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The denticity, flexibility, and steric hindrance of the ligand are key factors in deciding the mode and number of coordination around a metal ion on complex formation. The thermodynamic aspects of lanthanide complexation with various multidentate ligands provides a significant insight into understand the coordination chemistry of lanthanides in framing the relevant metal organic networks for the applications in biological, biochemical and medical aspects. The pyrazine carboxylic acids are known to form many structurally important complexes and further can form chelates with coordination number of eight for europium in which more water molecules can be knocked out from the primary coordination sphere than demanded by denticity of the ligand. The present studies aimed at ESI-MS characterization and determination of the thermodynamic parameters (log β, Δ

All atom molecular dynamics simulations and experiments were performed to understand the adsorption behavior of gadolinium (Gd3+) ion on the crown ethers grafted polystyrene (PS) surface. Two different types of crown ethers, viz., dibenzo crown ether (DBCE) and dicyclo hexano crown ether (DCHCE), were grafted separately on the PS surface to understand the adsorption behavior. We investigate the roles of Gd3+ ion concentration and grafting density (ρs) of the crown ether on the adsorption behavior of Gd3+ ion on the PS surface. The adsorption of Gd3+shows an increasing trend with increasing salt concentration, for all cases of crown ether grafting densities. The adsorption behavior follows the Langmuir isotherm model. The maximum amount of Gd3+ ion adsorption was observed to be 1.83 mg/g for DBCE and 2.02 mg/g for DCHCE at ρs = 2.07 mol/nm2. The maximum amount of Gd3+ ion adsorption on DBCE coated PS beads, in batch experiments, was found to be 1.76 mg/g, which is in good agreement with the theoretical res...

A combined experimental and theoretical study has been put forward to investigate the complexation behaviour of tetravalent Pu 4+ and hexavalent UO 2 2+ ions with the N,N-dihexyloctanamide (DHOA) ligand in different diluents. Solvent extraction experiments were conducted with Pu 4+ and UO 2 2+ ions using DHOA in different diluents, namely dodecane, toluene, chloroform, octanol, nitrophenyl-octyl ether (NPOE) and nitrobenzene (NB). The experimentally measured distribution coefficients (D) of both the Pu 4+ and UO 2 2+ ions followed the order: nitrobenzene > NPOE > octanol > chloroform > toluene > dodecane. In order to complement the experimental distribution data, the structure, energetic and thermodynamic parameters of the Pu 4+ and UO 2 2+ complexes with DHOA have been calculated using density functional theory. The free energy of extraction, Δ G ext , for the Pu 4+ and UO 2 2+ ions with DHOA was calculated using the Born–Haber thermodynamic cycle in the different experimentally studied diluents. The value of Δ G ext for the Pu 4+ ion in dodecane was found to be higher than that of the UO 2 2+ ion, as measured in the experiments. The value of Δ G ext was shown to increase with the increase in the dielectric constant of the diluent for both Pu 4+ and UO 2 2+ ions, similar to the experimental studies. Thus, the combined experimental and theoretical studies help to understand the underlying complexation mechanism of the UO 2 2+ and Pu 4+ ions with DHOA, which explains its selectivity towards the Pu 4+ ion.

Density functional theoretical modelling was performed to design and screen suitable macrocyclic crown ether functionalized resins for the isotopic enrichment of gadolinium. Theoretical calculations predict the complexation stability order of Gd3+ ion as follows: di-cylohexano-18-crown-6 (DCH18C6) > dibenzo-18-crown-6 (DB18C6) > benzo-15-crown-5 (B15C5), which was experimentally verified. The calculated isotopic separation factor value was shown to be the highest for DB18C6. From the theoretical analysis of both the stability and isotopic separation factor, DB18C6 is predicted to be the most promising candidate for isotopic separation of gadolinium. Hence, DB18C6 was functionalized with chloromethylated polystyrene (CMPS) resin. Subsequently, CMPS-grafted DB18C6 resin was synthesized and characterized. Furthermore, isotopic enrichment of gadolinium was carried out by performing column chromatographic experiments using CMPS-DB18C6 resin. The absorption capacity of the novel CMPS-DB18C6 resin for gadolinium was found to be 1 mg g−1. The separation coefficient, e × 103, was found to be 6.3, 8.9, 3.4, and 9.7 for Gd-155/158, Gd-156/158, Gd-157/158, and Gd-155/160 isotopic pairs, respectively, and thus hold promise for future isotopic enrichment technology.

