Gregory R. Choppin

Hypersensitivity in the electronic transitions of lanthanide and actinide complexes

The linear correlation of ligand basicity with oscillator strength of the lanthanide hypersensitive transitions has been shown to be of general utility and can be applied to lanthanide spectra of compounds in vapor, liquid solution or crystalline phases. A mechanism for hypersensitivity involving metal--ligand covalency via charge transfer levels is suggested. Physical interpretations of the tau/sub lambda/ parameters in the Judd--Ofelt theory have been presented based on covalency and symmetry arguments. The tau/sub 2/ parameters are apparently directly related to metal--ligand covalency while the tau/sub 4/ and/or tau/sub 6/ parameters seem to be primarily a function of the degree of symmetry in the coordinated complexes. 89 references.

Applications of lanthanide luminescence spectroscopy to solution studies of coordination chemistry

Abstract Luminescence spectroscopy is an important technique for the study of the coordination chemistry of the lanthanide ions in both the solution and solid state. This article concentrates on applications aimed at elucidating the coordination structure in solution phase systems. Luminescence lifetime measurements which allow the examination of the metal ion coordination environment in a wide variety of ligand systems are discussed. In addition, recent developments in the use of Eu(III) 7 F 0 → 5 D 0 excitation spectroscopy are presented. In the final section, the application of luminescence techniques to the examination of ligand sensitized lanthanide luminescence is reviewed briefly.

Luminescence study on determination of the hydration number of Cm(III)

A luminescence study of Cm(III) has shown a linear correlation between the decay constant kobs (the reciprocal of the excited-state lifetime) and the number of water molecules nH2o in the first coordination sphere of complexes. From measurements of kobs of Cm3+ in D2OH2O solutions and of Cm(III) doped lanthanum compounds, the following correlation for kobs (ms−1) vs. nH2o was established: nH2o = 0.65kobs − 0.88. This relationship was applied to study of the residual hydration of Cm(III) complexes of polyaminopolycarboxylate ligands. The hydration number of Cm(III) in these complexes is apparently larger than that of Eu(III).

Spectroscopic and chemical characterizations of molecular size fractionated humic acid

A sample of humic acid was divided by ultrafiltration into five fractions of different molecular size (F1; 300 000: F2; 100 000-300 000: F3; 50 000-100 000: F4; 10 000-50 000: F5; 1000-10 000 daltons). Characterization by IR, and CPMAS C-13 NMR spectroscopy indicated that the molecules of the fraction of >/=100 000 daltons were primarily aliphatic, while the smaller molecules of the >/=10 000 dalton fraction were predominantly aromatic. Titration (pH) data were consistent with an increase in the number of carboxylate groups per unit mass as molecular size became smaller. A comparative study with unpurified and purified (by treatment with ion exchange elution, acid precipitation and alkaline dissolution) humic acid samples showed chemical alteration with some loss of carboxyl groups in the humic acid.

Comparison of the solution chemistry of the actinides and lanthanides

Abstract The chemical behavior of the lanthanide and actinide elements in aqueous media is compared for similarities and differences. The stability of oxidation states and the thermodynamic parameters of complexation are evaluated to reflect how the decreased shielding of the 5f orbitals relative to that of the 4f orbitals causes the actinides to have somewhat different properties from those of the lanthanides. Although the interaction of the cations of both series with complexing ligands can be described satisfactorily by an ionic model, the freeenergy and enthalpy values of complexation of the trivalent actinides indicate a slightly greater covalency in bonding than that of the comparable lanthanide complexes. This is seen particularly in the metal-nitrogen-bonded systems.

Humics and Radionuclide Migration

Humic materials occur throughout the ecosphere in soils and waters, even in deep anoxic underground systems. The spectrum of molecular weights, the nature of the carbon skeleton and the types, positions and relative numbers of functional groups vary widely, in part depending on the origin and age of the humic material. Acid-base titration, C-13 CP/MAS solid-state nuclear magnetic resonance and ultrafiltration serve to define major operational characteristics of these materials. Humic materials sorb to surfaces and particulate matter in waters and can form colloids themselves. The structures and other general properties of humic substances are discussed briefly. Of importance in the migration of radionuclides in geological media is the strong complexing and redox interactions of humic materials to metal ions. The polyelectrolyte nature of humic molecules leads to very strong complexing which increases in strength with the increasing degree of ionization of the carboxylate groups. In addition, metal ions can be reduced to lower states; e. g., Pu(VI) to Pu (IV). Some unique problems are encountered in measuring the metal binding and/or redox by humic materials. However, such data is important as humic material can have significant effects on metal ion speciation and behavior in geologic systems even at 0.1 ppm levels. Measurements of actinide-humate interactions and their possible consequences on actinide migration are reviewed.

