Liquid-crystalline solvents, effect

Leigh and Mitchell have reported one of the largest liquid crystalline solvent effects... 

The role of liquid-crystalline solvents in affecting the rates and specificities of solute reactions is not clear. In some cases, no detectable influence of solvent order has been reported (2 -6) while in others, seemingly quite similar, large effects are found (1,7-16). In the extreme, different laboratories have published conflicting claims for the same reaction performed in the same solvent (2,17-19). The need for care in performing these experiments and in analyzing results from them cannot be emphasized too strongly. 

In general, the greater the similarity in size and shape between the solute and liquid-crystalline solvent molecules, the easier it is for the solute to incorporate itself into the liquid-crystalline phase. If the result of a chemical reaction depends on the solvent order, then the largest effects can be expected for those solute molecules which fit best into the liquid-crystal structure. Since the structure of -heptadecyl phenyl ketone is identical to -butyl stearate except for the terminal phenyl and butoxy groups, it exhibits the largest solvent effect of the ketones studied [730]. 

All in all, liquid-crystalline media are not generally useful solvents for controlling the rates and stereochemistries of chemical reactions. In each case, careful consideration of the fine details regarding the structure of educts and activated complex, their preferred orientations in a liquid-crystalline solvent matrix, and the disruptive effects that each solute has on the solvent order has to be made. A mesophase effect can only be expected when substantial changes in the overall shape of the reactant molecule(s) occur during the activation process [734],... 

A new formulation of the theory of paramagnetic shifts particularly suited to shifts in liquid crystalline solvents has been presented. (29) The Hamiltonian expression used allows for the inclusion of effects of preferential orientational distribution of the solute molecules. 

Other rate processes studied include isopropyl group rotation, (529) restricted rotations in phosphine derivatives of cyclopentadienyl complexes of iron and nickel, (530) and ring and nitrogen inversion in (en) complexes of praseodymium. (531) Molecular geometries of molecules in solution can be accurately determined from NMR spectra of the molecules oriented in a nematic phase of a liquid crystalline solvent. The effect of Eu(dpm), on the nematic phase spectrum of pyridine has been examined. The pyridine geometry is unaffected by the LSR. The observed LIS values can be separated into isotropic and anisotropic components. (532)... 

Rather small solvent effects, of ca. 1.5 Hz on /hc in C-enriched iodomethane, have been observed by Park et who measured this coupling in three different liquid crystalline solvents. The authors found that the solvent effects observed for the isotropic coupling are very similar to those measured for the dipolar couplings. 

Analysis of natural abundance deuterium distribution in organic molecules, an important step in the study of kinetic isotope effects associated with enzyme-catalyzed reactions, by the use of chiral anisotropic media has been explored. An aspect of this analysis is the discrimination of the enantiotopic deuterons in prochiral molecules and the quantification of isotopic fractionation on methylene prostereogenic sites. Towards this an approach has been presented which is based on the use of natural abundance 2-dimensional NMR experiments on solutes oriented by chiral liquid crystalline solvents and the separation of the deuterium signals based on the quadrupolar interaction. The case of 1,1 -bis(phenylthio)hexane derived by cleavage from methyl linoleate of safflower has been used to illustrate the method with (D/H)pro-R and (D/H)pro-S measured at the same methylene position of a fatty acid chain. Enantiomers of water soluble materials can be observed using deuterium NMR spectroscopy in the lyotropic mesophase formed by glucopon/hexanol/buffered water. ... 

A considerable number of liquid crystalline solvents have been investigated (Table 5). These studies showed that a necessary condition for a good separation power of the liquid crystal is a large persistence range of the mesophase [131]. The separation of meta- and para-xy ene presents a convenient test for the effectiveness of a column-packing material. The separation of these xylene isomers is a classical problem in G—LC which is also of considerable technical importance. The two isomers have about equal retention volumes in most ordinary stationary phases. 

Modem 2D techniques are applied, for example, in NMR on chiral solutes in chiral liquid crystalline solvents. NMR spectroscopy on such chiral host/chiral guest systems can be used for the spectral discrimination of enantiomeric solutes because the enantiomers show differential orientational ordering in the chiral environment. Based on this differential ordering effect (DOE), enantiomeric purity or excess can be quantitatively measured [92]. The DOE may also help to understand the intermolecular interactions involved in the chiral recogni-... 

For a partieular phase type, the magnitude of the observed effects on reactivity correlates with the structural compatibility of the solute and solvent, to a first approximation. Since liquid crystals are expected to favour specific orientations or conformations of reactants, reactions involving solutes, transition states or products that are most closely related structurally to the liquid crystalline solvent often show the largest effect. However, examples also exist where the reactivity of small solutes that are not solvent-like, or of solutes whose reaction involves only minor changes in shape, is significantly altered in a liquid crystalline solvent. This is... 

The largest reported effects of liquid crystalline solvents on thermal unimolecular reactivity involve cw-trans-olefin isomeriza-tions. Examples include the cis- to transisomerization of A, A -diacylindigos (4) [397] and the isomerization of merocya-nines (5) to indolinospiropyrans (6) [378, 379]. 

Experimental work In kinetics of polymerization In liquid crystalline media Is sketchy at best. Hopes have been formulated for the possibility of regulating stereo-placements and Inducement of topotactlc effects by free-radical polymerization of liquid crystalline monomers In bulk or In liquid crystalline solvents, due to the high degree of orientational order (35, 52). Such effects have yet to be established. Most of the reported data appear to be due to factors other than molecular orientation and are only Indirectly related to the liquid crystalline order of the medium. For example, factors such as Incomplete miscibility of monomer and solvent, phase separation of the polymer, enhancement of viscosity of the medium, and caging of molecules of initiator (53, 54, 55, 56) can be invoked to explain the observed kinetic effects. 

Many ccllulosic derivatives form anisotropic, i.e.. liquid crystalline, solutions, and cellulose acetate and triacetate are no exception. Various cellulose acetate anisotropic solutions have been made using a variety ol solvents. The nature of Ihe polymer -solvent interaction determines the concentration at which liquid crystalline behavior is initiated. The better the interaction, the lower the concentration needed to form the anisotropic, birclringenl polymer solution. Strong organic acids, eg, trifluoroac etic acid, are most effective and can produce an anisotropic phase with concentrations as low as... 

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