Prof Nik Kaltsoyannis
Our research focuses on the computational investigation of the electronic and geometric structure and reactivity of molecules from all areas of the periodic table. We employ a variety of techniques, primarily density functional theory and multiconfigurational ab initio methods. We are particularly interested in linking our research with experimental projects in order to achieve a more complete understanding than is possible from either approach working in isolation.
Our main area of research is heavy element chemistry, especially f elements. We were part of the ESPRC funded DIAMOND nuclear waste consortium (http://www.diamondconsortium.org/index.htm) which ran from 2008–2012, and will play a central role in the successor consortium “DISTINCTIVE” (2014–2018). We are also part of the Theoretical User Laboratory of ACTINET‑I3 (http://www.actinet-i3.eu/index.php?option=com_content&view=article&id=23&Itemid=23)
The QTAIM in heavy element chemistry
We have pioneered the application of the Quantum Theory of Atoms‑in‑Molecules (QTAIM,http://www.chemistry.mcmaster.ca/bader/) to the bonding in f element molecules. Assessing the extent of ionicity/covalency in f element bonding is not always straightforward, and the QTAIM is a very useful tool to this end, as set out in two of our recent papers: Dalton Transactions 39 (2010) 6719 and Dalton Transactions 40 (2011) 124. The image below is the molecular graph of [U(OPh)3]2(µ-η2:η2-N2), featured in another of our recent papers: Journal of the American Chemical Society 133 (2011) 9036. This is one of many examples of our application of the QTAIM to f element compounds; others include Inorganic Chemistry 51 (2012) 8557, Chemical Science 4 (2013) 1189 and Journal of the American Chemical Society 135 (2013) 5352.
In a contribution to the Forum on Aspects of Inorganic Chemistry Related to Nuclear Energy, we discussed the relevance of our QTAIM data on covalency in actinide compounds to the separation of the so‑called minor actinides (Americium and Curium) from lanthanide fission products in nuclear wastes (Inorganic Chemistry 52 (2013) 3407). Very recently, we have begun to assess the QTAIM as a tool to furnish bond strengths in heavy element compounds (Dalton Transactions 42 (2013) 13477).
Novel hydrogen storage materials
Hydrogen has been suggested as a clean energy carrier to be used in combination with hydrogen fuel cells in many forms of road vehicles from motor bikes to buses. However, the implementation of hydrogen as a fuel has met with several practical difficulties, and it has therefore been suggested to incorporate a material into the storage tank that binds to the hydrogen and increases the hydrogen storage capacity of the tank. A storage‑material–H2 binding enthalpy of between 20 and 40 kJmol-1 is ideal and, to achieve this, the incorporation of transition metal fragments into storage materials is being explored, such that the TM–H2 interaction occurs via the Kubas interaction. This process involves σ-donation from the filled H‑H σ-bonding orbital into an empty d orbital of a metal, and simultaneous π-back-donation from a filled metal d orbital into the vacant σ* anti-bonding orbital of the H2 molecule (similar to the synergic bonding described by the Dewar-Chatt-Duncanson model for the interaction of, for example, CO with transition metals). In collaboration with experimentalists at the Sustainable Environment Research Centre at the University of Glamorgan, we are probing the electronic structure of molecular models for the TM–H2 binding sites in potential hydrogen storage materials, and have established that Kubas binding is indeed occurring in these systems. The images below show ball and stick representations of the optimised geometries of H2‑free and H2‑bound molecules representing the benzyl disiloxy Ti(III) and dibenzyl siloxy Ti(III) binding sites in mesoporous amorphous silica based materials, as described in Journal of the American Chemical Society 132 (2010) 17296.
Some of our further work in this area, including the first study of the Kubas interaction using the QTAIM, may be found in Chemistry, A European Journal 18 (2012) 1750, Dalton Transactions 41 (2012) 8515 (cover article – see below) and Journal of Physical Chemistry C 116 (2012) 19134.
A long standing area of interest is the electronic structure of the iconic organometallic sandwich molecule cerocene (Ce(η8‑C8H8)2 below) and its actinide analogues. A hotly contested debate as to the oxidation state of the Ce atom has played out in the literature over the past two decades, with many experimental and theoretical techniques being trained on this target. In the mid 1990s, we were part of the experimental team that conducted X‑ray absorption spectroscopic studies (J. Am. Chem. Soc. 118 (1996) 13115) of cerocene and, more recently, conducted an EPSRC‑funded theoretical investigation of cerocene and An(η8‑C8H8)2 (An = Th‑Cm), which employed state of the art multiconfigurational quantum chemical techniques (J. Phys. Chem. A 113 (2009) 2896 and 8737). This computational work has conclusively established that cerocene is a Ce(IV) compound with a multiconfigurational ground state, has reconciled the conflicting experimental and theoretical data, and shown that the multiconfigurational character of the ground states of the actinide analogues increases as the 5f series is crossed.