Correlations among the Raman spectra and the conformational compositions of ethylene glycol, 1,2- and 1,3-propylene glycols

Abstract

• Raman bands about 800 cm -1 are markers of conformational compositions of the diols. • Experiments and DFT calculations show prevalence of gauche conformers for the diols. • The gas-phase approximation does not describe fully Raman spectra of the liquid diols. • DFT spectra of molecular clusters match well experimental spectra of liquid EG. • Intermolecular interactions lead to significant distortion of molecular conformations. Raman spectra of liquid ethylene glycol (EG), 1,2-propylene glycol (1,2-PG) and 1,3-propylene glycol (1,3-PG) were analyzed in terms of the conformational composition of the molecules. The analysis was based on comparison of the experimental spectra with the results of the density functional theory (DFT) calculations of the structure, energy and Raman spectra for 10 conformations of EG molecule, 27 conformations of 1,2-PG molecule and 25 conformations of 1,3-PG molecule. The gas-phase approximation calculations well describe the main features of the experimental spectra of the liquid substances in the region 600 – 1600 cm −1 . Calculations for clusters of EG molecules showed that an increase in the number of molecules in a cluster leads to significant improvement of the agreement between the calculated and experimental spectra. The conformations of molecules in the clusters are strongly distorted in comparison with the results of the gas-phase approximation. Thus, the intermolecular interactions influence significantly the conformational order and the Raman spectra of the diols.