Solvation of neodymium(III) perchlorate and nitrate in organic solvents as determined by spectroscopic measurements

Revisiting the assignment of innocent and non-innocent counterions in lanthanide(III) solution chemistry

Lanthanides are found in critical applications from display technology to renewable energy. Often, these rare earth elements are used as alloys or functional materials, yet access to them is through solution processes. In aqueous solutions, the rare earths are found predominantly as trivalent ions and charge balance dictates that counterions are present. The fast ligand exchange and lack of directional bonding in lanthanide complexes have led to questions regarding the speciation of Ln³⁺ solvates in the presence of various counterions and the distinction between innocent = non-coordinating and non-innocent = coordinating counterions. There is limited agreement as to which group counterions belong to, which led to this report. By using Eu³⁺ luminescence, it was possible to clearly distinguish between coordinating and non-coordinating ions. To interpret the results, it was required to bridge the descriptions of ion pairing and coordination. The data-in the form of Eu³⁺ luminescence spectra and luminescence lifetimes from solutions with varying concentrations of acetate, chloride, nitrate, sulfate, perchlorate and triflate-was contrasted to those obtained with ethylenediaminetetraacetic acid (EDTA^4-), which allowed for the distinction between three Ln³⁺-anion interaction types. It was possible to conclude which counterions are truly innocent (e.g. ClO₄^- and OTf^-) and which clearly coordinate (e.g. NO₃^- and AcO^-). Finally, a considerable amount of data from systems studied under similar conditions allowed the minimum perturbation arising from the inner sphere or outer sphere coordination in Eu³⁺ complexes to be identified.

Nd(III) hypersensitive peak as an optical absorption probe for determining nitric acid in aqueous solution: An application to aqueous raffinate solutions in nuclear reprocessing

A new method using Nd(III) absorption peak as a probe is described for the measurement of nitric acid concentration in aqueous solution. The hypersensitive peak of Nd(III) at 575.1 nm shows a substantial enhancement in the absorbance in comparison to other absorption peaks with increasing nitric acid concentration. The integrated area and absorbance of this hypersensitive peak show a linear dependency over a large dynamic range of 0.5-15.5 M of nitric acid. A methodology for the correction of spectral interference to the probing absorption peak of Nd(III) is also reported. The method is applied for the measurement of nitric acid in synthetic high level liquid waste solution and shown to be comparable to that obtained by titrimetric method. The present method can be easily adopted for the measurement of nitric acid concentration in aqueous raffinate solutions of nuclear reprocessing streams.

Trivalent Actinide Ions Showing Tenfold Coordination in Solution

Trivalent actinides generally exhibit ninefold coordination in solution. 2,6-Bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine (nPr-BTP), a tridentate nitrogen donor ligand, is known to form ninefold coordinated 1:3 complexes, [An(nPr-BTP)₃]³⁺ (An = U, Pu, Am, Cm) in solution. We report a Cm(III) complex with tenfold coordination in solution, [Cm(nPr-BTP)₃(NO₃)]²⁺. This species was identified using time-resolved laser fluorescence spectroscopy (TRLFS), vibronic side band spectroscopy (VSBS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). Adding nitrate to a solution of the [Cm(nPr-BTP)₃]³⁺ complex in 2-propanol shifts the Cm(III) emission band from 613.1 to 617.3 nm. This bathochromic shift is due to a higher coordination number of the Cm(III) ion in solution, in agreement with the formation of the [Cm(nPr-BTP)₃(NO₃)]²⁺ complex. The formation of this complex exhibits slow kinetics in the range of 5 to 12 days, depending on the water content of the solvent. Formation of a complex [Cm(nPr-BTP)₃(X)]²⁺ was not observed for anions other than nitrate (X^- = NO₂^-, CN^-, or OTf^-). The formation of the [Cm(nPr-BTP)₃(NO₃)]²⁺ complex was studied as a function of NO₃^- and nPr-BTP concentrations, and slope analyses confirmed the addition of one nitrate anion to the [Cm(nPr-BTP)₃]³⁺ complex. Experiments with varied nPr-BTP concentration show that [Cm(nPr-BTP)₃(NO₃)]²⁺ only forms at nPr-BTP concentrations below 10^-4 mol/L whereas for concentrations greater than 10^-4 mol/L the formation of the tenfold species is suppressed and [Cm(nPr-BTP)₃]³⁺ is the only species present. The presence of the tenfold coordinated complex is supported by VSBS, XPS, and DFT calculations. The vibronic side band of the [Cm(nPr-BTP)₃(NO₃)]²⁺ complex exhibits a nitrate stretching mode not observed in the [Cm(nPr-BTP)₃]³⁺ complex. Moreover, XPS on [M(nPr-BTP)₃(NO₃)](NO₃)₂ (M = Eu, Am) yields signals from both non-coordinated and coordinated nitrate. Finally, DFT calculations reveal that the energetically most favored structure is obtained if the nitrate is positioned on the C₂ axis of the D₃ symmetrical [Cm(nPr-BTP)₃]³⁺ complex with a bond distance of 413 pm. Combining results from TRLFS, VSBS, XPS, and DFT provides sound evidence for a unique tenfold coordinated Cm(III) complex in solution-a novelty in An(III) solution chemistry.

