Decoupling in the liquid crystals and solid state NMR of Fluorine containing organics

Abstract

The success of NMR methods in solids and liquid crystals is strongly related to more and more sophisticated strategies of spin decoupling. This is particularly true for liquid crystal samples where high resolved decoupled spectra are required. In the first part of this thesis we described the basic principles of spin decoupling and through numerical simulations based on appropriate spin modelling we provided new physical insight. Testing several decoupling schemes in fluorinated liquid crystals we found anomalous line broadenings of carbon resonances close to (^19)F. The underlying mechanisms of these broadenings were successfully explained in terms of 'H decoupling effects. We demonstrated that these broadening effects are related to the difficulty of (^1)H decoupling in the presence of strong (^1)H—(^19)F dipolar interactions. Employment of sophisticated decoupling methods drastically reduced or even fully eliminated the sources of these line-broadenings. In the second part of this thesis we extended the preceding work to spinning samples (both liquid crystals and solids). Analogous line-broadenings from decoupling effects are also at work here. However additional line-broadening mechanisms, such as magic angle spinning misset and (^19)F lifetime-broadening are also limiting factors of carbon linewidth. Quantification and assignments of dipolar splittings are vital to understand complex molecular conformations of liquid crystalline phases. To extract this information from ID NMR could be difficult. This difficulty arises from the complexity of ID spectra, and so 2D NMR methods have been explored. In the last part of this work we designed Separated-Local-Field sequences, showing that this class of experiments are particularly suited to quantitative use of C—F splittings in fluorinated liquid crystals.