Solid-state NMR

Anisotropic interactions

Interaction relvant to NMR are generally anisotropic, meaning the magnitude of the interactions depends on the orientation between the external magnetic field and the spin's environment (e.g., the molecule or crystal). Therefore, all NMR interaction are described by second rank tensors connecting two vector quantitites (e.g., one spin moment with the external field or two spin moments with each other).

Small molecules in solution may tumble fast enough due to Brownian motion that all orietations are effectivel averaged. Some interactions are consquently reduced to their isotropic mean value (e.g., chemical shift, J-coupling); this isotropic value is obtained by the trace of the interaction tensor. Other interactions are vanishing under fast tumbling since their respective interaction tensor is traceless.

Single crystals and powder spectra in solids

In large macromolecules or polymers, highly viscous or vitreous solutions, or in (poly-)crystalline/amorphous solids molecular tumbling is significantly arrested. Therefore, anisotropic interactions are not averaged. In single crystalline samples this causes a shift of (often narrow) resonance lines under rotation of the macroscopic crystal within the magnetic field. By controlled angular adjustment (e.g., by a goniometer), the interaction tensors can thus be measured and even be correlated with the crystallographic axes.

In the case of polycriystalline or amorphpus samples all possible orientations are observed at the same time. This causes inhomogeneous broadening of the nuclear resonances and a so-called powder spectrum is obtained. The envelope of each resonance line describes the weighted spectral range which would be swept by rotation of a single crystal around all axes. However, overlap of resonances of different nuclei is often a problem due to the large inhomogeneous breadth; also, no direct correlation between interaction tensor orientation and crystallographic axes is possible. Nevertheless, information about magnitude and orientation of the tensor elements may be obtained by more sophisticated experiments and spectral simulation.

Di- und quadrupolar interactions

Another important advantage of solid-state NMR is the direct influence of dipolar spin-spin interactions on the obtained NMR spectrum. In contrast to solution NMR, these dipolar interactions cause a splitting of resonance lines which allows for the direct and model-free measurement of distances or angles between nuclei with an inherently high accuracy. Similarly, electrical field gradients at the position of a high-spin nucleus may be obtained by analysis of quadrupolar splitting schemes. From this, for example, information about electron pair distributions can be obtained or hydrogen bond formation can be confirmed.

However, this abundance of additional couplings not present in solution NMR comes with certain disadvanteges. Overlapping resonances from typically hundreds or even thousands of different nuclei within a sample system result in spectral crowding. Additionally, insufficient averaging of strong dipolar couplings may cause a rapid loss of spin decoherence which in turn results in significantly increased (homogeneous) line widths.