[CuCl3]- and [CuCl4]2- Hydrates in Concentrated Aqueous Solution: A Density Functional Theory and ab Initio Study

In this work, structures and thermodynamic properties of [CuCl3] and [CuCl4]2 hydrates in aqueous solution were investigated using density functional theory and ab initio methods. Contact ion pair (CIP) and solvent-shared ion pair (SSIP) structures were both taken into account. Our calculations suggest that [CuCl3(H2O)(n)](-) clusters might favor a four-coordinated CIP structure with a water molecule coordinating with the copper atom in the equatorial position for n = 3 and 4 in aqueous solution, whereas the four-coordinated SSIP structure with one chloride atom dissociated becomes more stable as n increases to 5. For the [CuCl4](2-) cluster, the four-coordinated tetrahedron structure is more stable than the square-planar one, whereas for [CuCl4(H2O)(n)](2-) (n >= 1) clusters, it seems that four-coordinated SSIP structures are slightly more favorable than CIP structures. Our calculations suggest that Cu2+ perhaps prefers a coordination number of 4 in CuCl2 aqueous solution with high Cl concentrations. In addition, natural bond orbital (NBO) calculations suggest that there is obvious charge transfer (CT) between copper and chloride atoms in [CuClx](2-x) (x = 1-4) clusters. However, compared with that in the [CuCl2](0) cluster, the CT between the copper and chloride atoms in [CuCl3](-) and [CuCl4](2-) clusters becomes negligible as the number of attached redundant Cl ions increases. This implies that the coordination ability of Cl is greatly weakened for [CuCl3] and [CuCl4](2) clusters. Electronic absorption spectra of these different hydrates were obtained using long-range-corrected time-dependent density functional theory. The calculated electronic transition bands of the four-coordinated CIP conformer of [CuCl3(H2O)] for n = 3 and 4 are coincident with the absorption of [CuCl3](-) (aq) species (similar to 284 and 384 nm) resolved from UV spectra obtained in CuCl2 (ca. 10(-4) mol.kg(-1)) + LiCl (>10 mol.kg(-1)) solutions, whereas the calculated bands of [CuCl3(H2O)(n)](-) in their most stable configurations are not when n = 0 - 2 or n > 4, which means that the species [CuCl3] (aq) exists in those CuCl2 aqueous solutions in which the water activity is neither too low nor too high. The calculated bands of [CuCl4(H2O)(n)](2-) clusters correspond to the absorption spectra (similar to 270 and 370 nm) derived from UV measurements only when n = 0, which suggests that [CuCl4](2-) (aq ) species probably exist in environments in which the water activity is quite low.