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 (OH center dot center dot center dot Y, Y = Cl, OW) and carbon-hydrogen bonds (C-H center dot center dot center dot Y, Y = OW) between the components. The orientation of the hydrogen bonds is close to linear (-180 degrees), while the orientation of the carbon-hydrogen bonds is relatively weak (150 degrees-176 degrees). 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. (c) 2022 Elsevier B.V. All rights reserved.