Nuclear Magnetic Resonance of Liquid Crystals

ISBN-13: 9789400965195 / Angielski / Miękka / 2011 / 592 str.

J.W. Emsley

Opis

The liquid crystalline state has been known for about a century and has been studied by many techniques. Nuclear magnetic resonance has been used to study mesophases for thirty years, but it has been in very recent years that advances in this form of spectroscopy have led to a rapid growth in its applications to the study both of liquid crystals and of solutes dissolved in them. It has become apparent that no other method of studying liquid crystals can yield such a wealth of data and it is unrivalled as a means of probing the behaviour of the molecules in mesophases. There has also been a steady increase in the study of the shape of small molecules dissolved in liquid crystals via the analysis of their NMR spectrum. In fact, the study of solutes was until recently regarded as a separate activity to the study of liquid crystals themselves, but this artificial division arose only from the gap between the large amount of information that could be derived from the spectrum of a small molecule and the rather meagre data set obtainable from the spectra of liquid crystals. This gap has, however, narrowed and it is now possible to derive a very detailed picture of the structure and orientational ordering of the large molecules typical of those which form liquid crystals. There has also been a rapid growth of interest in the liquid crystalline state.

Spis treści

1. Quantitative Description of Orientational Order: Rigid Molecules.- Looking for a complete description.- A simple case: cylindrical molecules in a uniaxial phase.- The effect of symmetry.- Ordering matrix.- Orientational order parameters for more complex phases.- Connection with experiment.- Non-uniaxial phases.- Cartesian treatment.- Summary.- References.- 2. An Internal Order Parameter Formalism for Non-Rigid Molecules.- The internal order parameter formalism.- Observables for a molecule with one internal rotor.- Limiting cases.- Relation to Emsley-Luckhurst formalism.- Multi-rotor molecules.- Summary.- References.- 3. Molecular Field Theories of Nematics: Systems Composed of Uniaxial, Biaxial or Flexible Molecules.- The Maier-Saupe theory.- Biaxial molecules.- Flexible molecules: orientational order parameters.- Flexible molecules: the thermodynamic properties.- References.- 4. The Landau-De Gennes Theory of Liquid Crystals.- Landau theory of second order phase transitions.- The Landau-de Gennes theory of nematics.- Pretransitional behaviour.- A molecular approach.- References.- 5. NMR Spectra in Liquid Crystals the Partially Averaged Spin Hamiltonian.- Macroscopic alignment.- Spin Hamiltonian.- Spherical tensors.- Magnetic interactions in cartesian formalism.- Dipolar coupling.- Nuclear quadrupolar interaction.- Anisotropy of the shielding tensor.- Indirect couplings.- References.- 6. Studies of Solutes with Internal Rotors.- Theory.- Approximate methods.- Applications.- Molecules with a single locked conformer.- Molecules with two planar equivalent conformers.- Molecules with non planar equivalent conformers.- Molecules with two or more conformers.- Molecules with methyl rotation.- Molecules with ring puckering motions.- Conclusions and perspectives.- References.- 7. Molecular Structure from Dipolar Coupling.- The basic equations.- The procedure of structure determination.- Differences between spectra of isotropic and of oriented molecules.- Relations between direct couplings and molecular structure.- The computer programs for structure determination.- Types of molecules studied so far.- Basic theory for vibration corrections.- The continuum of directions with Saa = 0 in oriented molecules; the Saa = 0 cone.- The use of satellites for increased sensitivity.- The use of satellites in underdetermined cases.- Possible structure precision.- Limitation of structure precision due to anisotropy of the indirect coupling constants.- Limitation of structure precision due to correlation.- Vibration-rotation coupling.- Mean-field deformation.- Limitation of structure precision due to complex formation: the two-site approximation.- Summary.- References.- 8. On the Orientation of Small Molecules in Anisotropic Solvents.- Theory.- Dipolar couplings.- Quadrupolar couplings.- The hydrogen molecule.- The methane molecule.- External field gradients.- Solute-solvent interaction mechanisms.- References.- 9. Diffusive and Collective Motions in Liquid Crystal Phases.- Molecular motions in liquid crystals.- Motional processes and spin relaxation.- Correlation functions.- Projection operator techniques.- Choice of the projection operator.- The formal derivation of the diffusion equation.- Explicit form of the diffusion equation for cylindrical molecules.- Solution of the diffusion equations in liquid crystalline phases.