Knowledge Bank

“The infrared spectra of highly oriented polycrystalline films of form C stearic acid and vaccenic acid have been obtained using polarized radiation with the radiation beam perpendicular and at $45\circ$ angles to the principal crystal planes. The spectrum of stearic acid is discussed on the basis of the space group $C2h5=P21/b$ and orthorhombic substructure of the hydrocarbon $chains.1$ The splittings and dichroism of all bands which can be assigned agree with prediction. The only major bands which offer difficulties occur in the $1150-1350cm-1$ region in the form of an equally spaced series related to chain length. The dichroism and number of these bands suggest skeletal vibrations involving the carboxyl groups and the carbon chains. The crystal structure of vaccenic acid $[H3C-(CH3)5CH=(CH2)3COOH]$ has not been determined previously. Infrared spectra obtained on oriented crystalline films with polarized radiation indicate that the structure is, unlike saturated monocarboxylic acids, orthorhombic. Space symmetry $Vh16=Pnsm$ with four dimers in the unit cell, glide planes parallel and a mirror plane perpendicular to the main chains, is proposed. The main portions of the hydrocarbon chains are packed similarly to polyethylese, but the double bonds appear to lie in almost parallel planes.”