Hydrates of Copper Dichloride in Aqueous Solution: A Density Functional Theory and Polarized Continuum Model Investigation

In this work, the hydrates of copper dichloride in gas and aqueous phase have been investigated using the B3LYP method. Low-lying conformers of CuCl2(H2O)(n) clusters for n = 1 - 10 were obtained by an extensive conformation search. Contact ion pair (CIP) and solvent-shared ion pair (SSIP) with one dissociated chloride atom (SSIP/s) and SSIP with two dissociated chloride atoms (SSIP/d) all were considered. Our calculations present such a trend that a four-fold CIP conformer is more favorable for CuCl2(H2O)(n) cluster (n <= 7) and four-fold SSIP/s for n = 8-10 in the gas phase, while in aqueous solution, more stable structures are fivefold SSIP/s conformer for n = 7-9 and four-fold CIP conformer for n = 2-6. Hydrogen bond (HB) plays an important role in the CuCl2 solvation, especially HBs formed between the first and second solvation shell water molecules. Electronic absorption spectra of CuCl2(H2O)(n) clusters were obtained using long-range-corrected time-dependent density functional theory. The calculated electronic absorption peak around 270 nm of CIP conformers is coincident with the absorption of [CuCl2](aq)(0) species resolved from the spectra obtained in solutions of trace CuCl2 (ca. 10(-5) mol/kg) + LiCl (0 - 18 m), while those of SSIP/s (similar to 250 nm) and SSIP/d (similar to 180 nm) conformers probably correspond to the absorption spectra of [CuCl](aq)(+) and [Cu](aq)(2+) species, respectively. Natural bond orbital charge population analyses show that charge transfer (CT) between a central copper(II) atom and ligands (Cl and H2O) increases as the hydrated cluster expands, especially CT from Cu2+ to the first solvation shell, which enhances the strength of HBs. Such CT becomes more apparent for SSIP structure with the dissociation of chloride ion. OH stretching vibration frequencies of proton donor type water in CuCl2(H2O)(n) Clusters are obviously red-shifted in comparison to those of water clusters, due to CT between the central atom Cu and ligands. SSIP conformers have apparent IR absorption peaks of OR stretching vibration at similar to 3000 cm(-1) for the effect of half-dissociated chloride atoms.