Orientational order molecular structure relaxation

Monte Carlo and Molecular Dynamics simulations of water near hydrophobic surfaces have yielded a wealth of information about the structure, thermodynamics and transport properties of interfacial water. In particular, they have demonstrated the presence of molecular layering and density oscillations which extend many Angstroms away from the surfaces. These oscillations have recently been verified experimentally. Ordered dipolar orientations and reduced dipole relaxation times are observed in most of the simulations, indicating that interfacial water is not a uniform dielectric continuum. Reduced dipole relaxation times near the surfaces indicate that interfacial water experiences hindered rotation. The majority of simulation results indicate that water near hydrophobic surfaces exhibits fewer hydrogen bonds than water near the midplane. 

Interpretation of mechanical measurements in terms of molecular structure was until fairly recently confined essentially to identification of the temperatures of the major viscoelastic relaxations through extensional or torsional dynamic mechanical studies. Now, however, investigations of the elastic constants and their temperature dependence—allied with dynamic mechanical, creep and both wide and small angle X-ray diffraction— are yielding fairly detailed pictures of the interrelation of the crystalline and less well ordered regions of some oriented solid polymers. 

Liquid crystal are Intermediate between ordinary fluids, which are endowed with only short-range order or structure, and crystalline solids, which ideally have perfect order both with respect to the position and the orientation of their component molecules. Relaxation of the conditions for perfect ordering in various ways gives rise to mesophases of different sorts. The structures of small molecules that form mesophases are invariably highly anisotropic in shape, usually rod-like or lath-like, composed of rigid central section with some flexible end groups. One can associate a direction vector (for example, the molecular axis) with this non-spherical molecular shape. 

Linearly polarized fluorescence (LPF) and circularly polarized fluorescence (CPF) provide complementary techniques to LCLD and LCICD for the assignment of solute electronic transition moments and for investigating solute orientational ordering and liquid crystalline properties [1,2, 304, 326-328]. Additionally, these techniques can provide information on the electronic structure of excited state complexes (excim-ers and exciplexes) [329, 330]. Time-re-solved luminescence depolarization experiments have been used by various workers to study the ordering and mobility of molecules in liquid crystalline phases [317-319, 331, 332] the information obtainable from these studies in analogous to that obtained by NMR relaxation experiments. Since luminescence depolarization is the main result of probe molecular motions and is consequently very rapid, it leads to complica-... 

Since the first publications on this subject in 1963, NMR in liquid crystalline systems has been a wide and active field of research in many branches of organic and physical chemistry. In fact, NMR spectroscopy has revealed a powerful means of probing molecular structure, anisotropic magnetic parameters and dynamic behaviour of solute molecules dissolved in liquid crystals. Moreover, this technique has been successfully employed to investigate properties of mesophases themselves, such as their orientational ordering, translational and rotational diffusion and their effects on nuclear relaxation, and molecular organization in different liquid crystalline phases. 

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