[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 CuCl(3) and CuCl(4) 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 CuCl(3)(H(2)O)(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 CuCl(4) cluster, the four-coordinated tetrahedron structure is more stable than the square-planar one, whereas for CuCl(4)(H(2)O)(n) (n ≥ 1) clusters, it seems that four-coordinated SSIP structures are slightly more favorable than CIP structures. Our calculations suggest that Cu(2+) perhaps prefers a coordination number of 4 in CuCl(2) 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 CuCl(x) (x = 1-4) clusters. However, compared with that in the CuCl(2) cluster, the CT between the copper and chloride atoms in CuCl(3) and CuCl(4) clusters becomes negligible as the number of attached redundant Cl(-) ions increases. This implies that the coordination ability of Cl(-) is greatly weakened for CuCl(3) and CuCl(4) 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 CuCl(3)(H(2)O)(n) for n = 3 and 4 are coincident with the absorption of CuCl(3)(aq) species (∼284 and 384 nm) resolved from UV spectra obtained in CuCl(2) (ca. 10(-4) mol·kg(-1)) + LiCl (>10 mol·kg(-1)) solutions, whereas the calculated bands of CuCl(3)(H(2)O)(n) in their most stable configurations are not when n = 0 - 2 or n > 4, which means that the species CuCl(3)(aq) exists in those CuCl(2) aqueous solutions in which the water activity is neither too low nor too high. The calculated bands of CuCl(4)(H(2)O)(n) clusters correspond to the absorption spectra (∼270 and 370 nm) derived from UV measurements only when n = 0, which suggests that CuCl(4)(aq) species probably exist in environments in which the water activity is quite low.