Spectral Evidence of Bevel-Gear-Type Rotation of Benzene around Br in Solid p‑H₂: Infrared Spectrum of the C₆H₆Br Radical

Whether the structure of C₆H₆X (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 C₆H₆ upon photolysis in situ of a Br₂/C₆H₆/p-H₂ matrix at 3.2 K. In contrast to the C₆H₆Cl σ-complex reported previously, the observed infrared spectrum indicates that C₆H₆Br 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-C₆H₆Br₂ suggests that this gear-type motion allows the additional Br atom to attack C₆H₆Br only from the opposite side of the Br atom in C₆H₆Br.