Dynamic NMR studies of molecular motions and order in calamitic and discotic liquid crystals.

摘要

This dissertation reports a study of three kinds of liquid crystals using modern solid state NMR techniques: chiral rod-like liquid crystals, bent-core mesogens and disc-like liquid crystals. The properties and structures of liquid crystals are first introduced in Chapter 1. To understand the principles of different NMR phenomena, quantum mechanical theory is adopted to study different nuclear spin interactions and NMR techniques in Chapter 2.;The third part of the dissertation (Chapter 7-9) is concerned with 13C NMR techniques. After we introduce the quantum theory of different pulse sequences, theoretical models are presented to fit observations such as chemical shifts and dipolar splittings. Moreover high resolution liquid 13C NMR experiments are introduced to study some bent-core molecules. They are useful to assist the carbon peak assignments of these molecules. The structure and ordering information of liquid crystals can be determined in their mesophases.;Finally, a brief summary of the dissertation is given in the last chapter.;In the next part of this dissertation (Chapter 3-6), 2H NMR methods are used to investigate the dynamics and structures of some liquid crystal phases. This is first done using the spin relaxation study. The parameters obtained from the model simulation can describe the molecular motion and internal dynamics in the fast motion region. Secondly, we investigate the dynamic process of discotic mesophases and unwound smectic C* phase using the line shape simulation study. 2D 2H NMR exchange experiments are then performed to study the jump process in TGBA* phase and SmC* phase. The above investigation has demonstrated some powerful NMR methods for the dynamic study of liquid crystals.