Field-cycling NMR investigations of (13)C-(1)H cross-relaxation and cross-polarisation: the nuclear solid effect and dynamic nuclear polarisation

Proton tunnelling in the hydrogen bonds of the benzoic acid dimer: (18)O substitution and isotope effects of the heavy atom framework

Field-cycling (1)H NMR relaxometry has been used to measure the rate of concerted double proton transfer in the hydrogen bonds of (16)O and (18)O isotopologues of benzoic acid dimers. The experiments have been conducted in the solid state at low temperature 13.3 ≤ T ≤ 80 K where the dynamics are dominated by incoherent proton tunnelling. The low temperature tunnelling rate in the (16)O isotopologue is observed to be approximately 15% faster than in the (18)O isotopologue. The difference is attributed to an isotope effect of the heavy atom framework of the benzoic acid dimer resulting from displacements of the oxygen atoms that accompany the proton transfer. Sources of systematic uncertainty have been minimized in the design of the experimental protocols and the experiments are critically appraised in formally assigning the measured differences to an effect of mass on the tunnelling dynamics.

Methodology for solid state NMR study of cross relaxation and molecular dynamics in heteronuclear systems

This work presents the theoretical analysis of cross-relaxation and its applications to the study of molecular dynamics in the solid state heteronuclear systems in the laboratory frame. The solid state NMR experiments were carried out on a homemade pulse spectrometer operating at the frequency of 30.2 MHz and 28.411 MHz for protons and fluorines, respectively. It is worth noting that this spectrometer includes a specially designed probe which simultaneously works at two slightly differing frequencies for protons and fluorine nuclei with complete absence of their interference. Contrary to a large number of previous cross-relaxation studies, in our experiments proton spins can be polarized in the magnetic field B0 or excited by rf pulses, while fluorine spins are continuously saturated for a long time. The saturation of fluorines is maintained throughout the whole duration of the experiment. It leads to a simplification of the mathematical analysis of the experimental results.

1H-19F spin-lattice relaxation spectroscopy: proton tunnelling in the hydrogen bond studied by field-cycling NMR

Proton tunnelling in the hydrogen bonds of two fluorine substituted benzoic acid dimers has been investigated using field-cycling NMR relaxometry. The close proximity of the (19)F nuclei to the hydrogen bond protons introduces heteronuclear (19)F-(1)H dipolar interactions into the spin-lattice relaxation processes. This renders the (1)H magnetisation-recovery biexponential and introduces multiple spectral density components into the relaxation matrix characterised by frequencies that are sums and differences of the (19)F and (1)H Larmor frequencies. Using field-cycling NMR pulse sequences that measure the spin-lattice relaxation and cross-relaxation rates we demonstrate how some of these multiple spectral density components can be separately resolved. This leads to an accurate determination of the correlation times that characterise the proton tunnelling motion. A broad spectrum of relaxation behaviour is illustrated and explored in the chosen samples and the investigation is used to explore the theory and practise of field-cycling NMR relaxometry in cases where heteronuclear interactions are significant.

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.