Infrared identification of the sigma-complex of Cl-C6H6 in the reaction of chlorine atom and benzene in solid para-hydrogen

Abstract

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 pi- or sigma-form of the Cl-C6H6 complex. We took advantage of the diminished cage effect of para-hydrogen (p-H-2) to produce Cl in situ to react with C6H6 (or C6D6) upon photolysis of a Cl-2/C6H6 (or C6D6)/p-H-2 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 sigma-complex of Cl-C6H6. Observation of the s-complex rather than the pi-complex indicates that the s-complex is more stable in solid p-H-2 at 3.2 K. The spectral information is crucial for further investigations of the Cl + C6H6 reaction either in the gaseous or solution phase. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4790860]