Georges Wipff

Force field representation of the UO22+ cation from free energy MD simulations in water. Tests on its 18-crown-6 and NO3− adducts, and on its calix[6]arene6− and CMPO complexes

Abstract We report a theoretical study of the UO22+ cation in aqueous solution, using an empirical 1-6-12 representation of the nonbonded interaction energies. Based on free energy perturbation FEP calculations, we first derive new parameters for UO22+ which account for its hydration free energy, compared with the Sr2+ cation (ΔG = 13.6 kcal mol−1). With the new parameters, we simulate the UO22+-2NO3− salt and the UO22+.18-crown-6 adduct which both dissociate in water, and the CMPO.UO2(NO3)2 complexes of 1:1 and 1:2 stoichiometries in which the ligand remains bound to UO22+. The complex of UO22+ with the calix[6]arene6− hexanion remains inclusive, and predicted by free energy simulations to be more stable than the Sr2+ complex.

Solvation of M3+ lanthanide cations in room-temperature ionic liquids. A molecular dynamics investigation

We report a molecular dynamics study of the solvation of M3+ lanthanide cations (La3+, Eu3+ and Yb3+) in two room-temperature ionic liquids: [BMI][PF6] based on 1-butyl-3-methyl-imidazolium+,PF6− and [EMI][TCA] based on 1-ethyl-3-methyl-imidazolium+,AlCl4−. They reveal specific solvation as a function of the cation size and on the solvent. In [BMI][PF6] solution, the three studied M3+ cations are surrounded by six PF6− anions, while in [EMI][TCA] solution, they are surrounded by eight AlCl4− anions. The precise binding mode of the AlCl4− or PF6− solvent anions (denticity, number of coordinated halides and dynamics) depends on the M3+ cation size. The first shell PF6− anions rotate markedly during the dynamics while AlCl4− do not. In both liquids, this first ionic shell is surrounded by about 11–13 imidazolium cations. The solvation of the neutralizing NO3− counterions is similar in both liquids, and involves mainly imidazolium+ cations in the first shell (≈5.0 in [BMI][PF6] and 4.3 in [EMI][TCA]). When simulated in the gas phase, the first anionic shells are similar, but more rigid than in the solutions. Finally, free energy calculations are performed to compare the Eu3+/La3+ and Yb3+/Eu3+ solvation. In both ionic liquids, the smaller cations are better solvated, but the differences from one cation to the other are weaker than in water.

Demixing of Binary Water−Chloroform Mixtures Containing Ionophoric Solutes and Ion Recognition at a Liquid−Liquid Interface: A Molecular Dynamics Study

We report a series of molecular dynamics simulations on the demixing of “homogeneous” binary water−chloroform mixtures containing species involved in the assisted ion extraction process. We consider an ionophore L (L = 1,3-alternate calix4arene-crown6), uncomplexed salts of Cs+ and the LCs+ and LNa+ cation complexes with a lipophilic (Pic-) and a hydrophilic (Cl-) counterion, respectively, as being solutes. In all cases, the liquids separate rapidly, leading to two solvent slabs separated by a well-defined interface. However, the final state is very different, depending on the hydrophilic/hydrophobic balance of the solutes:  the Cs+ and NO3- ions of the CsNO3 salt are completely immersed in the aqueous phase, whereas Pic- anions display a strong adsorption at the interface. The LCs+ complex and the free ligand L, although more soluble in the organic phase than in water, also display a surfactant like behavior. Similar conclusions are obtained when L, LCs+, Cs+ Pic-, and Cs+ NO3- ions are simultaneously pr...

Complexation of alkali cations by calix[4]crown ionophores: Conformation and solvent dependent Na+/Cs+ binding selectivity and extraction: MD simulations in the gas phase, in water and at the chloroform-water interface

Abstract Calix[4]arenes bridged by crown5 and crown6 moieties represent a promising class of ionophores for big alkali cations. We present a theoretical demonstration of the possible modulation of the hostguest complementarity and recognition via the conformation of the host, and the solvent. Molecular dynamics and free energy calculations are reported for the 1,3-dimethoxy-p-tert-butyl and the p-H-derivatives in the cone, 1,3-alternate, and partial cone conformations, simulated in the gas phase and in water. In the gas phase, a decrease in binding affinity is calculated for the three forms of the complexes, from Na+ to Cs+. Intrinsically, the largest ions prefer clearly the 1,3-alternate conformers, while Na+ prefers slightly the cone conformers. In aqueous solution, the change in free energy for mutating the Na+ calix[4]-crown6 complex to the Cs+ complex depends markedly on the conformation of the calixarene, and is about 12 kcal/mol weaker for the 1,3-alternate than for the cone form. Taking into accou...

Insights into uranyl chemistry from molecular dynamics simulations.

First-principles and purely classical molecular dynamics (MD) simulations for complexes of the uranyl ion (UO(2)(2+)) are reviewed. Validation of Car-Parrinello MD simulations for small uranyl complexes in aqueous solution is discussed. Special attention is called to the mechanism of ligand-exchange reactions at the uranyl centre and to effects of solvation and hydration on coordination and structural properties. Large-scale classical MD simulations are surveyed in the context of liquid-liquid extraction, with uranyl complexes ranging from simple hydrates to calixarenes, and nonaqueous phases from simple organic solvents and supercritical CO(2) to ionic liquids.

