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

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

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 five-fold 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]0aq 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 (∼250 nm) and SSIP/d (∼180 nm) conformers probably correspond to the absorption spectra of [CuCl]+aq and [Cu]2+aq 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 OH stretching vibration at ∼3000 cm−1 for the effect of half-dissociated chloride atoms

High-Order Ca(II)–Chloro Complexes in Mixed CaCl₂–LiCl Aqueous Solution: Insights from Density Functional Theory and Molecular Dynamics Simulations

In this study, the structural characteristics of high-coordinated Ca–Cl complexes present in mixed CaCl₂–LiCl aqueous solution were investigated using density functional theory (DFT) and molecular dynamics (MD) simulations. The DFT results show that [CaCl_<i>x</i>]^2–<i>x</i> (<i>x</i> = 4–6) clusters are quite unstable in the gas phase, but these clusters become metastable when hydration is considered. The MD simulations show that high-coordinated Ca–chloro complexes are possible transient species that exist for up to nanoseconds in concentrated (11.10 mol·kg^–1) Cl^– solution at 273 and 298 K. As the temperature increases to 423 K, these high-coordinated structures tend to disassociate and convert into smaller clusters and single free ions. The presence of high-order Ca–Cl species in concentrated LiCl solution can be attributed to their enhanced hydration shell and the inadequate hydration of ions. The probability of the [CaCl_<i>x</i>]^2–<i>x</i>_aq (<i>x</i> = 4–6) species being present in concentrated LiCl solution decreases greatly with increasing temperature, which also indicates that the formation of the high-coordinated Ca–Cl structure is related to its hydration characteristics

[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)n]− for n = 3 and 4 are coincident with the absorption of [CuCl3]−(aq) species (∼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 (∼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

Direct Contact versus Solvent-Shared Ion Pairs in Saturated NiCl₂ Aqueous Solution: A DFT, CPMD, and EXAFS Investigation

In this work, a systematic investigation of the competition coordination of H₂O and Cl^– with Ni²⁺ in saturated NiCl₂ aqueous solution at room temperature was conducted using density functional theory (DFT), Car–Parrinello molecular dynamics (CPMD) simulations, and extended X-ray absorption fine structure (EXAFS) spectra. The calculated results reveal that the six-coordinated structure is favorable for [NiCl_<i>x</i>(H₂O)_<i>n</i>]^2–<i>x</i> (<i>x</i> = 0–2; <i>n</i> = 1–12) clusters in the aqueous phase. The hydration energy calculation shows that the six-coordinated solvent-shared ion pair (SSIP) ([Ni­(H₂O)₆(H₂O)_<i>n</i>−6Cl]⁺) is more stable than its contact ion pair (CIP) ([NiCl­(H₂O)₅(H₂O)_<i>n</i>−5]⁺) isomer as <i>n</i> ≥ 9 in the aqueous phase, and the six-coordinated solvent-shared ion pair with a dissociated double Cl^– (SSIP/d) ([Ni­(H₂O)₆(H₂O)_<i>n</i>−6Cl₂]⁰) is more preferable than its CIP ([NiCl₂(H₂O)₄(H₂O)_<i>n</i>−4]⁰) and solvent-shared ion pair with single dissociated Cl^– (SSIP/s) ([NiCl­(H₂O)₅(H₂O)_<i>n</i>−5Cl]⁰) isomers as <i>n</i> ≥ 11. The six-coordinated SSIP/d ([Ni­(H₂O)₆(H₂O)_<i>n</i>−6Cl₂]⁰) conformers are the dominant structures in a saturated NiCl_2(aq) solution (NiCl₂ concentration: ∼5.05 mol·kg^–1, corresponding to <i>n</i> ≈ 11). The CPMD simulations exhibited that there are six water molecules with Ni–O distance at ∼205.0 pm on average around each Ni²⁺ in the first hydration sphere, even in the saturated NiCl₂ aqueous solution (∼5.05 mol·kg^–1) at room temperature, and no obvious Ni–Cl interaction was found. The EXAFS spectra revealed that the first solvation shell of Ni²⁺ is an octahedral structure with six water molecules tightly bound in the NiCl_2(aq) solution with a concentration ranging from 1.00 to 5.05 mol·kg^–1, and there is no obvious evidence of Ni–Cl contact ion pairs. A comprehensive conclusion from the DFT, CPMD, and EXAFS studies is that there is no obvious direct contact between Ni²⁺ and Cl^–, even in saturated NiCl₂ aqueous solution at room temperature

Ionic solvation and association in LiCl aqueous solution: a density functional theory, polarised continuum model and molecular dynamics investigation

<p>In this work, the ionic solvation and association behaviours in the LiCl aqueous solution were investigated using density functional theory (DFT), a polarised continuum model and classical molecular dynamics simulations. DFT calculations of LiCl(H₂O)_1–6,8 clusters show that contact ion pair (CIP) and solvent-shared ion pair (SSIP) conformers of LiCl(H₂O)<i>_n</i> (<i>n</i> ≥ 4) clusters are generally energetic both in the gas phase and in the aqueous solution. Some SSIP conformers may be slightly more stable than their CIP isomers when at least eight water molecules are incorporated in the inner hydration shells of LiCl hydrates. The transformation between CIP and SSIP conformers is easy by overcoming a small energy barrier, which mainly results from the hydration shell reorganisation of Li⁺. Molecular dynamics simulations show that ion pairs or ion clusters can be found in the LiCl aqueous solution, and the probability of CIP conformers or ion clusters presented in the LiCl solution generally increases with rise in temperature. However, the presentation of ion pairs or ion clusters in the LiCl aqueous solution does not inevitably lead to the nucleation of LiCl crystallisation.</p