VIBRATIONAL SPECTROSCOPIC STUDY ON POLYMORPHISM OF ERUCIC-ACID AND PALMITOLEIC ACID - GAMMA-1-]ALPHA-1 AND GAMMA-]ALPHA REVERSIBLE SOLID-STATE PHASE-TRANSITIONS

The infrared and Raman spectra of four polymorphic phases (alpha, alpha1, gamma and gamma1) of erucic acid (cis-13-docosenoic acid) and those of two polymorphic phases (alpha and gamma) of palmitoleic acid (cis-9-hexadecenoic acid) were investigated. The gamma and gamma1 phases of erucic acid were analyzed on the basis of crystal structures determined by us. There were large spectral differences between gamma and gamma1 phases, which could be ascribed to the differences in the conformation of cis-olefin groups and the subcell structure. Two types of reversible solid state phase transitions (gamma --> alpha and gamma1 --> alpha1 transitions) were followed by the infrared and Raman spectra. It was concluded that the mechanism of the gamma --> alpha phase transition of erucic and palmitoleic acids is essentially the same as that of oleic acid previously reported by us [J. Phys. Chem. 90, 6371 (1986)], i.e. this phase transition is of order-disorder type accompanied by a conformational disordering at the methyl-terminal chain. Spectral changes on the gamma1 --> alpha1 transition suggested that a similar structural change took place during this transition but there were large structural differences between alpha and alpha1.