Conformational change in the C form of palmitic acid investigated by Raman spectroscopy and X-ray diffraction

•Temperature dependence Raman spectra in the C form of palmitic acid•The assignment of the observed bands was done based on the density functional theory.•The X-ray diffraction measurements showing a slight modification•The crystal undergone to a conformational change.

Fatty acids are substances found in most living beings in nature. Here we report the effect of the low temperature in the vibrational and structural properties of the C form of palmitic acid, a fatty acid with 16 carbon atoms. The Raman spectra were obtained in the temperature interval from 300 to 18 K in the spectral range between 30 and 3100 cm− 1. The assignment of the duly observed bands was done based on the density functional theory. On cooling, the main changes observed in the lattice mode region of the Raman spectra were interpreted as a conformational modification undergone by the palmitic acid molecules in the unit cell. The X-ray diffraction measurements were obtained from 290 to 80 K showing a slight modification in the lattice parameters at about 210 K. Differential scanning calorimetry (DSC) measurements were recorded between 150 and 300 K and no enthalpic anomaly in the DSC thermogram was observed. These techniques provided strong evidence of the conformational change in the molecules of palmitic acid at low temperatures.

Graphical abstractHere we report the effect of the low temperature in the vibrational and structural properties of the C form of palmitic acid. The Raman spectra were obtained in the temperature interval from 300 to 18 K. The assignment of the duly observed bands was done based on the density functional theory. On cooling, the main changes observed in the lattice mode region of the Raman spectra were interpreted as a conformational modification undergone by the palmitic acid molecules in the unit cell. The X-ray diffraction measurements showing a slight modification in the lattice parameters at about 210 K. These techniques provided strong evidence of the conformational change in the molecules of palmitic acid at low temperatures.Figure optionsDownload full-size imageDownload as PowerPoint slide