Liquid crystals oriented solutes studies

For UV and fluorescence measurements, the most commonly used liquid crystals are mixtures of the nematic 4 -alkylbicyclo-hexyl-4-carbonitrile s (CCH) (e.g., ZLI 1167 and 1695), which are transparent down to 200 nm and exhibit nematic ranges between =30 and 80 °C (see, for example, [315, 331, 333, 334]), and various cholesteric or compensated nematic phases of cholesteryl chloride/cholesteryl ester mixtures, which are transparent to =240 nm [310, 313, 326, 329]. Some use has also been made of 4 -(4-alkylcyclohexyl)benzo-nitriles (PCH-n), which are transparent to =290 nm [330, 335]. Several other meso-phases, including thermotropic smectics, discotics, and lyotropic phases, have low absorption in the UV region and have been used from time to time as well. The most commonly used liquid crystals in FTIR studies are the CCH-mixtures ZLI 1167 and 1695 [314, 321, 336, 337]. The orientation of the liquid crystalline solution is most commonly achieved either by cell surface treatment or the application of an electric or magnetic field. 

DNAs are soluble only in aqueous solutions and their fibrous crystals can be prepared by slow evaporation from the aqueous solution. Duplex structures in the fibers have been studied by X-ray diffraction [2,3] and sohd state NMR [4-6]. Orientation of DNA strands by using hydrodynamic flow gradients in the dilute aqueous solution [7,8] and lyotropic liquid crystal... 

The three-state RIS model of conformer statistics is used to analyze the 16 independent dipole coupling constants measured in a proton NMR study of n-hexane in a nematic liquid crystal solvent. The orientational ordering of the n-hexane molecule is treated in the context of the modular formulation of the potential of mean torque. This formulation gives an accurate description of alkane solute orientational order and conformer probabilities in the nematic solvent. Consequently, substantially more accurate calculated diplar couplings are obtained, and this is achieved without the need to resort to unconventionally high values of the trans-gauche energy difference E(g) in the RIS model. 

Liquid crystals, liposomes, and artificial membranes. Phospholipids dissolve in water to form true solutions only at very low concentrations ( 10-10 M for distearoyl phosphatidylcholine). At higher concentrations they exist in liquid crystalline phases in which the molecules are partially oriented. Phosphatidylcholines (lecithins) exist almost exclusively in a lamellar (smectic) phase in which the molecules form bilayers. In a warm phosphatidylcholine-water mixture containing at least 30% water by weight the phospholipid forms multilamellar vesicles, one lipid bilayer surrounding another in an "onion skin" structure. When such vesicles are subjected to ultrasonic vibration they break up, forming some very small vesicles of diameter down to 25 nm which are surrounded by a single bilayer. These unilamellar vesicles are often used for study of the properties of bilayers. Vesicles of both types are often called liposomes.75-77... 

M. Pavanello, B. Mennucci and A. Ferrarini, Quantum-mechanical studies of NMR properties of solutes in liquid crystals A new strategy to determine orientational order parameters, J. Chem. Phys., 122 (2005) 064906. 

Raman optical activity has only been measured so far in pure liquids and strong solutions. Crystals and powders are harder to study crystals must be polished and oriented carefully to eliminate artefacts, whereas multiple scattering in powders depolarizes the incident light. It would be of great interest to measure pure rotational, and rotational-vibrational, ROA in gases, but insufficient scattered intensity has so far prevented this. An additional complication in resonance scattering is that circular dichroism of the incident beam can contribute to the measured circular intensity difference. 

Liquid crystals can also be used since the temperature-dependent residual static dipole-dipole coupling caused by the anisotropic orientational order produces, in turn, a temperature-dependent static splitting of the proton spectrum. The first study used a 5% v/v solution of benzene in the room temperature mesophase ZLI-1132 liquid crystal in a 5 mm NMR tube. Over 60 lines were present in the spectrum (66 at 360 MHz and 64 at 600 MHz). The authors measured the frequency separation of the outermost lines as a function of temperature at 360 MHz, and fitted this to a third-order polynomial, giving ... 

Structural parameters and interatomic distances derived from electron diffraction <77JST(42)121> and x-ray diffraction studies <76AX(B)3178> were given in CHEC-I. The molecular structure of pyrazine has been determined by combined analyses of data obtained by gas-phase electron diffraction (ED) and liquid-crystal NMR (LCNMR) <88JA2758>. The NMR spectrum gives structural information because the solute is partially oriented in the liquid-crystal solvent. The structural parameters determined from the ED, LCNMR data and in a joint analysis of both are listed in Table 2. There the C—C bonded distance is fixed since LCNMR data give no information on the absolute size of the molecule. Since pyrazine itself has no dipole moment, it should not show a microwave (pure rotation) spectrum. 

---