Spectral Evidence of Bevel-Gear-Type Rotation of Benzene around Br in Solid p -H 2 : Infrared Spectrum of the C 6 H 6 Br Radical.

Whether the structure of C 6 H 6 X (X = halogen), an intermediate in the halogenation of benzene, is an open or a bridged form has been debated. We produced Br to react with C 6 H 6 upon photolysis in situ of a Br 2 /C 6 H 6 / p -H 2 matrix at 3.2 K. In contrast to the C 6 H 6 Cl σ-complex reported previously, the observed infrared spectrum indicates that C 6 H 6 Br is an open-form π-complex. Furthermore, lines of the two CH out-of-plane bending modes associated mainly with even- and odd-numbered carbons, predicted near 672 and 719 cm -1 , merged into a broad line at 697.3 cm -1 , indicating that these modes become nearly equivalent as Br migrates from one carbon atom to another. Quantum-chemical calculations support that the benzene ring performs a bevel-gear-type rotation with respect to Br. Observation of only trans - ortho - and trans - para -C 6 H 6 Br 2 suggests that this gear-type motion allows the additional Br atom to attack C 6 H 6 Br only from the opposite side of the Br atom in C 6 H 6 Br.