Ethylene Glycol Dihedral Angle Dynamics: Relating Molecular Conformation to the Raman Spectrum of the Liquid

The central OCCO dihedral of the ethylene glycol (EG) molecule exists in both trans and gauche geometries in the liquid. The presence of the trans conformer had been inferred from the Raman spectra by interpreting the occurrence of bands in the Raman spectra that were absent in the infrared as evidence for inversion symmetry and hence the trans conformation. The validity of this interpretation is questionable as not all conformations of the EG molecule, where the OCCO dihedral is trans, possess inversion symmetry. We show here that the resolution of the apparent paradox is intimately related to the conformation and dynamics of not just the central OCCO but also the two terminal dihedrals of the EG molecule. Using ab initio molecular dynamics simulations, we show that changes in conformation associated with the three dihedral angles are not infrequent; a number of events are observed during the course of the simulations allowing for a straightforward estimate of the kinetic parameters. More importantly, these parameters allow us to address and resolve the problems in interpreting the Raman spectra and consequently relate molecular conformation to the Raman spectrum of the EG molecule in the liquid state.