Solid-state13C NMR study on order → disorder phase transition in oleic acid

The reversible solid-state phase transition between the gamma and alpha phases of oleic acid was followed by C-13 solid-state NMR spectroscopy. All peaks in the gamma and a phases were assigned based on the corresponding crystal structures determined by X-ray diffraction method (Abrahamsson and Rydei-stedt-Nahringbauer, 1962, and Kaneko et al., 1996). As to the transition behavior, there was a significant difference between the two hydrocarbon chains straddling cis-olefin group. On the gamma --> alpha phase transition at 271 K, the methyl-sided methylene carbons exhibited upfield shifts due to the gauche effect. Characteristic peaks shifts due to the conformational change of the cis-olefin group were observed for cis-olefin carbons. On the other hand, the peaks due to carboxyl-sided chains showed no large shifts on this transition. Each oleic acid carbon atom showed a discontinuous decrease in T-1, except for the methyl carbon, which was in the extreme narrowing limit even in the gamma phase. However, the T-1 values of the methyl-sided carbons were significantly smaller than those of the carboxyl-sided carbons in the alpha phase. The conformational disordering in the methyl-sided chains was accompanied by a remarkable increase of molecular motion in this moiety.