Hydration structures of the squarate dianion C_4O_4~(2-). A combined molecular dynamics simulation and quantum ab initio study

摘要

Molecular dynamics (MD) simulations are used to determine the structure of the first solvation shell around a single monocyclic oxocarbon dianion C_4O_4~(2-) in aqueous solution. The simulations were carried out using a fixed-geometry model for the oxocarbon with excess partial charges equally distributed over the oxygen atoms and the well-known SPC/E model for water. Quantum ab initio calculations for an isolated oxocarbon at different levels of approximation indicate that the such a description of the squarate dianion should be fairly accurate for the study of solvation structures. Analysis of a complete set of solute-solvent site-site radial distribution functions and hydrogen (H)-bonding distributions obtained from the MD simulations, indicates a well-defined first solvation shell consisting of approximately 18 water molecules. About 12 of these molecules are tightly H-bonded to the oxocarbon (an average of 3 molecules per oxygen atom) forming a highly symmetric solute-solvent complex, while the remaining six water molecules are more loosely distributed above and below the oxocarbon plane. The structure of a cluster consisting of a dianion and 12 water molecules was then examined through ab initio calculations at the Hartree-Fock 6-31G(d,p) level. The optimized ab initio structure of the cluster is in excellent agreement with the MD results.