NMR structure analysis is the main technique used for the determination of configuration, conformation, and constitution of organic compounds. Classical procedures are based on the combined use of chemical shifts, spin-spin coupling constants, and nuclear Overhauser effects (NOEs).
In many cases these data are not sufficient and an alternative approach is required. This is based on the measurement of additional anisotropic NMR parameters in appropriate alignment media. Under such conditions the following anisotropic parameters are accessible: residual chemical shift anisotropy (RCSA), residual dipolar couplings (RDCs) and residual quadrupolar couplings (RQCs).
The experimental RDCs are further used to solve different structural problems. During RDC analysis, an alignment tensor is calculated for a sterically fixed orientational model. Agreement between experimental and back calculated RDCs allows to solve the constitution, to assign the configuration, and the conformation of the molecule. The majority of the organic molecules we have recently been working with, contains some certain degree of flexibility. In such cases, the straightforward approach, which assumes a single rigid conformation, fails. Therefore, we have started to evaluate our experimental RDCs with a specialized molecular dynamics approach (see more in the Computational chemistry).