R.A. Wind

Experimental setup for enhanced 13C NMR spectroscopy in solids using dynamic nuclear polarization

Abstract If a solid contains free electrons the dynamic nuclear polarization (DNP) technique can be applied to enhance the 13C signal by one or two orders of magnitude. The method demands irradiation near the electron Larmor frequency. In this article the microwave part of the experimental setup is described as well as two probes in which DNP can be combined with 13C FID or 13C1H cross polarization experiments. In one of the probes magic-angle spinning can also be performed. The magnetic field strength used is 1.4 T. Measurements are shown of enhanced 13C FID and CP spectra of a low volatile bituminous coal, and of polystyrene doped with a stable radical.

A quantitative investigation of the dynamic nuclear polarization effect by fixed paramagnetic centra of abundant and rare spins in solids at room temperature

The existing theory of the dynamic nuclear polarization (DNP) effect by fixed paramagnetic centra in solids is extended to make it also valid for systems where spin-diffusion among the polarized spins is neglectable. The theory is quantitatively compared with measurements at room temperature of the 1H- and 13C-DNP enhancement of polystyrene doped with free radicals. Easy-to-use formulas are derived which can be used to get an estimate of the DNP enhancement.

The influence of symmetry-restricted spin diffusion on the spin-lattice relaxation of molecular groups in solids

The influence of symmetry-restricted spin diffusion on the spin-lattice relaxation of rapidly reorienting molecular groups in solids is considered. Each group consists of three identical spin-12 nuclei at the corners of an equilateral triangle. First, the spin-lattice relaxation of isolated three-spin systems is considered. Then symmetry-restricted spin diffusion is treated and its influence on the spin-lattice relaxation in solids is discussed. The resulting coupled spin-lattice relaxation equations explain phenomena like non-exponential relaxations in solids. Previous works concerning the spin-lattice relaxation in the symmetry-restricted spin diffusion model are discussed briefly.

Structural information of undoped trans-polyacetylene obtained by 13C 2D NMR combined with dynamic nuclear polarization

Abstract By means of 13C 2D NMR in combination with dynamic nuclear polarization at room temperature, the orientation of the chemical-shift tensor and the carbon-carbon bond lengths in undoped trans-polyacetylene are determined. It is proven that the tensor element σ33 has its principal axis perpendicular to the molecular plane, and that the σ11 axis makes an angle of 43 ± 5° with respect to the single bond and 78 ± 6° with respect to the double bond. It is found that the single bond length is 1.45 ± 0.01 A and the double bond length 1.38 ± 0.01 A. The C−C=C bond angle is 120.7 ± 1.5°. No evidence is found for fast bond alternation caused by mobile solitons, which indicates that the soliton motion is largely restricted to an area close to trap sites.

Determination of the NMR relaxation times T1z and T1D of solids by means of continuous-wave experiments

In this paper an experimental and theoretical investigation has been performed on three types of continuous-wave lock-in experiments, from which the relaxation times T1z and T1D of solids can be obtained: the work involves the saturation behaviour of the magnitude of the absorption and dispersion derivative, the saturation behaviour of the peak distance of the absorption derivative, and the recovery from saturation. Especially the latter method provides an easy way of determining T1Z and T1D if they are larger than a few seconds. Though the recovery generally consists of a mixture of two exponentials, under certain conditions they can be detected separately and the relaxation times can be obtained from the respective time constants. As this experiment takes relatively little time, this method can well compete with existing pulse experiments. Moreover, this paper also provides an experimental verification of the Provotorov theory, extended to the case of lock-in detection and non-negligible chemical shift. The experiments have been performed on MoF6.

Applications of Dynamic Nuclear Polarization in Coal Research

The Nuclear Magnetic Resonance signals of coal can be enhanced to an appreciable amount by means of Dynamic Nuclear Polarization (DNP).

Investigation of Methyl Reorientations in 1,4-Dimethoxy- 2,6-dimethylbenzene by NMR and Inelastic Neutron Scattering

Abstract In this paper a solid state NMR relaxation study and inelastic neutron scattering (INS) experiments (the latter in the solid and liquid phase) are reported for a molecule containing methyl groups in different surroundings, namely 1,4-dimethoxy-2,6-dimethylbenzene. The results are compared with each other and with those of a liquid state NMR study reported earlier. It is found that in this way both the shapes and magnitudes of the various hindering potentials can be determined. The result is that the methyl groups 1, 2 and 4 possess a threefold cosine-shaped potential with heights given by 7.7, 9.0 and 15.0 kJ/mol according to the solid state NMR measurements (from the INS study the values 7.5, 7.5 and 16.0 kJ/mol were obtained). For methyl group 6 a mixture of a three-and six-fold symmetry had to be taken into account in order to explain the experiments. Except for methyl group 4 the liquid state NMR results deviate from those obtained with the other two techniques. A possible cause for this discrepancy is discussed. For the molecular structure it is concluded that the reorientations of the methyl groups 1 and 2 are influenced by mutual hindering and that the angle between the plane through the C4-O-C bond and the aromatic plane is small, whereas for the C1-O-C bond this angle is estimated to be about 70°.

Free precession decay of nuclear pairs in solids

Abstract The nodes in the free precession decay of proton pairs in a rigid lattice, with dipolar interaction between the protons of a pair, are determined by both the distance between the protons and the possible motion of the pairs. Interaction between the pairs does not alter the place of these nodes. When using the Fourier transform of a cw lock-in experiment, the amplitude of the modulation field also appears to have no influence on the nodes, whereas they are only slightly altered because of saturation of the spin system. From the nodes, proton-proton distances of gypsum, cyanamide and methylene chloride have been determined.