Magnetism, Spectroscopy, Theory
Probing Covalency in Actinide Molecules: A Computational Toolbox for Magnetic Resonance
Understanding, predicting, and controlling the behaviour of actinide (An) ions in solution and the compounds they form in the solid state are crucial steps to keeping the nuclear energy option open and in the management of spent fuel and legacy wastes. This necessitates a fundamental understanding of An chemistry, wherein one of the central questions is the extent of covalent bonding of the An elements. Our knowledge of the chemistry of An elements has progressed much more slowly than for the other elements, largely due to the inherent difficulties in handling highly radiotoxic materials. The advancement of computational techniques is an excellent route to circumvent expensive and sometimes hazardous experiments with An species, however there are simply not enough direct experimental techniques capable of reporting on fundamental concepts like covalency, and without such data, computational methods cannot be validated.
Two spectroscopic techniques that are able to directly report on the covalency of An species are nuclear magnetic resonance (NMR) and pulsed electron paramagnetic resonance (EPR). Both techniques are able to measure the hyperfine interactions (electron spin – nuclear spin) between the An electrons and the ligand nuclei, which can be decomposed into the amount of An spin density on the ligands, and the anisotropy of the magnetic moment. The former provides a direct measure of covalency while the latter provides crucial information on the molecular electronic structure; we have recently published the first pulsed EPR study of An materials (A. Formanuik et al., Nature Chem., 2017, 9, 578), demonstrating this approach. However, the analysis of such data is not trivial and, while such experiments continue, it is vital that reliable methods for calculating the hyperfine interactions in An molecules are developed.
Current approaches are usually based on density functional theory (DFT), which may be completely inappropriate for An molecules with significant relativistic effects and orbital degeneracies. Therefore, this project seeks to develop a computational toolbox for the calculation of hyperfine interactions based on complete active space self-consistent field spin-orbit (CASSCF-SO) calculations. These methods include the most important relativistic effects as well as explicitly accounting for orbital degeneracy, thus providing an accurate description of An molecular electronic structure. The successful candidate will: (i) learn the CASSCF-SO method and how to determine the electronic structure of An complexes, (ii) aid in development of a method for the accurate calculation of hyperfine interactions, and (iii) benchmark the computational results by modelling and interpreting experimental NMR and EPR data collected at The University of Manchester and by our collaborators.
Funding and availability
This position is funded by the STFC/MOD for 3 years from September 2018. It is open to UK/EU citizens only.