RESUMO
In this study we present the results of a first principles molecular dynamics simulation of a single 1-ethyl-3-methyl-imidazolium chloride [C(2)C(1)im][Cl] ion pair dissolved in 60 water molecules. We observe a preference of the in plane chloride coordination with respect to the cation ring plane as compared to the energetic slightly more demanding on top coordination. Evaluation of the different radial distribution functions demonstrates that the structure of the hydration shell around the ion pair differs significantly from bulk water and that no true ion pair dissociation in terms of completely autonomous solvation shells takes place on the timescale of the simulation. In addition, dipole moment distributions of the solvent in distinct solvation shells around different functional parts of the [C(2)C(1)im][Cl] ion pair are calculated from maximally localized Wannier functions. The analysis of these distributions gives evidence for a depolarization of water molecules close to the hydrophobic parts of the cation as well as close to the anion. Examination of the angular distribution of different OH(H(2)O)-X angles in turn shows a linear coordination of chloride accompanied by a tangential orientation of water molecules around the hydrophobic groups, being a typical feature of hydrophobic hydration. Based on these orientational aspects, a structural model for the obvious preference of ion pair association is developed, which justifies the associating behavior of solvated [C(2)C(1)im][Cl] ions in terms of an energetically favorable interface between the solvation shells of the anion and the hydrophobic parts of the cation.
RESUMO
Different cluster sets containing only 2-fold coordinated water, 2- and 3-fold coordinated water, and 2-fold, 3-fold, and tetrahedrally coordinated water molecules were investigated by applying second-order Møller-Plesset perturbation theory and density functional theory based on generalized gradient approximation functionals in the framework of the quantum cluster equilibrium theory. We found an improvement of the calculated isobars at low temperatures if tetrahedrally coordinated water molecules were included in the set of 2-fold hydrogen-bonded clusters. This was also reflected in a reduced parameter for the intercluster interaction. If all parameters were kept constant and only the electronic structure methods were varied, large basis set dependencies in the liquid state for the density functional theory results were found. The behavior of the intercluster parameter was also examined for the case that cooperative effects were neglected. The values were 3 times as large as in the calculations including the total electronic structure. Furthermore, these effects are more severe in the tetrahedrally coordinated clusters. Different populations were considered, one weighted by the total number of clusters and one depending on the monomers.
RESUMO
With the aid of the quantum cluster equilibrium method, we calculate thermodynamic properties for a new water cluster set containing 2-fold and additional tetrahedrally hydrogen-bonded water molecules on the basis of accurate correlated electronic structure calculations. The addition of clusters with 4-fold coordinated water molecules leads to an improved thermodynamical description of the liquid phase in comparison to experimental values. The comparison of the obtained isobars from the pure 2-fold cluster set with the mixed cluster set shows improved results for the mixed set. Furthermore, the results of the liquid-phase entropy calculation compare excellently with experiment if the mixed cluster set is applied. The calculated populations allow us to determine hydrogen bond numbers, resulting in a temperature-dependent average hydrogen bond number. We observe a decreasing average hydrogen bond number of 2.77 at 274 K to 2.26 at 373 K and a dominance of 75% 2-fold hydrogen-bonded water molecules at room temperature for the mixed cluster set.