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DNP NMR

PCCP DNP coverThe Department of Physics at the University of Warwick has magnetic resonance research groups that study nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR). Dynamic nuclear polarization enhanced NMR (DNP NMR) is a method that combines these two techniques.

NMR spectroscopy has increased in importance as it is an element specific probe that can distinguish very small changes in the surroundings of different sites (e.g. the number of corners by which an SiO4 unit is connected into a structure) which has become important throughout the sciences. Its major drawback is the intrinsically relatively weak signal due to the small thermally derived population differences between nuclear energy levels. NMR of solids was revolutionised with the implementation of cross-polarisation (CP) that transferred magnetisation from nuclei with high magnetic moments (e.g. 1H) to more dilute nuclei with smaller magnetic moments (e.g. 13C) that yielded a factor of ~ 4 increase in the 13C NMR signal strength. Today there is very significant effort with a wide range of approaches to try and increase the size of the NMR signal still further and considerable investment to achieve even a few tens of percent increase.

Dynamic nuclear polarisation enhanced NMR (DNP NMR) is a technique that uses unpaired electron spins to boost the NMR signal by as much as 100,000. Although the effect has been known from theory and experiments at low magnetic fields for some time, it is only now that this can be put into practise, with the whole experiment carried out at high magnetic field. This is possible now because high field magnets of sufficient flexibility and robustness can be manufactured, and the production of microwaves (similar to those in a microwave oven although much higher in frequency) at high frequencies and with sufficient power for DNP to work at up to 395 GHz is becoming feasible. The DNP Project at Warwick seeks to bring this technology together in a new instrument to now carry out DNP at magnetic fields up to 14.1 T on solid materials and to develop the technology to use both continuous and pulsed microwave DNP at these magnetic field strengths.

Huge gains in sensitivity will result from both the DNP effect itself which in thermal equilibrium could offer potential enhancements of the ratio of the gyromagnetic ratio of the electron to that of the nucleus, a factor of > 2500 for 13C , combined with magic angle spinning (MAS) operation at ~ 90 K further increasing the enhancement via the thermal Boltzmann factor. The instrument would produce DNP at NMR frequencies much beyond those yet reported and thus allow modern high resolution solid state NMR experiments to be undertaken with gains over conventional NMR of 100-1000 routinely expected.

Quadrupolar nuclei (especially those with non-integer spins), which make up > 75 % of the NMR-active nuclear isotopes, have largely been precluded from DNP NMR because the nuclear resonances are too broad at current DNP-accessible magnetic fields (B0). Second-order quadrupolar broadening demands the use of high B0 and Warwick's proposed instrument would have sufficiently high B0 to open up their study by DNP. The wide frequency capability of the instrument would provide new insight into the physics of high-field DNP allowing, for the first time, an optimum technology to be developed in this emerging field. The versatility of the proposed instrument means that, with the same equipment, one could also carry out world-leading pulsed EPR and ENDOR experiments. The project is driven by multidisciplinary applications in areas of huge importance and as diverse as structural biology and fuel cell/electrochemistry technology. The DNP approach will allow NMR to be considered where hitherto sensitivity would have prohibited its use because of the sample size and/or the number of spins of interest are limited in number.

Before arriving in Warwick, Gavin Morley and collaborators demonstrated the first ENDOR-DNP measurements:

- GW Morley et al., Efficient dynamic nuclear polarization at high magnetic fields, Physical Review Letters 98, 220501 (2007)

- GW Morley, K Porfyrakis, A Ardavan & J van Tol, Dynamic nuclear polarization with simultaneous excitation of electronic and nuclear transitions, Applied Magnetic Resonance 34, 347 (2008)

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The high symmetry of the N@C60 molecule shown here permits ENDOR-DNP, even when the molecule is at low temperatures where it is unable to rotate.