The extraordinary extraction of metal ions in imidazolium ionic liquid phase containing neutral extractants was most often attributed to the cation exchange mechanism in addition to the usual solvation-type mechanism. The cation exchange mechanism results in an irreversible loss of the ionic liquid cation, which eventually leads to the pollution of aqueous phase also. To minimize the limitations of imidazolium based ionic liquid, strongly hydrophobic ionic liquids containing tetra-alkyl ammonium ion have been introduced and studied for actinide separation. Even though these tetra-alkyl ammonium ionic liquids did not undergo cation exchange, but showed exceptional extraction of actinides for the reason, which was not established so far. To unravel the unique role of these hydrophobic ionic liquids, the extraction behavior of U(VI) was studied in a solution of tri-n-octylmethylammonium nitrate ([N1888 ][NO3]) in conjunction with a neutral ligand, 2-hydroxy-N,N-dioctyl acetamide (DOHyA). The extraction of U(VI) was studied as a function of various extraction parameters and the organic phase obtained after extraction was probed by FTIR and Raman spectroscopy to elucidate the mechanism of U(VI) extraction in ionic liquid phase. The result revealed that U(VI) was extracted into ionic liquid phase by a couple of extraction modes, namely through metal–ligand coordination and by loss-less anion exchange mode and these pathways did not cause any undue burden to the organic and aqueous phases, unlike the traditional imidazolium ionic liquids. Further, DFT studies were performed to elucidate the underlying mechanism of U(VI) complexation in ionic liquid phase. The calculated values of binding energy (ΔE) and Gibbs free energy (ΔG) for the complexation of U(VI) with DOHyA in dodecane and tetra-alkyl ammonium ionic liquids medium were found in good agreement with the experimentally determined complexation trends of U(VI) ion with DoHyA. The DFT calculations further established that the cation exchange mechanism by both the ionic liquids, [N1888][NO3] and [N1888][NTf2] was not feasible, but anion exchange mechanism could be possible as observed in the experimental studies.

Three benzene-centered tripodal diglycolamides (Bz-T-DGAs) were synthesized and evaluated for actinide, lanthanide, and fission product ion extraction. 1,3,5-Triethylbenzene-based tripodal DGA (LI) showed high distribution ratio (D) values for Am3+ and Eu3+ in a mixture of 95% n-dodecane and 5% iso-decanol at 3 M HNO3. Eu/Am separation factors, in the range of 8–10, were obtained at 1 M HNO3 which decreased at higher acidities with the exception of LII which did not show much change. Benzene-1,3,5-triamide-based tripodal DGA (LII) exhibited a high D-value for Pu4+ compared to the other ligands. Slope analysis showed the formation of 1 : 1 or 1 : 2 complexes is dependent on the ligand. The nature of the complexes was further studied with luminescence spectroscopy (Eu complexes) and DFT calculations (Am complexes).

The unusually high selectivity of the Cs+ ion over the Na+ ion with bis(2-propyloxy)calix[4]crown-6 (BPC6) compared to dicyclohexano-18-crown-6 (DCH18C6) has been investigated using generalized gradient approximated (GGA) BP86, hybrid B3LYP and meta hybrid TPSSH density functionals, employing split valence plus polarization (SVP) and triple zeta valence plus polarization (TZVP) basis sets in conjunction with the COSMO (conductor like screening model) solvation approach. The calculated theoretical selectivity of the Cs+ ion over the Na+ ion was found to be in accordance with the experimental selectivity obtained using solvent extraction experiments in ionic liquids (IL) and octanol. The distribution constant of the Cs+ ion, DCs with DCH18C6 in the 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMIMTF2N) IL phase was found to be significantly large than that in octanol. The experimentally measured DCs value was found to be very large compared to the value of DNa in the IL phase. The presence of the BMIM cation in the recorded UV-visible spectra of the raffinate phase with and without DCH18C6 indicates the BMIM cation exchange with Cs+ and Na+ ions, thus supporting the dual mode of extraction. The dual mode of metal ion extraction observed in the experimental study was complemented by density functional theoretical study. The calculated free energy of extraction, ΔGext, for the metal ion was found to be higher in IL compared to octanol. Further, preferential selectivity of the Cs+ ion over the Na+ ion was established from the free energy difference, ΔΔGext, between the two competing metal ions. The unusually high selectivity of Cs+ over the Na+ ion by BPC6 in IL compared to DCH18C6 is also demonstrated by the free energy difference, ΔΔΔGext, between the two competing ligands which was shown to be free from the complicated metal ion solvation energy.

