Structures of 18-crown-6/Cs+ complexes in aqueous solutions by wide angle X-ray scattering and density functional theory

Abstract

• The oxygen atoms of 18-crown-6 and the hydrogen atoms of water molecule form strong hydrogen bonds in aqueous solution. • Cs + is located directly above the 18-crown-6 ring plane, about 1.15 Å from the centroid of 18-crown-6. • The higher the concentration of CsCl, the lower the complexation percentage of 18-crown-6 to Cs + . • Cs + complexing with 18-crown-6 mainly hydrates with four water molecules. Wide angle X-ray scattering (WAXS) and density functional theory (DFT) were employed to study the structures of aqueous 18-crown-6/CsCl solutions with CsCl concentrations of 0.25, 0.50 and 1.00 mol·L -1 . The empirical potential structure refinement (EPSR) modelling was adopted to get the pair distribution functions (PDFs), coordination numbers ( CN ), coordination number distributions (CND), angle distribution functions (ADFs), independent gradient model (IGM) and spatial density functions (SDFs) for the 18-crown-6 hydration, the complexation of 18-crown-6 and Cs + , the hydration of 18-crown-6/Cs + complex, and the association of Cl - and 18-crown-6/Cs + complex. Judging from the distance and orientation of water molecules, and the IGM between 18-crown-6 and water molecules, the oxygen atoms of 18-crown-6 (Oc) and the hydrogen atoms of water molecules (Hw) form strong hydrogen bonds, and these water molecules were distributed on both sides of the 18-crown-6 ring plane. However, the hydrogen atoms of 18-crown-6 (Hc) hardly form hydrogen bonds with the oxygen atoms of water molecules (Ow). Cs + is located directly above the 18-crown-6 ring plane of 0.35 to 2.05 Å away from the centroid of 18-crown-6, and forms 18-crown-6/Cs + complex with six Oc at r Cs + - O c = 3.06 Å. When the concentrations of CsCl are 0.25, 0.50 and 1.00 mol·L -1 , the complexation percentage of 18-crown-6 to Cs + are about 100%, 58% and 54%, respectively. No classic 2:1 “sandwich” 18-crown-6/Cs + complex structure was found in the solutions we surveyed. Both interaction energies and solvation stability energies reveal that the hydrates of 18-crown-6/Cs + complexes were formed in aqueous solutions. Cs + uncomplexed with 18-crown-6 hydrates with eight water molecules in aqueous solution, while Cs + can also hydrate with about four water molecules when it complexes with 18-crown-6. The results of atomic pair distances, the dipole distance of water molecules, the SDF and the IGM indicate that although water molecules can be located on both sides of the 18-crown-6 ring plane when the hydrates are formed, they hydrate with Cs + on the side where Cs + only existed. Cs + of 18-crown-6/Cs + complex associates with Cl - with the r Cs + - Cl - = 3.79 Å by electrostatic attraction.