Infrared spectra from 650 to 1350 cm -I of crystalline homologous methyl esters from methyl propionate to methyl stearate have been observed. Prominent bands of methyl propionate are assigned to fundamental modes assuming C s symmetry. A basis is suggested for the well known observation that methyl esters exhibit a characteristic triplet in the 1200 cm -1 region. The assignment of methyl propionate is taken as a starting point for evaluation of the spectra of higher homologues, which are qualitatively interpreted in terms of vibrational coupling between group frequencies. Spectral changes produced by unsaturation are briefly discussed and analytical applications evaluated.
THE infrared spectra of crystalline carboxylic acids have been studied by numerous investigators . In recent years it has become important to study the spectra of carboxylic acids in the form of their methyl esters, because separation techniques such as gas-liquid chromatography are most conveniently applied to methylesterified samples. The methyl esters of normal saturated carboxylic acids up to a chain length of Cls melt close to or below room temperature. Branching and unsaturation tend to lower the melting point further. In the liquid state or in solution molecules containing long (CH~), chains exhibit profound rotational isomerism and give rise to broad clusters of unresolved absorption bands which are not suitable for characterization of closely related compounds such as members of a homologous series .
This communication discusses the infrared spectra of saturated unbranched methyl esters from methyl propionate to methyl stearate, as obtained at temperatures well below the melting points of the compounds.
EXPERIMElVTAL
The esters were gas-chromatographicaUy purified commercial samples. The chromatographic data indicated that the amount of homologous impurities in the final specimens did not exceed a few p.p.m. A low-temperature cell of well known