Structure and molecular anisotropy of sorbic acid, CH ₃ . CH : CH . CH : CH . COOH

A preliminary study of sorbic acid has shown that the monoclinic unit cell contains eight molecules, these being linked in pairs by hydrogen bonds about the symmetry centres of the crystal, with the long chain axis of the molecule in or near to the (010) plane and inclined at between 10 $^{\circ} $ and 15$^{\circ} $ to the a axis in the obtuse angle $\beta $. The molecular chains must be rotated so as to bring some of the atoms out of the (010) plane, and an approximate measure of this rotation is provided by measurements of optical and magnetic anisotropy, which give $\pm $ 35 $^{\circ} $ as the angle between the b axis and the normal to the molecular plane. The magnetic anisotropy due to resonance in the conjugated carbon chain is about half as large as that in the benzene ring and falls into line with previous observations on non-cyclic conjugated compounds. The orientation of the molecules has been confirmed by observation of the size and shape of the diffuse spots occurring on well-exposed Laue photographs taken with radiation from a copper target, and the usefulness of these photographs as a secondary method of structure determination has been emphasized.