Z-Bonds in Choline Chloride/Water Deep Eutectic Solvent: X-ray/Neutron Scattering and Density Functional Theory Calculations

Z-bond, a new weak interaction that couples H-bond and electrostatic interactions, plays an important role in ionic liquid and deep eutectic solvent (DES) formation. However, little direct experimental observation of the Z-bonds is available. In the present work, X-ray scattering (XRS) and isotope-substituted neutron scattering (ISNS) multi-data reverse driven all atomic modeling [empirical potential structure refinement (EPSR)] was employed to elucidate the microstructure of choline chloride (ChCl)/3H₂O DES. The results show that Z-bonds are the determinative driving force for Ch⁺ solvation, while H-bonds directly drive Cl^- solvation. Density functional theory (DFT) calculations confirm both bond motifs and quantify their strengths. H-bonds facilitate the formation of longer chains and larger rings, whereas Z-bonds predominantly result in the formation of medium-length chains and smaller rings. The size distribution of chains and rings formed by Z-bonds significantly surpasses that of H-bonds. Thus, the Z-bonds result in a lower diffusion coefficient of Ch⁺ [(0.0336 ± 0.0011) × 10^-5 cm²/s] than that of Cl^- [(0.0651 ± 0.0013) × 10^-5 cm²/s], emphasizing the efficacy of Z-bond structures in the modulation of transport properties.