Soft-donor ligands for the separation of actinides from lanthanides
An important problem in the nuclear power industry is associated with the separation of two radioactive components of spent nuclear fuel. These components are characterised as long-lived minor actinide (Np, Am, Cm) and short-lived lanthanide species, respectively. These components can be managed and utilised in very different ways and their efficient separation therefore has important environmental and economic benefits. The chemical bonding in actinide systems is believed to be subtly different to that of their lanthanide counterparts due to the greater spatial delocalisation of the 5f orbitals in the former. This leads to greater covalent character in An-ligand bonds and allows carefully selected soft-donor ligands to preferentially bind the An(III) ion. The simulation of An(III) and Ln(III) complexes, however, present a significant computational challenge and we employ state-of-the-art relativistic quantum chemical methods in order to better understand the electronic structure.
Modelling the electronic structure of macrocyclic d- and f-element complexes
The porphyrins, sometimes described as the “pigments of life” due to their central role in photosynthesis and the transport of oxygen in the cardiovascular system, are a class of molecules that have become ubiquitous in modern life. They find application in a diversity of fields, where examples include their use as photosensitisers in the photodynamic therapy of certain cancers and as essential components of a class of dye-sensitised solar cells. Expanded porphyrins have also shown promise as potential actinide detectors and sensors. We perform a variety of quantum chemical simulations of porphyrins complexes of the d- and f-block elements for application in spintronics, renewable energy and nuclear waste remediation. The porphyrin ligand is amenable to extensive modification and substitution, and we take advantage of this to ‘tune’ specific physical and chemical properties of our complexes so as to optimise their use in the aforementioned applications.
Robust measures of covalency in complexes of the f-elements
The quantification of covalency in f-element complexes is of both fundamental scientific interest and critical industrial importance. In the nuclear power industry, strategies for the remediation of spent nuclear fuel are based on the chemical separation of minor actinides from lanthanides which, in turn, is believed to be dependent on a difference in covalent character. Metal–ligand covalency is, however, difficult to quantify in complexes containing open shell ions due to the presence of strong electron correlation, manifesting itself in the form of multiconfigurational character in the electronic wavefunction. Traditional views of covalency fail in the context of a multiconfigurational wavefunction due to the breakdown of the independent particle approximation. In order to allow the question of covalency in such multiconfigurational systems to be considered, we instead turn to the (physically observable) total electron density. The topology of the electron density can be interrogated so as to give us an unambiguous partitioning of a molecule into atomic components and allows the degree of electron sharing between atoms to be quantified.
The spectroscopy of actinide complexes
The combination of relativity, electron correlation and spin-orbit coupling can place simulations of the absorption and emission spectra of actinide complexes beyond the reach of standard quantum chemical methods. We employ the complete-active-space self-consistent-field (CASSCF) approach in order to consider all of these contributions to both the ground and excited state electronic structure of uranium complexes. Much of this work is carried out on open shell complexes and can consider more than 50,000 individual transitions. The results of these calculations help us to interpret and assign the spectra produced by experimental collaborators, and to therefore deepen our understanding of the fundamental processes responsible for the spectroscopic characteristics of these complexes.
Novel materials for the removal of radionuclides from aqueous environments
The treatment and remediation of groundwater and other aqueous environments that have been contaminated by human-made radioactive ions (such as nuclear fission products and highly active transuranic elements) is an essential task in the cleanup of legacy nuclear power facilities. The recent accident at the Japanese Fukushima Daiichi nuclear power plant, in which radionuclides were released into the environment, highlights the fact that such treatment can also be required in a wider environmental context. Recent experimental work has revealed that graphene oxide (GO) is an extremely effective material for the removal of radionuclides from aqueous environments (such as liquid nuclear waste or contaminated groundwater) through surface absorption. Furthermore, GO flakes coagulate so as to form large particles that can be easily removed from solution, are non-toxic and biodegradable. Our work focusses on how the physical and chemical properties of the flakes can be modified so as to optimise their ability to absorb radionuclides. Our quantum chemical studies are to be complemented by large scale classical simulations of GO flakes in order to better understand their ability to coagulate, and therefore to investigate the effect of the optimisation of absorption properties on this ability.