Infrared spectroscopic evidence for the coexistence of two molecular configurations in crystalline fatty acids

Abstract

Infrared spectra of 14 normal fatty acids have been obtained in the range of temperatures from room temperature to liquid-helium temperature. Great temperature dependencies are observed in their spectra, especially in the regions of the characteristic frequencies of the carboxyl group, and the band progression of the CH2 wagging modes. Many bands decrease in intensity with decreasing temperature and disappear near liquid-helium temperature. Close to the positions of these lost bands, alternative bands, which increase in intensity with decreasing temperature, are found. The results support the postulate proposed in our earlier work that the two distinct configurations, cis and trans forms for Cβ–Cα–C=O group, coexist in the crystalline state, and indicate that (1) the cis configuration is the lower energy (enthalpy) form for the odd acids (C3–C11) and for the acids containing four or six carbon atoms; and (2) the trans configuration is the lower energy form for the even-numbered (C8–C20) carbon series. Differences of enthalpy and entropy between cis and trans forms of CH3(CH2)12COOH are estimated to be about 200 cal/mole and 3.2 cal/(mole deg), respectively.