Infrared And Raman Studies Of Some Monounsaturated Olefinic And Acetylenic Fatty-Acids And Their Derivatives

The Raman spectra (800–3200 cm–1) of carbon tetrachloride solutions of the 15 isomeric methyl cis-octadecenoates, the 15 isomeric trans-octadecenoic acids, methyl octadec-17-enoate, and of the 16 isomeric octadecynoic acids have been obtained. Provided that the double bond is not conjugated with the carbonyl group, and does not occur in the terminal position, the double-bond vibration gives rise to a strong Raman band at 1656 ± 1 cm–1 for the cis-compounds, and to a strong band at 1670 ± 1 cm–1 for the trans-compounds. Conjugation with the carbonyl group lowers the value by 8 cm–1, and for the terminal double bond the value is 1642 cm–1. The triple-bond vibration gives rise to two Raman bands at 2232 ± 1 cm–1(s) and 2291 ± 2 cm–1(m). When the triple bond is conjugated with the carbonyl group only one band at 2245 cm–1 is observed. The terminal triple bond gives a single band at 2120 cm–1. From a comparison of the i.r. and Raman spectra of the methyl cis- and trans-octadec-8-enoates an assignment is suggested for the vibrational modes of the cis- and trans-CHCH– units.