Tunnelling magnetic resonances: dynamic nuclear polarisation and the diffusion of methyl group tunnelling energy.

School of Physics & Astronomy, University of Nottingham, University Park, Nottingham NG7 2RD, UK.
Journal of Magnetic Resonance (Impact Factor: 2.3). 05/2009; 199(1):10-7. DOI:10.1016/j.jmr.2009.03.013
Source: PubMed

ABSTRACT The dynamic nuclear polarisation (DNP) of (1)H spins arising from methyl tunnelling magnetic resonances has been investigated in copper-doped zinc acetate dihydrate using field-cycling NMR spectroscopy at 4.2K. The tunnel resonances appear in the field range 20-50 mT and trace out the envelope of the electron spin resonance spectrum of the Cu(2+) ion impurities. By investigating the DNP line shapes as a function of time, the cooling of the methyl tunnel reservoir has been probed. The role of spectral diffusion of tunnelling energy in determining the DNP line shapes has been investigated through experiments and numerical simulations based on a theoretical model that describes the time evolution of the (1)H polarisation and the tunnelling temperature. The model is discussed in detail in comparison with the experiments. All effects have been studied as a function of Cu(2+) ion concentration.

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    ABSTRACT: Using low temperature dynamic nuclear polarisation (DNP) in conjunction with dissolution makes it possible to generate highly polarised nuclear spin systems for liquid state applications of nuclear magnetic resonance spectroscopy. However, in its current implementation, which requires the transfer of the solute between two different magnets, the hyperpolarisation strategy is limited to spin systems with relatively long longitudinal relaxation time constants. Here we describe the design and construction of a dedicated spectrometer for DNP applications that is based on a magnet with two isocentres. DNP enhancement is carried out in the upper compartment of this magnet in a low temperature environment at 3.35 T, while a 9.4 T isocentre in the lower compartment is used for high resolution NMR spectroscopy. The close proximity (85 cm) of the two isocentres makes it possible to transfer the sample in the solid state with very little loss of spin polarisation. In first performance tests this novel experimental set-up proved to be superior to the strategy involving two separate magnets.
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A J Horsewill