Solid-state NMR spectroscopy can provide valuable information about molecular and supra-molecular structures, packing arrangements, as well as molecular dynamics. In order to exploit this rich potential, the development of advanced techniques and their application is of great importance. The solid-state NMR group therefore devotes considerable efforts to the development and application of new methods.
In the solid state, anisotropic spin interactions such as the chemical shift anisotropy or dipolar couplings contain unique spatial and dynamic information. However, these interactions lead to broad and overlapping spectral lines, which obscure the familiar isotropic chemical information known from solution-state NMR spectroscopy. In order to obtain high-resolution, it is necessary to remove the broadening due to this anisotropic interactions. The most convenient way to achieve this goal is rapid sample rotation at the so-called “magic” angle (MAS) relative to the static magnetic field, and at spinning speeds up to 35 kHz the anisotropic spin interactions are mostly refocused and the isotropic chemical shift information is regained. However one would like to access the spatial and dynamic information inhered to the anisotropic interactions. This can be achieved by combining fast MAS with appropriate recoupling techniques, based on the irradiation of sample rotation-synchronized radio frequency impulses. In this way, new solid-state NMR experiments can be designed which access spatial information, such as local proximities, as well as molecular dynamics.
A particularly successful experimental solid-state method is ¹H double-quantum NMR spectroscopy under fast MAS in solids. This method is based on the indirect observation of double-quantum (DQ) coherences, which are correlated non-equilibrium states involving two dipolar coupled nuclear spins. Since the efficiency of DQ excitation strongly depends on the dipolar coupling between the two spins, these DQ experiments can be used to determine the dipolar coupling constant. In rigid samples, the dipolar coupling can be used to determine the distance between the coupled spins, and hence elucidate, e.g. the nature of hydrogen-bonded intermolecular bridges, whereas in mobile systems with known chemical structure the anisotropy of the molecular dynamics, as described by a local order parameter, can be probed.
¹H double quantum NMR spectrum of nematic model compound.
Even at very high static magnetic fields up to 20 Tesla (¹H-frequency: 850 MHz) and using the fastest MAS frequencies available the resolution of ¹H DQ NMR spectroscopy is often still inadequate. In such cases, coherent heteronuclear NMR methods offer significantly improved chemical selectivity by taking advantage of the larger spread of chemical shifts in rare nuclei such as ¹³C. These methods, which also utilizes the recoupling approach described above, then allow the identification of heteronuclear spatial proximities and, through the determination of dipolar couplings, an analysis of molecular dynamics. Both advanced NMR methods described in here have the major advantage of being applicable to as-synthesized samples, i.e. there is no requirement for isotopic labeling.