Luminescence study on the solvation of Cm(III) in binary aqueous solvent mixtures

The preferential solvation of Cm(III) in binary aqueous mixtures of MeOH,tBuOH, DMSO, MeCN and acetone is studied at varied solvent composition. Solvation is investigated by time-resolved laser fluorescence spectroscopy and emission spectra and fluorescence lifetime data are obtained. At high mole fractions of organic solvent, preferential solvation of Cm(III) increases in the order: Me2CO <tBuOH ≈ MeOH < MeCN < H2O < DMSO. Thermodynamic data are derived from the spectroscopic results showing small positive standard Gibbs energies for the transfer of Cm(III) into the various solvent mixtures except for DMSO mixtures where negative values are found. The spectroscopically obtained enthalpies of transfer are fitted to a solvation model using the model parameters Δ Δ Hº12and (αn+βN)º, rendering valuable information on the interaction of Cm(III) with the solvent molecules. Cm(III)-solvent interaction increases in the order:tBuOH < MeCN < MeOH < DMSO.

Luminescence study on solvation of americium(III), curium(III) and several lanthanide(III) ions in nonaqueous and binary mixed solvents

The luminescence lifetimes of An(III) and Ln(III) ions [An=Am and Cm; Ln=Nd, Sm, Eu, Tb and Dy] were measured in dimethyl sulfoxide(DMSO), N,N-dimethylformamide(DMF), methanol(MeOH), water and their perdeuterated solvents. Nonradiative decay rates of the ions were in the order of H2O > MeOH > DMF > DMSO, indicating that O-H vibration is more effective quencher than C-H, C=O, and S=O vibrations in the solvent molecules. Maximal lifetime ratios τD/τHwere observed for Eu(III) in H2O, for Sm(III) in MeOH and DMF, and for Sm(III) and Dy(III) in DMSO. The solvent composition in the first coordination sphere of Cm(III) and Ln(III) in binary mixed solvents was also studied by measuring the luminescence lifetime. Cm(III) and Ln(III) were preferentially solvated by DMSO in DMSO-H2O, by DMF in DMF-H2O, and by H2O in MeOH-H2O over the whole range of the solvent composition. The order of the preferential solvation, i.e., DMSO > DMF > H2O > MeOH, correlates with the relative basicity of these solvents. The Gibbs free energy of transfer of ions from water to nonaqueous solvents was further estimated from the degree of the preferential solvation.

Spectroscopic system for direct lanthanide photoluminescence spectroscopy with nanomolar detection limits

A new spectroscopic system for direct photoluminescence of lanthanide ions (Ln(III)) through electronic transitions within the 4f(n) manifold is described. The system is based on an injection seeded frequency tripled (lambda = 355 nm) Nd:YAG pump laser coupled with a master oscillator power oscillator (MOPO). The MOPO delivers an average pulse energy of approximately 60 mJ/pulse, is continuously tunable from 425 to 690 nm (Signal) and 735 to 1800 nm (Idler) with a linewidth of <0.2 cm(-1), and has a pulse duration of 10-12 ns. Aqueous solutions containing two polyaminocarboxylate complexes, ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA), and Ln(3+) aqua ion for several lanthanides including Eu(III), Tb(III), Dy(III), and Sm(III)) are used as steady-state and time-resolved photoluminescence standards. The versatility of the instrument is demonstrated by excitation scans over a broad visible range for aqueous solutions of complexes of Eu(III), Dy(III), Sm(III), and Tb(III). The Eu(III) excitation band ((7)F(o)-->(5)D(o)) is recorded over a range of complex concentrations that are 1000-fold less than reported previously, including Eu(EDTA) (1.00 nM), Eu(DTPA) (1.00 nM), and Eu(III) aqua ion (50.0 nM). Emission spectra are recorded in the visible range for Ln(III) complexes at pH 6.5 and 1.00 mM. Excited-state lifetimes for the standards were constant as a function of concentration from 10.0 nM to 1.00 mM for Eu(EDTA) and Eu(DTPA) and from 100 nM to 1.00 mM for Eu(III) aqua ion. Photoluminescence lifetimes in H(2)O and D(2)O are recorded and used to calculate the number of bound water molecules for all complexes.