A Study of Long Chain Organic Compounds by Electron Diffraction

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Abstract
A layer of lard deposited upon a metal surface gives an electron diffraction pattern consisting of segments of diffuse inclined bands. These arise from line gratings made up of long hydrocarbon chains. The mean inclination of the axes of these chains to the surface normal is calculated to be 27°, and the separation of carbon atoms 2.50A. The saturated glyceride, tri‐stearin, yields a diffraction pattern of diffuse bands parallel to the surface, unlike the diffraction pattern from lard, although the latter is predominantly a mixture of glycerides. We believe that the inclination of the long hydrocarbon chains in lard is due to lack of saturation, although this hypothesis has not been tested. Diffraction patterns from stearic acid deposited upon a metal surface also contain inclined bands. These are relatively well defined, and have inclinations which differ from one sample to another. This variation is attributed to the existence of large crystals the c axes of which are differently inclined to the plane of incidence. We calculate that this axis is always inclined 33° to the surface normal. A similar investigation of methyl stearate yields the angle 25°. For both stearic acid and methyl stearate the separation between diffraction bands corresponds to the atomic spacing 2.50A. This is in good agreement with the known distance between alternate carbon atoms in a zigzag hydrocarbon chain.

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