Vapor–solvent shift of the lowest frequency vibration of p-benzoquinone and toluquinone and the consequences for the vibrational and electronic spectral assignments

Far‐infrared (20–230 cm−1) absorption spectra of p‐benzoquinone‐h4,‐d4 and of toluquinone have been measured. In the vapor phase, the lowest frequency vibration of these three compounds is found at 88.9, 87.5, and 82.3 cm−1, respectively. In the condensed phase the frequency increases by approximately 20 cm−1. These measurements clarify assignments, based on this vibrational mode, of the visible absorption spectrum as well as of other vibrations. It is proposed that intermolecular forces are responsible for the large frequency shift and that the observed width in solution (≂24 cm−1) is homogeneous, reflecting rapid relaxation processes of this large amplitude low frequency bending mode.