We use density functional theory (B3LYP) and the COSMO continuum solvent model to characterize the structure and stability of the hydrated Cu(II) complexes [Cu(MeNH_2)(H_2O)_(n−1)]^(2+) and [Cu(OH)_x(H_2O)_(n−x)]^(2−x) (x = 1−3) as a function of metal coordination number (4−6) and cluster size (n = 4−8, 18). The small clusters with n = 4−8 are found to be the most stable in the nearly square-planar four-coordinate configuration, except for [Cu(OH)_3(H_2O)]^−, which is three-coordinate. In the presence of the two full hydration shells (n = 18), however, the five-coordinate square-pyramidal geometry is the most favorable for Cu(MeNH_2)^(2+) (5, 6) and Cu(OH)^+ (5, 4, 6), and the four-coordinate geometry is the most stable for Cu(OH)_2 (4, 5) and Cu(OH)_3^− (4). (Other possible coordination numbers for these complexes in the aqueous phase are shown in parentheses.) A small energetic difference between these structures (0.23−2.65 kcal/mol) suggests that complexes with different coordination numbers may coexist in solution. Using two full hydration shells around the Cu^(2+) ion (18 ligands) gives Gibbs free energies of aqueous reactions that are in excellent agreement with experiment. The mean unsigned error is 0.7 kcal/mol for the three consecutive hydrolysis steps of Cu^(2+) and the complexation of Cu^(2+) with methylamine. Conversely, calculations for the complexes with only one coordination shell (four equatorial ligands) lead to a mean unsigned error that is >6.0 kcal/mol. Thus, the explicit treatment of the first and the second shells is critical for the accurate prediction of structural and thermodynamic properties of Cu(II) species in aqueous solution.

Computational study of copper(II) complexation and hydrolysis in aqueous solutions using mixed cluster/continuum models.

Solvent- and catalyst-free synthesis of an azine-linked covalent organic framework and the induced tautomerization in the adsorption of U(VI) and Hg(II)

The hydrated structure of the Cu(II) ion has been a subject of ongoing debate in the literature. In this article, we use density functional theory (B3LYP) and the COSMO continuum solvent model to characterize the structure and stability of [Cu(H2O)n]2+ clusters as a function of coordination number (4, 5, and 6) and cluster size (n = 4−18). We find that the most thermodynamically favored Cu(II) complexes in the gas phase have a very open four-coordinate structure. They are formed from a stable square-planar [Cu(H2O)8]2+ core stabilized by an unpaired electron in the Cu(II) ion dx2−y2 orbital. This is consistent with cluster geometries suggested by recent mass-spectrometric experiments. In the aqueous phase, we find that the more compact five-coordinate square-pyramidal geometry is more stable than either the four-coordinate or six-coordinate clusters in agreement with recent combined EXAFS and XANES studies of aqueous solutions of Cu(II). However, a small energetic difference (~1.4 kcal/mol) between the five- and six-coordinate models with two full hydration shells around the metal ion suggests that both forms may coexist in solution.

Hydration of copper(II): new insights from density functional theory and the COSMO solvation model.

The geometric and electronic structures of the aqua, chloro, acetato, hydroxo and carbonato complexes of U, Np and Pu in both their (VI) and (V) oxidation states, and in an aqueous environment, have been studied using density functional theory methods. We have obtained micro-solvated structures derived from molecular dynamics simulations and included the bulk solvent using a continuum model. We find that two different hydrogen bonding patterns involving the axial actinyl oxygen atoms are sometimes possible, and may give rise to different An–O bond lengths and vibrational frequencies. These alternative structures are reflected in the experimental An–O bond lengths of the aqua and carbonato complexes. The variation of the redox potential of the uranyl complexes with the different ligands has been studied using both BP86 and B3LYP functionals. The relative values for the four uranium complexes having anionic ligands are in surprisingly good agreement with experiment, although the absolute values are in error by ∼1 eV. The absolute error for the aqua species is much less, leading to an incorrect order of the redox potentials of the aqua and chloro species.

The geometric structures, vibrational frequencies and redox properties of the actinyl coordination complexes ([AnO(2)(L)(n)](m); An = U, Pu, Np; L = H2O, Cl-, CO32-, CH3CO2-, OH-) in aqueous solution, studied by density functional theory methods

First principles molecular dynamics simulations of the hydration shells surrounding UO(2)(2+) ions are reported for temperatures near 300 K. Most of the simulations were done with 64 solvating water molecules (22 ps). Simulations with 122 water molecules (9 ps) were also carried out. The hydration structure predicted from the simulations was found to agree with very well-known results from x-ray data. The average U=O bond length was found to be 1.77 A. The first hydration shell contained five trigonally coordinated water molecules that were equatorially oriented about the O-U-O axis with the hydrogen atoms oriented away from the uranium atom. The five waters in the first shell were located at an average distance of 2.44 A (2.46 A, 122 water simulation). The second hydration shell was composed of distinct equatorial and apical regions resulting in a peak in the U-O radial distribution function at 4.59 A. The equatorial second shell contained ten water molecules hydrogen bonded to the five first shell molecules. Above and below the UO(2)(2+) ion, the water molecules were found to be significantly less structured. In these apical regions, water molecules were found to sporadically hydrogen bond to the oxygen atoms of the UO(2)(2+), oriented in such a way as to have their protons pointed toward the cation. While the number of apical waters varied greatly, an average of five to six waters was found in this region. Many water transfers into and out of the equatorial and apical second solvation shells were observed to occur on a picosecond time scale via dissociative mechanisms. Beyond these shells, the bonding pattern substantially returned to the tetrahedral structure of bulk water.

Equatorial and apical solvent shells of the UO22+ ion

http://www.vitorge.name/pierre/publis/06sib_ma.pdf

A theoretical study of uranyl solvation: Explicit modelling of the second hydration sphere by quantum mechanical methods

Gibson and coworkers have recently shown that the PaO2 2+ ion, which formally contains Pa(VI), can be prepared in the gas phase.1 As far as we are aware, this was the first report of the preparation of a “hypervalent” Pa compound. However, the energy necessary to produce this species is large, as the ionization energy of the “actinyl” ion PaO2 + that contains Pa(V) was calculated to be 16.6 eV. One would normally imagine that such high-energy species cannot exist as stable compounds. Yet the ionization energy of UO2 + is estimated 2 to be 15.2 eV, not much less than 16.6 eV, and the uranyl ion, UO2 2+, which formally contains U(VI), is of course stable in both the solid state and in aqueous solution (at low pH). Since the ionization energy of ThO2 is “only” 8.7 eV, 3

http://www.vitorge.name/pierre/publis/09mar_p.pdf

B. Siboulet

Papers

A theoretical study of uranyl solvation: Explicit modelling of the second hydration sphere by quantum mechanical methods

Can stable molecular compounds of Th(V) and Pa(VI) exist