Chemistry Staff Seminar Series Seminar

Charge storage using metal nitrides
Dr Andrew Hector

Where metal nitrides have been examined for charge storage applications in aqueous supercapacitors and as conversion electrodes in lithium-ion batteries they have shown promise. Using precursor-based routes to produce nanocrystalline molybdenum nitride we have shown significant capacities in aqueous acid conditions that are maintained during fast and slow charge/discharge cycles can be achieved. The capacity of titanium nitride in aqueous basic conditions can be enhanced to a large degree by electrochemical surface aging and this capacity also survives extensive cycling. Nickel nitride is found to have capacity and cycling behaviour for sodium-ion negative electrodes that is competitive with the best materials currently in the literature.

Singlet NMR
Prof Malcolm H Levitt

In nuclear magnetic resonance (NMR) it is common to store information in the nuclear spin system, and retrieve at a later time. This is used, for example, in the measurement of flow and diffusion by NMR and MRI. It was generally taken for granted that the relaxation time T1 sets a rough upper limit on the time for which information can be stored. However in 2003 our group demonstrated that in some cases, information may be stored in nuclear spin systems for more than an order of magnitude longer than T1. The effect involves singlet nuclear spin states, which are non-magnetic correlated states of nuclear pairs, and which are protected against many important relaxation mechanisms. In one case we showed the storage of information in 15N-labelled nitrous oxide for more than half an hour.

I will discuss the phenomenon of singlet nuclear spin order in a variety of contexts - including gas-phase parahydrogen, small molecules in solution, and some examples in solids. I will discuss how nuclear singlet spin order may be generated from magnetization, how it is maintained, and how it is converted back into observable magnetization.

Our latest work in the field includes the generation of hyperpolarized nuclear singlet order using dynamic nuclear polarization, a technique which is capable of enhancing NMR signals by up to 5 orders of magnitude, but which is limited by the decay of the polarization due to relaxation processes. We have been particularly interested in the properties of nearly equivalent spin pairs, where the hyperpolarized singlet order is long-lived in high magnetic field, even without any external intervention. We have demonstrated repeated detection of hyperpolarized 13C singlet order in solution, and repeated use for MRI experiments, over a total time of 30 minutes. We are working with the synthetic groups of Richard and Lynda Brown to develop agents that may be used to transport hyperpolarized nuclear spin order for many hours, in room-temperature solution.
We have also studied the nuclear singlet states of parawater molecules trapped inside fullerene cavities, in the solid state. In a collaboration with the group of Richard Whitby, we have detected the conversion between ortho and parawater, using cryogenic NMR.