The infrared spectra of a number of aliphatic monocarboxylic acids in the 350–50 cm−1 range are reported and several consistencies in band frequencies noted. An attempt to assign the low frequency vibrational modes of some of the simpler acids has been made based on infrared and Raman intensities. It is concluded that these molecules consist of relatively strongly coupled dimer molecules in the pure liquid and that the spectra reflect this dimer structure.
J. E. Saunders, F. F. Bentley, and J. E. Katon, "Low-Frequency Infrared Spectra of Aliphatic Monocarboxylic Acids," Appl. Spectrosc. 22, 286-294 (1968)