Gregory Choppin's
Research Interests

Actinide and Lanthanide Complexation

A major area of study is the thermodynamic changes related to the stability of complexes with inorganic and organic ligands. Actinide and lanthanide cations form strong ionic complexes and the dehydration of the cations during the complexation process is a major factor in the overall enthalpy changes. The variation of inner/outer sphere complexation with ligand basicity is also being studied. Our studies of the kinetics of complexation of these elements by multidentate ligands have indicated two major pathways: one involving hydrogen ion catalysis, and one independent of pH. These kinetic studies may give an understanding of the importance of structural effects of the ligands. Many of the complexes that we have studied are useful in the separation chemistry of the actinide elements in nuclear reprocessing.

Environmental Behavior of the Actinides

A major concern in the development of the nuclear power industry is the storage and disposal of the transuranium elements. These elements could escape to the environment either through an accident in a reactor or by leakage from a disposal site and pose a serious hazard. We are studying the binding of the transuranium elements by potential complexants in the environment. These will allow better modeling of the probable environmental behavior of these elements. These studies require thermodynamic and kinetic data since it is uncertain which of these two factors will play the major role in either the migration or the retention of the transuranium elements in soil.

NMR and Optical Spectroscopy of the Lanthanides and Actinides

The thermodynamic and kinetic studies have led to models of complex structure in aqueous solution. These models are being further tested by NMR and optical spectroscopy. Paramagnetic lanthanides offer useful probes for information on details of the structure of the ligands when complexed. An area of focus is on lanthanide complexes of value in medical MRI techniques.
The optical spectra (f - f transitions) of the lanthanides provide another useful probe to obtain information on the symmetry and the nature of the bonding of the complexes. Although the bonding in these complexes is predominantly ionic, the very small covalent contribution seems to have a strong influence on the spectral properties. Extensive use is made of such techniques as potentiometric titration, calorimetry, NMR spectroscopy, laser excited fluorescence spectroscopy, stopped-flow kinetics spectrometry, and Raman spectrometry.