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A proton magnetic resonance investigation of molecular motion in oleic acid and elaidic acid Ware, Donald Robert

Abstract

A study of the n.m.r. absorption of oleic acid has been carried out between 77°K. and 287°K. A sharp increase in line width and second moment is observed below 100°K. indicative of an increase in the intramolecular interactions. A narrow line, indicative of liquid-like motion, is observed at temperatures above 208°K. The second moment and line width decrease rapidly as the melting point is approached, dropping to liquid-like values at 287°K. The experimental data are compared with the theoretical data obtained from the known crystal structure. From this comparison, a crystal transition is proposed to take place below 100°K., and heteronuclear premelting is proposed to take place above 208°K. It is likely that end methyl group rotation takes place at all temperatures but there is no extensive torsional oscillation or vibration below 26o°K. and there is little apparent change in the cell dimensions between 260°K. and 100°K. Elaidic acid has been studied between 77°K. and 315°K. The line width and second moment increase slightly below 170GK. and a narrow component is observed above 225°K. The line width and second moment fall sharply only at the melting point, decreasing to a liquid-like line. As no crystal structure is available for elaidic acid, the experimental data are compared with experimental and theoretical data for oleic acid and stearic acid. Some form of molecular vibration or hindered rotation is proposed to account for the transition at 170°K., and heteronuclear premelting is proposed to take place above 225°K. The difference in the behaviour of oleic acid and elaidic acid near the melting point is explained in terms of the molecular shape.

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