Magnetic spectroscopies (NMR, EPR, SQUID)

Magnetic spectroscopies (NMR, EPR, and SQUID) investigate the behavior of substances under the influence of an external magnetic field. The magnetic properties of the substances under study are largely determined by the three-dimensional arrangement of atomic nuclei, the properties of the surrounding electrons, and their interaction with each other. The methods used here non-destructively measure the magnetic properties of the substances in their ground state. The main focus of our research is on coordination compounds of f-elements (actinides and lanthanides), which predominantly possess unpaired electrons.

For elucidating the constitution, configuration, and conformation of various molecules in solution as well as in the solid state, NMR spectroscopy is currently the most significant method. In addition to structure determination, dynamic processes on a molecular level can also be addressed over a wide time range from microseconds to seconds. NMR spectroscopy of f-element compounds is primarily dominated by additional shifts, so-called paramagnetic shifts, resulting from interactions between the unpaired electrons of the metal ions and the observed nuclear spins of the ligand molecules. These are extracted by comparing the obtained spectra with a diamagnetic reference. Depending on the type of interaction, statements can be made about the nature of the bond between the metal ion and surrounding ligands.

The electronic properties of the metal ions can be determined using EPR spectroscopy. Special focus lies on the highly accurate determination of fundamental parameters such as g-factors, fine and hyperfine structure, as well as their respective anisotropies. These physical properties provide insights into the distribution of electron density in the molecule and the interaction of paramagnetic centers with each other and with the surrounding ligands. Magnetic properties are direction-dependent in external magnetic fields. The investigation of solutions (isotropic orientation of dissolved molecules), frozen solutions, powders, and single crystals (known alignment of spins in the molecule) thus allows the resolution of various phenomena.

A SQUID magnetometer determines the macroscopic magnetic susceptibility of a sample over a wide temperature range and in the presence of external fields of variable strength. This macroscopic manifestation of the magnetic response to an external magnetic field is particularly dependent on the oxidation state of the metal ions in coordination compounds and thus on their electronic properties. SQUID is the most sensitive of the magnetic methods and also provides information about the type of magnetism, e.g., paramagnetism or ferrimagnetism.

The development of suitable mathematical models to understand these electronic interactions is carried out in close collaboration with theoretical chemistry. However, the basis for all practical work is the synthesis of model compounds that yield as isostructural complexes as possible across the series of f-element compounds (Coordination chemistry with organic ligands). Precise knowledge of the solution speciation of the complexes (TRLFS, Synthesis of f-element compounds and their coordination chemistry with organic ligands) and characterization of the structure (XRD, XAS), are fundamental to this research.

For the determination of the magnetic properties of f-element compounds, the Institute for Resource Ecology has access to two NMR spectrometers (Agilent 400 MR, Agilent 600 DD2) for the investigation of samples in solution (lanthanides and early actinides). Another NMR spectrometer (Agilent VNMRS 400) is installed in the radiochemical control area and is used, with appropriate safety measures, particularly for the investigation of transuranic compounds. State-of-the-art multinuclear 1D and 2D NMR methods are used to study complexes of actinides with biologically, environmentally, and repository-relevant ligands. For the measurement of EPR spectra, a Bruker ELEXSYS E500 X-band EPR spectrometer is installed in the control area, where samples of the actinides including the transuranics can be measured. The magnetic methods at the institute are complemented by a QuantumDesign MPMS3 SQUID magnetometer, which is also installed in the control area and can measure samples of the actinides and specifically the transuranics.