The temperature dependence of methyl tunnelling motion in three acetates

NMR at pressures up to 90 GPa

The past 15 years have seen an astonishing increase in Nuclear Magnetic Resonance (NMR) sensitivity and accessible pressure range in high-pressure NMR experiments, owing to a series of new developments of NMR spectroscopy applied to the diamond anvil cell (DAC). Recently, with the application of electro-magnetic lenses, so-called Lenz lenses, in toroidal diamond indenter cells, pressures of up to 72 GPa with NMR spin sensitivities of about 10¹² spin/Hz^1/2 has been achieved. Here, we describe the implementation of a refined NMR resonator structure using a pair of double stage Lenz lenses driven by a Helmholtz coil within a standard DAC, allowing to measure sample volumes as small as 100 pl prior to compression. With this set-up, pressures close to 100 GPa could be realised repeatedly, with enhanced spin sensitivities of about 5 × 10¹¹ spin/Hz^1/2. The manufacturing and handling of these new NMR-DACs is relatively easy and straightforward, which will allow for further applications in physics, chemistry, or biochemistry.

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

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.

Methyl tunnelling, reorientation and NMR relaxation in solid acetates

Rotational excitations of methyl groups in six solid acetates have been investigated by 1H nuclear magnetic resonance (NMR) relaxation time measurements at 15 MHz and 30 MHz and at temperatures between 10 K and the melting point. Hindering barriers between 1.6 kJ/mol and 3.7 kJ/mol have been found that could be correlated to the tunnelling frequencies observed by inelastic neutron scattering. A consistent description of the relaxation rate dependences from the classical regime at high temperatures to the quantum-mechanical regime at low temperatures is possible by Haupt's equation. The rotational potentials are mainly determined by inter-molecular interaction with an important influence of water of crystallization, if present.