NMR Spectroscopy has been proven to be a powerful tool for structure determination in solid state. However, without proper structural models, interpretation of NMR observables remains qualitative. Thus the first, crucial step in structure determination with solid-state NMR is to provide reasonable starting models. For setting up structural models, we mainly utilize empiric descriptions and force field methods, as to achieve cost efficient sampling of the hyperspace.
These models can then be refined using more accurate methods like DFT calculations. Depending on the system in question, we use either a cluster approach or periodic boundary conditions. Once a structural model is known, we can calculate NMR parameters from the structure to compare them to the measurement. This may involve calculating observables like the chemical shielding, the alignment of the chemical shielding anisotropy tensor or the J-coupling, or simply distance patterns for calculating exchange rates in cross polarization experiments or simulating REDOR curves. In the long run this shall support the development of routines to automatically perform the setup, adaption and fitting of structural models to the experimental data.
Top: Experimantal 13C-NMR-spectrum for melon (blue). Calculated values for the isotropic chemical shielding are given for the different carbon sites.
Right: Volume plot of the electron density (of the respective Kohn-Sham-States) for the three highest bands in benzene (blue, green and grey). The orange mesh represents the sum of the three bands, which gives idea of the electron densitiy for the PI-system.
Bottom: Three-dimensional plot of the nuclear independent shielding values within the molecular plane of benzene. Blue denotes negative shielding values around -10 ppm, red denotes positive shielding values around +10 ppm. Highest values above +10 ppm are truncated.