Hydrates of Cu²⁺ and CuCl⁺ in Dilute Aqueous Solution: A Density Functional Theory and Polarized Continuum Model Investigation

Abstract

In this work, structures, and properties of Cu2+ and CuCl+ hydrates in the gas and aqueous phases have been investigated using the B3LYP method. Contact ion pair (CIP) and solvent-shared ion pair (SSIP) were both taken into account for CuCl+ hydrates. Our calculations show that [Cu(H2O)n]2+ clusters favor a very open four-coordinated structure for n = 5−12 in the gas phase, while a five-coordinated conformer is favored for n ≥ 8 in the aqueous phase. An approximate complete solvation shell of Cu2+ in the aqueous phase needs more than 12 water molecules, while that of CuCl+ in the aqueous phase needs only about eight water molecules. For [CuCl(H2O)n]+ clusters, the most stable structure is a four-coordinated CIP conformer for n = 4−7 in the gas phase and a five-coordinated CIP conformer for n = 8−10 in the aqueous phase. However, the five-coordinated CIP/h conformer (CIP conformer that the axial chloride atom tends to dissociate) of [CuCl(H2O)n]+ clusters becomes more favorable as n increases to 11. As the hydration process proceeds, the charges on the copper atom of [Cu(H2O)n]2+ clusters decrease, while those of [CuCl(H2O)n]+ clusters increase (probably due to the dissociation of Cl−). The d−d electron transition and partial charge transition band around 160 nm of the five-coordinated conformer of [Cu(H2O)n]2+ clusters and those bands (∼170 and ∼160 nm) of SSIP or five-coordinated CIP/h conformers of [CuCl(H2O)n]+ clusters are coincident with the absorption of [Cu]2+(aq) species (∼180 nm) resolved from the spectra obtained in trace CuCl2 (ca. 10−5 mol·kg−1) + LiCl (0−18 mol·kg−1) aqueous solution, while those of five-coordinated CIP conformers of [CuCl(H2O)n]+ clusters (n = 8 and 9) around 261 and 247 nm correspond to the absorption of [CuCl]+(aq) species (∼250 nm). Our calculated electronic spectra indicate that the typical peak of copper(II)−chloride complexes changes from 180 to 250 nm, and 275 nm, as the process of Cl− coordination. For [Cu]2+(aq), [CuCl]+(aq), and [CuCl2]0(aq) species, the central Cu(II) atom prefers five-coordination