Interaction of humic and fulvic acids with Th(IV)

Abstract The binding of Th(IV) to three humic and two fulvic acids has been studied using a solvent extraction technique. The results are interpreted to indicate the formation of two types of thorium binding sites in the humic polymer containing one or two carboxylate groups. High complex stability is observed and thorium humate binding constants increase with increasing ionization of the humic (fulvic) acid polyelectrolyte. The results are interpreted using a modified Born equation for electrostatic interaction. Thermodynamic results indicate that the great stability of these complexes is derived from a very favorable complexation entropy.

Mixed Ca2+/UO22+/CO32- complex formation at different ionic strengths

Fluorescence spectroscopy was used to study the Ca2+/UO22+/CO32- solution system at different ionic strengths. Fluorescent titrations confirmed formation of Ca2UO2(CO3)3 complex for which the values of log β213 were measured at 0.1, 0.3, 0.7, 1.0 and 3.0 m (NaClO4) at pcH = 8.1. Specific Interaction Theory (SIT) was used to correlate the log β213 values as a function of ionic strength. The value obtained for log β0213 (I = 0) was 29.8 ± 0.7, indicating that Ca2UO2(CO3)3 was the predominant specie at the pH 8 in calcium rich, oxic waters.

STRUCTURE AND THERMODYNAMICS OF LANTHANIDE AND ACTINIDE COMPLEXES IN SOLUTION.

Several questions of importance in the study of lanthanide and actinide coordination compounds are reviewed. There is considerable evidence that in aqueous solution the primary coordination number is nine for the ions La(ni) through Nd(m) and eight for the ions heavier than Gd(m). While it seems that some degree of covalency exists in the metal—ligand bonding a model of electrostatic bonding seems satisfactory for explaining the structure and formation of the complexes. The dehydration of the lanthanide ions upon complexation largely determines the enthalpy and entropy data. However, there seems to be a compensation effect in the hydration parts of those terms such that the free energy changes seem to reflect the metal—ligand reaction unobscured by hydration factors. A number of individual inorganic and organic ligand systems are reviewed from these viewpoints. The lanthanide and actinide elements constitute two families of metals which often exhibit very similar chemical behaviour. This similarity is most easily observed with the cations in oxidation state iii, the most common one for all the elements of the lanthanide series and for the transpiutonium elements of the actinide series. This paper reviews the present state of understanding of the complexes formed in aqueous solution by the trivalent ions of these two families ofelements. The coordination chemistry of these elements has been studied neither as intensively nor as extensively as that of the transition metal ions. One reason for this is that these elements have been less available generally than the transition metal elements. Although they are fairly abundant, a lack of industrial application until a relatively few years ago caused many of the lanthanide elements to be fairly expensive. Increased interest in the use of these elements in phosphors and lasers and as catalysts as well as improvements in the methods of separation has caused a substantial reduction in their cost. The availability of the transuranium elements of the actinide series was limited to a few nuclear energy laboratories where greater emphasis was placed on research of either a more nuclear or more applied nature. This latter situation has changed as the transpiutonium programme of the US Atomic Energy Commission has resulted in production of relatively large amounts of the elements as heavy as californium (Z = 98). However, the 23

Covalency in f-element bonds

Abstract The actinide concept proposed by Seaborg in 1945 involves a close relationship between the trivalent 4f (Ln) and the 5f (An) elements. The trivalent ions of these two families have very similar properties due to strong ionic bonding and similar ionic radii. Their similarity led to the successful formation of element 95–103 over the next 30 years by accurate prediction of the chemistry of the undiscovered elements. In the early 1950s, a small degree of covalency was proposed in the bonding of the lanthanide elements. However, controversy continues today on the degree of covalency in the bonds of the 4f and 5f elements. The interpretation of data in terms of covalency in the M–L bonds is reviewed with emphasis on which orbitals are involved.