Raman spectra and hydroxyl frequencies of some fatty acids

Summary

The Raman spectra of formic, acetic, dichloracetic and trichloracetic acids have been obtained with a filter of iodine in CCl₄ in order to record the O-H frequencies. Trichloracetic acid has also been studied in different states of aggregation. The changes in the Raman spectra for various states are indicated by the change in the intensities of lines 433, 450, 676 and 833; 450 is present in the solid only, and 433 in solution only while both are present in the liquid. The frequency 676 is sharp and strong in the solid, but weaker and more diffuse as we pass from the solid to solution. The frequencies 1434, 676, 455 and 102 which vanish or become very weak on dilution represent the undissociated molecule, the frequencies 1330, 833, 952, 433 which are strengthened on dilution represent the monomer, the frequencies 202, 284 and 952 which remain unaffected represent both the ion and the undissociated molecule while the frequencies 695, 734, 1750 which appear on great dilutions represent the ionic frequencies. O-H bands have been recorded for all the acids; in the case of trichloracetic acid the band shows a triplet structure the centre of the band being at 2996. The shift of the hydroxyl frequency from the normal value of 3608 is greater in trichloracetic than in formic or acetic acid which is in accordance with Venkateswaran’s hypothesis.

The spectra of formic and acetic acids are analysed for the monomer and polymer molecule, and the force constants for the various linkages are suggested.