆. Observation of the σ-complex rather than the π-complex indicates that the σ-complex is more stable in solid p-H₂ at 3.2 K. The spectral information is crucial for further investigations of the Cl + C ₆H₆ reaction either in the gaseous or solution phase.