Quantum chemical characterisation of molecular magnetism in actinide complexes

The prediction of paramagnetic NMR (pNMR) chemical shifts in molecules containing heavy atoms presents a significant challenge to computational quantum chemistry. The importance of meeting this challenge lies in the central role that NMR plays in the structural characterisation of chemical systems. Hence there is a need for reliable assignment and prediction of chemical shifts. We have developed code in MATLAB that facilitates this, based on the decomposition of the magnetic moment matrices by irreducible tensor operators (ITO) as outlined in the work of van den Heuvel and Soncini. This work firstly presents a published paper that studies the effect of zero-field-splitting (ZFS). It is shown that the inclusion of ZFS can produce substantial shifts in the predicted chemical shifts. The computations presented are typically sufficient to enable assignment of experimental spectra. However for one case, in which the peaks are closely clustered, the inclusion of ZFS re-orders the chemical shifts making assignment quite difficult. We also observe, and echo, the previously reported importance of including the paramagnetic spin-orbit hyperfine interaction for carbon-13 and silicon-29 atoms, when these are directly bound to a heavy element and thus subject to heavy-atom-light-atom (HALA) effects. Then we study the magnetic properties of U(DOTA), and three axially substituted variants. This is achieved by calculating the main magnetic matrices and paramagnetic NMR (pNMR) spectra. U(DOTA) has two suggested assignments for its proton spectra and our calculations allow a definitive assignment. The complications due to large spin-orbit coupling (SOC) and the interchange between the square antiprism (SAP) and twisted square antiprism (TSA) conformers are discussed. The axial symmetry of the molecule allows the tensor decomposition technique to be used to model the Zeeman contribution, and this results in strong correlation between calculated and experimental results. Experimental assignments for the axially substituted variants do not distinguish between protons attached to the same carbon atom, and we are able to separate these. The binding of a water ligand has little effect on the calculated spectra, but binding an anionic ligand results in a compression of the spectral range which our results duplicate. Binding an anionic ligand also causes the anisotropy axis to rotate by 90 degrees. These effects are examined with reference to spin density. Finally carbon-13 spectra are predicted for future experimental verification. The applicability of the new method of calculating paramagnetic NMR shifts is extended to another class of very heavy element compounds --- some substituted actinocene complexes. The initial results aimed at assigning published experimental NMR data are presented and our implementation of the ITO model is considered as promising.