Far infrared spectra and vibrational assignments of substituted benzenes

The vibrational absorption spectra of some substituted benzenes have been measured in the range 50 to 450 cm-1. The compounds were measured as liquids, in solutions, as crystalline solidsat low temperature, and in polyethylene matrices. The extension of the infrared spectrum to very low frequencies has made it possible to determine new values for many fundamental vibrations. An assignment of all the vibrational frequencies in the low-frequency range has been made, from the infrared and Raman data, for p-dihalogeno-benzenes, p-halogenotoluenes, p-halogeno-nitrobenzenes, and for some mono-substituted benzenes. Some measurements have been made on the marked variation of intensity of the lowest frequency bending mode of p-dihalogeno-benzenes.