Theoretical calculations of extraction selectivity: Alkali cation complexes of calix[4]-bis-crown6 in pure water, chloroform, and at a water/chloroform interface

We report theoretical calculations of ion extraction selectivity by ionophores, based on molecular dynamics simulations coupled with the free energy perturbation technique. This method is applied to the Calix[4]‐bis‐crown6 (L) ionophore, which displays remarkable selectivity for Cs+ over Na+ extraction from an aqueous to a chloroform phase. Using a thermodynamic cycle, we model the cation extraction selectivity of L from water to chloroform and calculate a peak for Cs+, in agreement with the experiment. This high Cs+ ionophoricity is accounted for mostly by differential solvation effects, with standard 1–6–12 pairwise potentials without need of “special π interactions” with the ionophore. The effect of a picrate (Pic−) counterion on structures and selectivities is investigated. Finally, we report simulations on the L ionophore free and on the LCs+ and LCs+Pic− complexes at the water/chloroform interface. We find that all these species are “adsorbed” at the interface like surfactants instead of diffusing spontaneously to the organic phase.

Water versus acetonitrile coordination to uranyl. Density functional study of cooperative polarization effects in solution.

Optimizations at the BLYP and B3LYP levels are reported for mixed uranyl-water/acetonitrile complexes [UO(2)(H(2)O)(5-n)(MeCN)(n)](2+) (n = 0-5), in both the gas phase and a polarizable continuum modeling acetonitrile. Car-Parrinello molecular dynamics (CPMD) simulations have been performed for these complexes in the gas phase, and for selected species (n = 0, 1, 3, 5) in a periodic box of liquid acetonitrile. According to structural and energetic data, uranyl has a higher affinity for acetonitrile than for water in the gas phase, in keeping with the higher dipole moment and polarizability of acetonitrile. In acetonitrile solution, however, water is the better ligand because of specific solvation effects. Analysis of the dipole moment of the coordinated water molecule in [UO(2)(H(2)O)(MeCN)(4)](2+) reveals that the interaction with the second-shell solvent molecules (through fairly strong and persistent O-H···N hydrogen bonds) causes a significant increase of this dipole moment (by more than 1 D). This cooperative polarization of water reinforces the uranyl-water bond as well as the water solvation via strengthened (UO(2))OH(2)···NCMe hydrogen bonds. Such cooperativity is essentially absent in the acetonitrile ligands that make much weaker (UO(2))NCMe···NCMe hydrogen bonds. Beyond the uranyl case, this study points to the importance of cooperative polarization effects to enhance the M(n+) ion affinity for water in condensed phases involving M(n+)-OH(2)···A fragments, where A is a H-bond proton acceptor and M(n+) is a hard cation.

Density Functional Theory Study of Uranium(VI) Aquo Chloro Complexes in Aqueous Solution

Mixed uranyl aquo chloro complexes of the type [UO2(H2O)xCly]2-y (y = 1, 2, 3, 4; x + y = 4, 5) have been optimized at the BLYP, BP86, and B3LYP levels of density functional theory in vacuo and in a polarizable continuum modeling bulk water (PCM) and have been studied at the BLYP level with Car-Parrinello molecular dynamics (MD) simulations in the gas phase and in explicit aqueous solution. Free binding energies were evaluated from static PCM data and from pointwise thermodynamic integration involving constrained MD simulations in water. The computations reveal significant solvent effects on geometric and energetic parameters. Based on the comparison of PCM-optimized or MD-averaged uranyl-ligand bond distances with EXAFS-derived values, the transition between five- and four-coordination about uranyl is indicated to occur at a Cl content of y = 2 or 3.

Do Perchlorate and Triflate Anions Bind to the Uranyl Cation in an Acidic Aqueous Medium? A Combined EXAFS and Quantum Mechanical Investigation

Structural properties of uranyl cations in acidic aqueous perchlorate and triflate solutions were investigated using uranium LIII -edge extended X-ray absorption fine-structure spectroscopy (EXAFS) in conjunction with quantum mechanical calculations of gas-phase model complexes. EXAFS spectra were measured in aqueous solutions of up to 10 M triflic and 11.5 M perchloric acid, as well as mixtures of perchloric acid and sodium perchlorate. In no case is the perchlorate anion coordinated to UO2(2+). The number of equatorial water molecules bound to UO2(2+) is always about five. In the case of the 10 M CF3SO3H solution, an inner-sphere complexation of the triflate is observed with a U-S radial distance of 3.62 Å. These results are in qualitative agreement with quantum mechanical calculations of model uranyl complexes, according to which the interaction energies of anions follow the order perchlorate

Competitive Complexation of Nitrates and Chlorides to Uranyl in a Room Temperature Ionic Liquid

By coupling EXAFS, UV-vis spectroscopy, and molecular dynamics and quantum mechanical calculations, we studied the competitive complexation of uranyl cations with nitrate and chloride ions in a water immiscible ionic liquid (IL), C(4)mimTf(2)N (C(4)mim(+): 1-butyl-3-methyl-imidazolium; Tf(2)N(-) = (CF(3)SO(2))(2)N)(-): bis(trifluoromethylsulfonyl)imide). Both nitrate and chloride are stronger ligands for uranyl than the IL Tf(2)N(-) or triflate anions and when those anions are simultaneously present, neither the limiting complex UO(2)(NO(3))(3)(-) nor UO(2)Cl(4)(2-) alone could be observed. At a U/NO(3)/Cl ratio of 1/2/2, the dominant species is likely UO(2)Cl(NO(3))(2)(-). When chloride is in excess over uranyl with different nitrate concentrations (U/NO(3)/Cl ratio of 1/2/6, 1/4/4, and 1/12/4) the solution contains a mixture of UO(2)Cl(4)(2-) and UO(2)Cl(3)(NO(3))(2-) species. Furthermore, it is shown that the experimental protocol for introducing these anions to the solution (either as uranyl counterion, as added salt, or as IL component) influences the UV-vis spectra, pointing to the formation of different kinetically equilibrated complexes in the IL.