Raman spectra of benzene and benzene- d6 crystals

Abstract

Raman spectra have been studied for the benzene and benzene- d 6 crystals at various temperatures between 4·2° and 273°K. The low-frequency Raman bands due to lattice vibrations have been found to be extremely sharp at 4·2°K. Decrease of their frequencies with an increase of temperature is rather small in the temperature range below 77°K, and considerably larger at higher temperatures. A comparison between the corresponding bands of the benzene and benzene- d 6 crystals has indicated that all the observed low-frequency Raman bands arise from rotational lattice vibrations (librations). Assignments have been made for these bands with reference to F ruhling's assignments for the single crystal of benzene at 273°K and to a molecular packing diagram of the benzene crystal. With several plausible assumptions the root mean square (r.m.s.) amplitudes of rotational oscillations of the molecule around its principal axes have been estimated from the Raman data. They have been compared with X-ray values obtained by Cox et al. for the benzene crystal at 270°K. The Raman results show maximum rotational oscillation around the two-fold axis of the molecule nearly parallel to the b axis of the crystal. This is inconsistent with the X-ray results, which indicate maximum oscillation around the six-fold axis of the molecule. The molecular packing diagram seems to support the Raman results. The r.m.s. amplitudes decrease to 2–3° at 4·2°K for both the benzene and benzene- d 6 crystals. The Raman spectra due to intramolecular vibrations have also been studied. Splittings due to crystal field have been found for the bands ν 7( e 2 g and ν 9( e 2 g ). The gas-phase inactive fundamental ν 5( b 2 g appeared in the crystal Raman spectra at about 1010 and 860 cm −1 for benzene and benzene- d 6. Raman measurement for the mixed crystal of benzene and benzene- d 6 has indicated that site-group splitting is responsible to the observed doublet of ν 7( e 2 g in the crystal spectra.