Nematic solvents solutes

In the held of thermotropic cholesterics, the most promising approach seems to be that reported by Nordio and Ferrarini22 23 for calculating helical twisting powers. It allows one to tackle real molecules with rather complex structures and to describe them in detail. The model is currently being extended to include a better description of nematic solvents and specific solute-solvent interactions. Once tested also for conformationally mobile molecules, this model could allow the prediction of the handedness of single-component cholesterics, and, in the held of induced cholesterics, very interesting information on solute molecules could be obtained. 

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. 

That both phenomena arise as a consequence of macroscopic solvent order and not Intimate solvent-solute Interactions Is clear Saeva and 01In (75) have shown that solute LCICD spectra can be observed In twisted nematic phases only Nakazaki et al. (76) find an excess of one enantiomer of hexahelicene Is produced photochemlcally from achiral precursors In twisted nematic phases no LCICD spectra or optical Induction occurs In untwisted nematic phases and the handedness of the twist can be correlated with the sign of the LCICD and the preferred product enantiomer. Furthermore, Isotropic phases of cholesteric mixtures display no discernible LCICD spectra (12, 67) and the enantiomeric excesses In products of photolablle reactants In Isotropic phases are near zero (51). 

Recently, the surface tensor model has been used together with the dielectric continuum model to calculate the orientational order parameters of solutes in nematic solvents [8,9,27], Figure 2.32 shows the theoretical results for anthracene and anthraquinone in nematic solvents with different dielectric anisotropy. Considering only the surface tensor contribution, positive Szz and Sxx and negative are obtained, with Szz > Sxx > Syy. This corresponds to what could be expected on the basis of the molecular shape the long axis (z) is preferentially aligned with the director, and the normal to the... 

J. W. Emsley, S. K. Heeks, T. J. Home, M. H. Howells, A. Moon, W. E. Palke, S. U. Patel, G. N. Shilstone and A. Smith, Multiple contributions to potentials of mean torque for solutes dissolved in liquid-crystal solvents. A comparison of the orientational ordering of anthracene and anthraquinone as solutes in nematic solvents, Liq. Cryst., 9 (1991) 649-660. 

Perhaps, it is appropriate to remind the reader that sophisticated analytical papers using NMR appeared already 40+ years ago. Thus 1H (60 MHz) NMR spectra of cyclopropane molecules containing a single 13C in natural abundance were analyzed quantitatively,99 for the solute in a nematic solvent at 348 K, revealing the several direct Dhh and indirect fHH spin-spin interactions, as well as Jch on the labelled carbon. The presence of the T3C atom of course causes inequivalence of the protons, yielding 2 of one type and 4 of another. Work on (CH2)3 continues for example see Ref. 100. 

In any case however, antipodal helices cause countercurrent spectra of the optical rotation, so that the observation of just a single Cotton effect is sufficient to discriminate the antipodes and, in case, enantiomeric solutes. For such an experiment the choice of the infrared spectral range is no longer dictated by the structure period but by the presence of suitable transition moments. The low demand for the chiral solute to be characterized (Korte, 1978) is exemplified by Fig. 4.6-14. In the 20 im wide sample cell an area of 3 mm times 3 mm was filled with approximately 200 pg solution containing circa 0.2 pg of either S-(-) or R-(-i-) Thalidomide (Contergan) in a nematic solvent. In the spectral interval shown, at least three oppositely shaped ACE are found, the pronounced one around 836 cm is related to the 7 (C- H), phenyl-H out-of-plane vibration of the... 

Experimental orientational data have been obtained from proton NMR spectra of 1,4-difluorobenzene, 1,4-dichlorobenzene, 1,4-diboromobenzene and naphthalene dissolved in several nematic solvents at different tempera-tures. The results indicate a relationship between the sign and magnitude of dielectric anisotropies of the nematic solvents and the temperature dependence of the orientational biaxiality ratio of the solutes dissolved in them. ... 

The solute molecule is dissolved in the liquid crystal solvent at low concentration. A variety of nematic solvents are available, some of which are nematic at room temperature. Representative high-resolution proton NMR spectra are given in Figure 1. Because the solvent order depends on composition and temperature, it is important that temperature and composition gradients at the NMR probe be minimized if the narrow line widths of a few hertz are to be obtained. The spectra of Figure 1 show the rapid increase of spectral complexity with the number of nuclei. The spectra become almost continuous and uninterpretable at about 10 spins. Simplified proton NMR spectra can be obtained by partial deuterium substitution and decoupling.6 This has been described for cyclohexane, but has not been used extensively. Proton double resonance is also a useful experimental technique for the identification of spectral lines.6... 