- Side-chain dynamics.- Collective motions.- References.- 10. Density Matrix Formalism for NMR Studies of Liquid Crystals.- Basic theory.- Free precession.- RF pulses: flip angles and phase shifts.- Multiple quantum coherence.- References.- 11. Nuclear Spin Relaxation.- Theoretical framework.- A case study: one spin I = 1.- Relaxation calculations.- Relaxation measurements.- Instrumental requirements.- Results for systems with a single deuteron.- Sample rotation.- Multispin systems.- Deuterium.- 13C-H relaxation.- Other nuclei.- Conclusions.- References.- 12. Multiple Quantum NMR in Liquid Crystalline Phases.- The averaging of magnetic interaction tensors.- Multiple quantum NMR.- Time domain cross sections.- Projections of two dimensional spectra.- The complexity of the multiple quantum spectra of an oriented chain.- Multiple quantum NMR experiments on oriented chain.- Spectral simulations.- Discussion.- Conclusion.- References.- 13. Dynamics of Molecular Processes by NMR in Liquid Crystalline Solvents.- Dynamic 1H NMR in liquid crystals.- Ring inversion in s-trioxane.- Bond shift in cyclooctatetraene.- Bond rearrangement in bullvalene.- Dynamic deuterium NMR in liquid crystals.- Ring inversion of cyclohexane.- Ring inversion in p-dioxane.- Bond rearrangement in bullvalene.- Ring inversion of bridged perinaphthalenes.- Dynamic lineshapes in liquid crystalline solutions.- Dynamic lineshape for I = 1/2 systems.- Application of group theory.- Dynamic lineshapes for I = 1 nuclei.- Summary and conclusions.- References.- 14. Discotic Liquid Crystals and their Characterization by Deuterium NMR.- Chemical structure and classification of discotic mesophases.- Deuterium NMR in discotic mesophases.- Uniaxial discotic mesophases.- Alignment in a magnetic field.- Molecular order parameter.- Chain conformation in the mesogen.- Biaxial discotic mesophases.- The Drd mesophase.- The Dt mesophase.- Discotic mesophases as solvents.- Molecular translational diffusion in discotics.- References.- 15. Measurement of Orientational Ordering by NMR.- Rigid solute molecules.- Choice of interaction.- Dipolar coupling.- Quadrupolar splittings.- Comparison with theoretical models.- Non-rigid molecules.- Location of axes.- Local ordering matrices.- The ordering of alkyl chains.- Comparison with theory.- Non-rigid solutes.- Carbon-13 studies of orientational order.- Conclusion.- References.- 16. Determination of Biaxial Structures in Lyotropic Materials by Deuterium NMR.- Analysis of the spectral patterns.- Hexagonal phase.- Ribbon phase.- Ripple phase.- Discussion.- References.- 17. Phase Biaxiality in the Cholesteric and Blue Phases.- 2H NMR spectral patterns.- Distortion and spatial modulated diffusion.- Blue phases.- Discussion.- References.- 18. Phase Biaxiality in Some of the Smectic Phases.- General theoretical considerations.- SG phase.- SC phase.- Discussion.- References.- 19. Orientational Order Parameters and Mechanisms of Phase Biaxiality.- Ordering in uniaxial phases.- Ordering in biaxial phases.- Biaxial mechanism of the cholesteric phase.- Biaxiality from rotational freeze-out parameters.- Discussion.- References.- 20. Amphiphilic Molecules in Lyotropic Liquid Crystals and Micellar Phases.- Relevant aspects of NMR.- Order parameter curves.- Infinite lamellae in lamellar phases.- Infinite cylinders in hexagonal phases.- Spheroïds in micellar phases.- Oblate spheroïds in nematic phases.- Discussion.- Comments about the thermodynamics.- Conclusion.- References.- 21. Structure of the Hydrophobic-Hydrophilic Interface in Lyotropic Liquid Crystals: Counterion Binding Studies.- Theory of counterion quadrupolar splittings.- Sodium-23 studies.- Non-ionic surfactants.- Charged surfactants.- Potassium-39 studies.- Effects of water content and temperature.- Effects of solubilization of alcohols.- Effects of hydrocarbons.- Chemical equilibrium model of counterion binding.- Comment.- References.- 22. Solubilizate-Bilayer Interactions in Lamellar Mesophases.- Solubilization of alcohols.- X-ray diffraction measurements.- 2H NMR measurements.- Solubilization of hydrocarbons.- X-ray diffraction measurements.- NMR measurements.- Comparison of bilayer thickness as calculated from X-ray diffraction and 2H NMR measurements.- Distribution of hydrocarbon solubilizates in the bilayer.- Response of the bilayer structure to solubilizate.- Appendix 1.- X-ray diffraction measurements.- The partial molecular surface area.- Appendix 2.- The calculation of the effective length of a hydrocarbon chain from its order profile.- References.- 23. NMR of Liquid Crystalline Lipids in Biological Membranes.- Some characteristics of biological membranes.- Deuterium NMR spectra of membranes.- Acholeplasma Laidlawii B: a simple biological membrane.- 31P NMR of membranes.- Synopsis.- References.