In liquid crystalline solvents

Lowenstein and Igner86 have determined the quadrupole coupling tensor of 33S in dimethyl sulphone, both in liquid crystalline solvents and in CHCI3 solutions. In solution, an approximate value of xc has been measured from 2H y and Tj. 33S y values are as follows 1.63MHz for rj — 0 and 1.41 MHz for q — 1. In liquid crystals, x is about 1.8 MHz. These values seem to be in agreement within... 

Similar rate decelerations in liquid-crystalline solvents have been observed for the thermal cis trans isomerization of a bulky tetrasubstituted ethene in cholesteric phases [728], On the other hand, the activation parameters for the thermal cis trans isomerization of less-dipolar substituted azobenzenes show no dependence on the solvent order. This indicates that the cis isomers and their corresponding activated complexes present a similar steric appearance to the solvent environment [729]. This result is more consistent with an isomerization mechanism which proceeds by inversion rather than by rotation cf. Eq. (5-40) in Section 5.3.2 and [527-529, 561]. The latter reaction represents a nice example of the use of liquid-crystalline solvents as mechanistic probes [729]. 

As a consequence of the alignment of solute molecules in liquid-crystalline solvents, the ratio of products formed in competitive reaction pathways can be different from that observed in isotropic liquids. This is illustrated by the Norrish type II photolysis of alkyl phenyl ketones with varying alkyl chain length in the isotropic, smectic, and... 

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. 

The 2D C-NMR spectrum of bullvalene was considered by Huang and coworkers. The deuterium NMR spectrum of partially deuterated bullvalene was studied over a temperature range — 35 to +115° C in liquid crystalline solvents. The spectrum ranged from four distinct quadrupole doublets (relative intensities 3 3 3 1) on account of four non-equivalent groups of deuterium. After coalescence at ca. — 15°C one doublet is observed at high temperature. Similar activation parameters to those cited above were deduced. 

Diehl P (1996) Structure of Rigid Molecules Dissolved in Liquid Crystalline Solvents. In Grant DM, Harris RK (eds) Encyclopedia of Nuclear Magnetic Resonance. Wiley, New York, p4591... 

Another approach to the simplification of proton NMR spectra of solutes dissolved in liquid crystalline solvents is by selective, partial deuteration... 

Martin et al.3H measured the diffusion of methylene chloride (CH2C12) in liquid crystalline solvents by following the decay of the double quantum coherence. Zax and Pines39 measured diffusion of benzene in a nematic liquid crystalline solution using a pulse sequence (Fig. 22) which non-selectively excites MQC, permits this to evolve in the presence of a magnetic field gradient, then uses a series of A identical gradient-delay periods to selectively provide an echo which arises solely from the A-quanlum coherence in the evolution period t. ... 

For leading references on NMR in liquid crystalline solvents see (a) J. W. Emsley and J. C. Lindon, NMR Spectroscopy Using Liquid Crystal Solvents, Pergamon Press, Oxford, UK, 1975 (b) J. W. Emsley, ed., Nuclear Magnetic Resonance of Liquid Crystals, D. Riedel Publishing Co., Dordrecht, The Netherlands, 1985 (c) E. E. Burnell and C. A. de Lange, eds., NMR of Orientationally Ordered Liquids, Kluwer Academic Press, Dordrecht, The Netherlands, 2003. 

Molecular Structure from NMR in Liquid Crystalline Solvents... 

A theoretical formalism for calculating dynamic multiple quantum NMR line shapes has been developed and used to calculate the expected line shapes of 1,3,5-trioxane dissolved in liquid crystalline solvents and undergoing ring inversion <1988JCP25>. It was shown that the resulting line shapes can be used to derive the kinetic parameters of the dynamic process. 

Liquid crystals have been used for many years as solvents in NMR studies. Though tumbling of solute molecules in isotropic solvents has the great advantage of simplifying NMR spectra, additional information about the structures of solute molecules can be gained from analysis of the more complicated spectra obtained for oriented solute molecules in liquid crystalline solvents. 

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]. 

To conclude this section on 2D NMR of liquid crystals, some studies of more exotic liquid crystalline systems are pointed out. Polymer dispersed nematic liquid crystals have attracted much attention because of their applications as optical display panels. Deuteron 2D quadrupole echo experiments have been reported [9.28] in the isotropic and nematic phases of / -deuterated 5CB dispersed in polymers. A similar technique was used [9.29] to study two model bilayer membranes. Both studies allow determination of the lineshape F(u ) due to quadrupolar interactions and the homogeneous linewidth L(u ) of the individual lines [9.28]. The 2D quadrupole echo experiment has also been used [9.30] to separate chemical shift and quadrupolar splitting information of a perdeuterated solute dissolved in a lyotropic liquid crystal. The method was compared with the multiple-quantum spectroscopy that is based on the observation of double-quantum coherence whose evolution depends on the chemical shift but not on the quadrupolar splitting. The multiple-quantum method was found to give a substantial chemical shift resolution. The pulse sequences for these methods and their treatment using density matrix formalism were summarized [9.30] for a spin 1=1 system with non-zero chemical shift. Finally, 2D deuteron exchange NMR was used [9.31] to study ring inversion of solutes in liquid crystalline solvents. 

The NMR spectra of quadrupolar nuclei in liquid crystalline solvents are dominated by the quadrupolar interaction, which leads to splitting of magnitude 2 fl ( 10 -10 Hz for H) as given by Eq. (4),... 

Subsequent to the 1982 compilation of Lounila and Jokisaari [237], a large number of studies report chemical shift anisotropies for various nuclei in liquid crystalline solvents. Representative examples include ACTh in dichloromethylphosphine [243], methylisothiocyanide [244], and norborna-diene [141] A(Tc in chloro- and bromoform [245], bis(trimethylsilyl)diacetylene [246], and pyridine, pyrazine, pyridazine, and pyrimidine [247] ACTh and ACq in butyne [248], fluoro- [245,249], bromo- [245], and iodomethane [244, 245, 250], ethylene [251], methylisocyanide [252], dimethyl-mercury [253], benzene [250, 254, 255], 1,3,5-trichloro- [99, 256], 1,3,5-tribromo-[244], and 1,3,5-trinitrobenzene [244] Afluoro-methane [249] Aa in dimethylmercury [253] AcTsein carbon diselenide [258] AcTje in tellurophene [259] and AOxe [260]. 

The most studied reaction in liquid crystalline solvents is the NorishA ang type II reaction of alkyl and aryl alkyl ketones (Scheme 1). This reaction has been used extensively, mainly by Weiss and coworkers, to probe the properties of liquid crystals, since the distribution of fragmentation and cyclization products and the distribution of stereoisomeric trans- and cw-cyclobutanols are sensitive to the dynamics and conformational equilibria of the 1,4-biradical intermediate produced in the primary photochemical decay process of the ketone [352, 353, 360, 368-376]. Generally, in liquid crystals there is a preference for fragmentation over cyclization of the biradical, due to a preference for the biradical conformer that can only react by fragmentation. When cy-... 

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