Finally, in addition to predictions for the increments in Miesowicz and Leslie viscosities, the Brochard theory predicts [Brochard, 1979] the increment Syi in the viscosity associated with the twist distortion of a nematic solvent on dissolution of a polymeric solute ... 

Pashkovskii, E. E., and Litvina, T. G., Twist viscosity coefficient of a dilute solution of the main-chain mesogenic polymer in a nematic solvent an estimation of the anisotropy and the rotational relaxation time of polymer chains, J. Phys. II, 2, 521-528 (1992a). 

Janik, B., Samulski, E. T., andToriumi, H., Flexible solutes in a uniaxial field a H NMR study of n-aUcanes in a nematic solvent, J. Phys. Chem., 91, 1842-1850 (1987). 

Observed structures of a lyotropic material are classified into three categories nematic, smectic, and cholesteric. Nematic and cholesteric mesophases can be readily identified by microscopic examination. The existence of a smectic mesophase is not well defined and is only suggested in some cases. Solvent, solution concentration, polymer molecular weight, and temperature all affect the phase behavior of lyotropic polymer solutions. In general, the phase transition temperature of a lyotropic solution increases with increasing polymer molecular weight and concentration. It is often difficult to determine the critical concentration or transition temperature of a lyotropic polymer solution precisely. Some polymers even degrade below the nematic isotropic transition temperature so that it is impossible to determine the transition temperatures. Phase behavior is also affected by the polymer molecular conformation and intermolecular interactions. 

Recently, there are several reportsA.) discussing the twist mode of the binary system of the thermotropic liquid crystals composed of a nematic solvent and a cholesteric solute. Another interest is in attempting to pursue the structural similarity between thermotropic liquid crystals and lyotropic one, especially for the dependence of cholesteric pitch on the temperature. 

The extrapolated line of log S-log C crossed each other at a critical concentration Cq at which S stays constant and independent of temperature. These results suggest that the temperature dependence of the cholesteric pitch would inflect at the concentration higher than Cq This is analogous to the behavior of thermotropic liquid crystals composed of cholesteric solute and nematic solvent, where the sign of dS/dT reverses at a critical concentration. It is understood that the behavior of both thermotropic and lyotropic liquid crystals is comparable provided that the nematic substances of the former are substituted with the solvents of the latter. The critical concentration Cq is about 0.41 vol/vol and this value is very close to the concentration at which the side chains on neighboring molecules of the polymer come to contact each other ( refer to fig.5 ). From these results, it is expected that the origin or mechanism of twist would change at this concentration Cq. The... 

The nematic phase has point group symmetry Dooh- If we add some amount of chiral, e.g., right-handed molecules, the symmetry is reduced from Dooh to Dqo (symmetry of a twisted cylinder). Such a phase is called chiral nematic phase. Chiral molecules used as a dopant (solute) in nematic solvent considerably modify the nematic surrounding and the overall structure becomes twisted with a helical pitch Pc, incommensurate with a molecular size a, na (n is an integer) and usually Po a. Typically, a < 10 nm, Pq = 0.1-10 pm. 

The concentration at which lyotropic phase is formed depends on the chemical stmcture and molecular mass of the polymer, the solvent nature, and the temperature. Generally, the effective concentration is in the range 10-20% by weight of the polymer in the solvent at room temperature. All polyamides form nematic lyotropic solutions, and the introdurtion of either a lateral substituent on the diamine, or of 1,3-phenylene units (in very small amoimts) or of flexible spacer (espedally polymethylene fragments) can increase the solubility of the polymers in the less a ressive solvent. 

R. BACGHMAN. I meant the (dianges between e isotropic solutian and the nematic phase solution. P. W. MORGAN. Ws have not observed any differences between isotropic and nematic solutions. I would not expect the spectra of isotrx ic and nematic solutions to differ exc t insofar as they are affected by orientation and concentration. Ihe polymer chains in the isotropic solution axe presumably fully extended and associated with the solvent in much the same way as in the nematic phase. The transition from isotrc lc to anisotropic state, dxich occurs at the critical concentration, but inccmpletely, does not require the chains to become more fully extended. One could look for changes in solvent association for isotropic solutions in base of varying concentration. 

Fig. 5.22. Schematic illustrations of the trajectory of a polymer solute chain in isotropic solvent and in a nematic solvent relative magnitudes of the parallel and perpendicular (to the vertical direction and n, respectively) components of the radius-of-gyration tensor R are indicated.

They may, however, point in any direction perpendicular to it. Accordingly, the nematogens with negative diamagnetic anisotropy do not orient uniformly in a magnetic field, but exhibit a lower order with respect to an external axis than the nematic phases with positive anisotropy. Nevertheless, it is possible to obtain high resolution nmr spectra in liquid crystalline solvents of this type [26]. High resolution nmr spectra have also been observed in the nematic soap solution [13]. 

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