Polarized infrared spectra of some crystalline fatty acids

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Abstract
Summary: Polarized infrared spectra of thin films of representative samples of form C′, B′, and A′ n‐alkyl monocarboxylic acids have been observed. The data obtained on C′ heptadecanoic acid confirm that the type C and type C′ structures are very similar and that the major difference is in the packing of the CH3 end‐groups. The results on type B′ and A′ acids indicate that meaningful polarization data can be obtained on triclinic crystals of n‐alkyl carboxylic acids, provided that careful consideration is given to the sample orientation and to the symmetry characteristics of certain portions of the molecules. The P1 space structure of these crystals and the orthorhombic substructure of the B′ configuration as well as the triclinic substructure of the A′ configuration were confirmed. Some strong infrared bands of the highly irregular triclinic A′ structure did not seem to be appreciably polarized. The data on group‐frequency bands which did show clear‐cut polarization were in agreement with prediction. The spectra of form A′, B′, and C′ crystals did not coineide with data obtained on KBr pellets. It is concluded that in these pellets fatty acids exist in a form which is not easy to duplicate under more conventional conditions.