The Band Spectrum of Benzene: Existence of a Small Vibrational Frequency in the Normal State

A vibrational frequency of 160 cm^-1 in the band spectrum of benzene, postulated by Henri but called into question by Kronenberger and Pringsheim, is discussed from the point of view of the available spectral and Raman data but no definite conclusion as to its origin in the normal state of the molecule is reached. Experimental evidence is then brought forward to show that the observed intensity distribution of absorption bands having this separation corresponds closely to the distribution calculated from the Boltzmann law with the addition of certain reasonable assumptions, on the basis that the 160 cm^-1 frequency occurs in the normal state. Further checks are obtained by repeating the measurements and calculations for higher temperatures. The possible origin of this small vibrational frequency in the benzene molecule is briefly discussed.