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The University of Southampton
MagRes@SotonResearch

Long-lived nuclear spin states

The ability to preserve spin polarisation for several minutes or hours opens many new possibilities in NMR and MRI, from enhancing NMR studies of translational dynamics up to tracing and imaging of molecular tags in materials or in-vivo.

the persistence of spin memory

NMR experiments exploit modes of nuclear spin order which represent particular configurations of the nuclear spin orientations. Most NMR experiments involve Zeeman order which represents a net orientation of the nuclei along an applied magnetic field. A small amount of Zeeman order is generated through natural thermal processes by allowing the sample to equilibrate in a strong magnetic field. Although the thermal Zeeman order is very small (typically only ~10-5), all conventional NMR and MRI techniques rely on it. Its small size makes NMR a low sensitivity technique when compared to optical spectroscopy and many other methods. The process by which thermal Zeeman order is established in a magnetic field is called spin-lattice relaxation and typically follows approximately first-order kinetics with a time constant denoted T1. In a typical solution, T1 ranges from tens of milliseconds (water in body tissues) up to a few tens of seconds (selected 13C or 15N-labelled substrates).

In 2004 we demonstrated that coupled clusters of nuclear spins support classes of nuclear spin order (called long-lived states, LLS) which in some circumstances have much longer lifetimes than T1. One subclass of such states occurs in molecules containing coupled pairs of spins-1/2 and is called singlet order. This may be viewed as a spin configuration in which pairs of magnetic nuclei are oriented in opposite directions such as their net magnetisation cancels out. The decay time constant for singlet order is denoted TS and may greatly exceed T1. Values of TS have been observed which are > 10 min for proton pairs, > 25 min for nitrogen-15 pairs and >1 hour for carbon-13 pairs.

In suitable systems, the two forms of nuclear spin order (Zeeman and Singlet) may be interconverted by using suitable sequences of applied magnetic fields.

This is an highly active research area where our efforts are focused on:

 

  1. developing theoretical and experimental tools for handling long-lived nuclear spin states
  2. developing nuclear spin relaxation models to understand and predict the relaxation decay rates of those states
  3. design and synthesis of new molecules supporting long-lived states, in collaboration with synthetic chemists
  4. exploiting long-lived states in the context of chemical reactions and NMR imaging
  5. combining hyperpolarisation techniques (para-H2 and dissolution-DNP) with long-lived states to prepare long-lived reservoirs of hyperpolarisation
  6. exploiting the use of long-lived states as tags in imaging experiments
  7. exploiting the use of long-lived states to characterise transport phenomena in porous media
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