The Vibrational Spectra of Molecules and Complex Ions in Crystals. II. Benzene

Abstract

The infra-red spectrum of crystalline benzene has been studied at −12°C, −65°C and −170°C as well as the liquid at 28°C. The lines are extremely sharp, the mean line width for fundamentals being 7 cm−1 in the crystal. In three cases, it is less than the spectral slit width. All out-of-plane degenerate modes are split by about 10 cm−1. Selection rules are obeyed and agree with the reported x-ray structure. All ungerade fundamentals are observed directly, some for the first time. Fifty-two combination bands are observed. The frequency assignments of Ingold et al. are confirmed except for the B2u species. Evidence is cited for re-assigning ν14 from 1648 cm−1 to 1310 cm−1 and ν15 from 1110 cm−1 to 1150 cm−1. Ingold's isotopic data does not conflict with this change. Combination frequencies involving torsional lattice modes are apparently observed at −170°C.