Hydrogen bonds in aqueous choline chloride solutions by DFT calculations and X-ray scattering

Choline chloride (ChCl) is a common hydrogen bond acceptor (HBA) for deep eutectic solvents (DESs), and the hydrogen bonds between ChCl and H2O impact the physicochemical properties of DES-water mixtures. In the present work, the hydrogen bonds in aqueous ChCl solutions were investigated combining with Raman spectroscopy, density functional theory (DFT) calculations, X-ray scattering and empirical potential structure refinement (EPSR). The Ch+ in aqueous ChCl solutions mainly exists as the gauche conformation. The aqueous ChCl solutions were found to be stabilized by both classical hydrogen bonds (O-H⋯Y, Y = Cl, OW) and carbon-hydrogen bonds (C-H⋯Y, Y = OW) between the components. The orientation of the hydrogen bonds is close to linear (∼180°), while the orientation of the carbon-hydrogen bonds is relatively weak (150°∼176°). The existence of carbon-hydrogen bonds elucidates the high coordination of quaternary ammonium group with H2O. Carbon-hydrogen bonds are found in the partial radial distribution functions, the angle distribution functions and the atoms in molecules analysis. Hydrogen bonds and carbon-hydrogen bonds can affect the physicochemical properties of aqueous ChCl solutions and DES-H2O mixtures. This fundamental understanding of the aqueous ChCl solutions will enable future developments in functional design and application of choline-based DESs.