Molecular dynamics Simulations of the squarate dianion (C4O42-) in aqueous solution

A molecular dynamics simulation study of the structure and dynamics of aqueous solutions of the squarate oxocarbon dianion (C4O42-) is presented. Analyses of the solute-solvent radial distribution functions and hydrogen (H)-bonding distributions indicate a well-defined hydration shell consisting of approximately 18 water molecules. About 12 of these molecules are tightly H-bonded to the oxocarbon (an average of three molecules per oxygen atom) forming a highly symmetric solute-solvent aggregate, whereas the remaining six water molecules (not bonded to the ion) are more loosely distributed above and below the oxocarbon plane. The mean residence time for molecules that are H-bonded to the solute is estimated to be larger than 20 ps, whereas molecules that are not directly bonded to the ion are frequently exchanged with the bulk and remain within the first solvation shell for times of the order of a few picoseconds. For the dynamics, we find that the translational motions projected along the squarate plane and perpendicular to it are comparable to each other. The rotational diffusion coefficients for the main symmetry axes also indicate that the spinning and tumbling motions of the oxocarbon are roughly isotropic despite the shape of the solute. The squarate is found to perform fast librational motions inside the solvente cage, with a characteristic frequency of approximately 70 cm(-1), in close agreement with recent experimental Raman band shape analysis. These results are discussed in the light of the hydration structure.