The preferential selectivity of dicyclohexano-18-crown-6 (DCH18C6) for bivalent Sr(+2) ion over tetravalent Th(+4) ion was investigated using generalized gradient approximated (GGA) BP86 and the hybrid B3LYP density functional, employing split valence plus polarization (SV(P)) and triple-zeta valence plus polarization (TZVP) basis sets in conjunction with the COSMO (conductor-like screening model) solvation approach. The calculated theoretical selectivity of DCH18C6 for Sr(+2) ion over Th(+4) ion was found to be in accord with the selectivity for Sr(+2) ion over Th(+4) ion observed when performing liquid-liquid extraction experiments in different organic solvents. While 1:1(M:L) stoichiometric complexation reactions can be used to predict the preferential selectivity of Sr(2+) ion over Th(4+) ion, the results obtained are not consistent with the experimental results observed upon increasing the dielectric constant of the solvent. The calculated theoretical gas-phase data for the free energy of complexation, ∆G, fail to explain the selectivity for Sr(+2) ion over Th(+4) ion. However, when 1:2 (M:L) stoichiometric complexation reactions (reported in previous X-ray crystallography studies) are considered, correct and consistent results for the selectivity for Sr(+2) ion over a wide range of dielectric constants are predicted. The distribution constant for Sr(2+) and Th(4+) ions was found to gradually increase with increasing dielectric constant of the organic solvent, and was found to be highest in nitrobenzene. The selectivity data calculated from ∆∆G ext are in excellent agreement with the results obtained from solvent extraction experiments.

The structures, energetics, thermodynamic parameters and vibrational spectrum of hydrated cationic rubidium clusters incorporating a single rubidium ion and up to 32 water molecules using second order Moller–Plesset and hybrid B3LYP density functional are presented. The predicted equilibrium rubidium–oxygen distance of 2.99 A at the present level of theory is in excellent agreement with the diffraction result of 2.98 A for a hydrated rubidium ion cluster. From geometry optimizations it has been found that Rb+ ion has seven water molecules in the first hydration sphere for n ≥ 24 which support the earlier experimental and simulation findings. The number of hydrogen bonds is found to be increased with an increase in the hydrated cluster size. The present calculated values of interaction enthalpies are in good agreement with the reported gas phase experimental results up to n = 5. The hydration number of rubidium metal ion is predicted to be 7 as seven water molecules are directly linked to the metal ion independently for a large cluster (n > 24) having three layer of solvation representing the bulk solvent limit. Calculation of the vibrational frequencies shows that the formation of a Rb+-water cluster induces significant shifts from the normal stretching modes of isolated water. From the ab initio molecular dynamics simulation, the average first shell coordination number was found to be 6.31 at both PW91 and PBE levels of theories, which is in close agreement with the QM predicted value (CN = 7) and experimental results (CN = 6.4–7.4). The average second shell coordination number is found to be ~ 21–25.

This paper presents the results of quantum chemical and classical molecular dynamics (MD) simulations of the microhydration states of the Sr 2+ ion. The quantum chemical results strongly suggest a coordination number (CN) of 8 for the first hydration shell of Sr 2+ , which is in quantitative agreement with data available from X-ray absorption fine structure (XAFS) measurements. The calculated theoretical Sr O bond distance of 2.59 A is also in excellent agreement with the XAFS results (2.60 A). Classical MD simulations are conducted on various water models to predict the hydration structure of the Sr 2+ ion. The CN is found to be in the range of 8–9 using SPC, TIP3P, and TIP4P-2005 water models, with the probability more skewed toward 8. MD–EXAFS study and coordination number analyses reveal that TIP4P-2005 is the best model potential for simulating water molecules to reproduce the experimentally observed absorption spectra and coordination numbers.

The binding interaction of alkali metal ions (charged guest) within the narrow cavity of crown ethers (neutral host) of different cavity size has been studied using quantum chemical density functional theory. Different conformational structures, binding energies and various thermodynamic parameters of free crown ethers and their metal ion complexes have been determined with the B3LYP functional using a large split valence 6-311++G(d,p) basis set. Geometry optimization was performed using guess structures obtained from semi-empirical PM3 optimized structures without imposing any symmetry restriction. The calculated values of binding enthalpy increase with increase in cavity size, i.e., with increase in donor O atoms and are found to be in good agreement with gas phase experimental results. We have demonstrated the effect of micro-solvation on the binding interaction between the alkali metal ions (Li + and Na + ) and the macrocyclic crown ethers by considering micro-solvated metal ions up to six water molecules directly attached to the metal ion. A metal ion exchange reaction involving the replacement of lithium ion in metal ion–crown ether complexes with sodium ion contained within a metal ion–water cluster serves as the basis for modeling binding preferences in solution. An attempt has been made to study the effect of micro-solvation on the binding interaction of metal ions with crown ethers by considering two water molecules attached to metal ion–crown ether complexes. The calculated O H stretching frequency of H 2 O molecule in micro-solvated metal ion–crown complexes is less blue-shifted in comparison to hydrated metal ions. The calculated IR spectra can be compared with an experimental spectrum to determine the presence of micro-solvated metal ion–crown ether complexes in extractant phase.