How Many Water Molecules in the Hydration Shell of 18-Crown-6? Monte Carlo Simulations Based on ab Initio-Derived Potential Energy Surface

The solvation structure of 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6) in aqueous solution has been investigated using the Metropolis Monte Carlo scheme. Simulations have been performed for a system containing one 18-crown-6 molecule and 500 water molecules. An intermolecular potential function between 18-crown-6 and water has been newly developed, on the basis of ab initio calculations. The MCY potential model was employed to describe water−water interactions. Two water molecules were found in the first hydration shell of the ligand: one molecule was above the ligand plane and another was below it. Each water molecule forms mobile hydrogen bonds with the 18-crown-6 molecule; i.e., the O atom of the water molecule is localized above the ligand cavity, whereas the two H atoms were found to coordinate nonlocally to two of the three O atoms lying on each side of the ligand. In addition, precise orientation and detailed characteristics of the solvation have been comprehensively investigated and displayed.