Solvents nematic

It is occasionally desirable to retain a small proportion of molecular orientation, in order to quantitate the dipolar interactions present, whilst minimizing their contribution to the linewidth. Partial orientation may be achieved by using a nematic solvent. In large, magnetically anisotropic molecules it may occur naturally at the highest magnetic fields. 

The conformation of thietane-1-oxide (185), determined from its H NMR spectrum taken in a nematic solvent, was found to have the oxygen equatorial and to be strongly puckered with an angle of puckering of about 38°, in agreement with the results of a microwave study170. 

As mentioned in the introduction, the first empirical correlation between the absolute configuration of dopants and the handedness of induced cholesterics was proposed in 1975.20 The first attempt to find a general correlation was a few years later Krabbe et al.58 related the sense of the cholesteric to a stereochemical descriptor of the dopant based on the effective volume of the substituents and listed many compounds following this rule. However, exceptions were described at that time,59 and, furthermore, this approach neglects the role of the structure of the nematic solvent in determining the sense of the cholesteric. It is well known that chiral compounds may induce cholesterics of opposite handedness in different nematics.60,61... 

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. 

Fig. 7. Contour plot of the 2D PDLF experiment performed on a sample of 2-fluorotoluene dissolved in the nematic solvent ZLI1695. The right projection shows the PDLF spectrum obtained under 17 kHz decoupling with PMLG. (Reproduced by permission of Elsevier Science.)...

Fig. 10. Experimental and calculated 1H NMR spectra of PBLN in three different nematic solvents at room temperature. (Reproduced by permission of American Chemical Society.)...

The H magnetic resonance spectra of pyrrole- 15N in two nematic solvents have been reported91. The dipolar coupling constants were used to derive relative intemu-clear distances, and it was found that the N—H bond distance relative to the distance... 

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. 

The order and mobility of a labeled flexible alkyl spacer in the linear thermotropic polymeric nematic liquid crystal poly(2,2 -dimethyl-4,4 -dioxyazoxybenzenedodecanedioyl-dj0) (poly[oxy(3-methyl-1,4-phenylene)azoxy 2-methyl-1,4-phenylene)oxy(1,12-dioxo-1,12-dodecanediyl-d2oll) is explored with deuterium NMR. The quadrupol splittings of the spacer methylene segments in the nematic melt of the polymer are reported as a function of the temperature and are contrasted with observations on model compounds solubilized in a nematic solvent. 

The possibility of cis-trans equilibria exists, but has not been explored in depth. One indication for such equilibria comes from the NMR investigation of the parent nickel dithiolene in a nematic solvent only a rapidly equilibrating compound could have produced the observed spectral pattern.35 In the preparation of some mesomorphic dithiolenes related to the one cited above, two products were initially obtained, which during further work-up converted into the trans isomer only.38 In Pt complexes of this type, the existence of two 195Pt- H couplings was taken as an indication36 that the cis and trans isomers coexisted. 

As customary, acronyms will be used for nematic solvents throughout the text 5CB, (d-w-pentyl -cyanobiphenyl EBBA, vV-(/ -ethoxybenzylidene)-//-butylanyline ZLI1132, eutectic mixture of l,4-(fra 5,-4/- -alkylcyclohexyl)-cyanobenzene and, 4- trans-Ar - -pentylcyclohexyl)-cyanobiphenyl. 

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. 

H. Goto, K. Akagi, Optically Active Conjugated Polymers Prepared from Achiral Monomers by Polycondensation in a Chiral Nematic Solvent, Angetv. Chem. Int. Ed. 2005, 44, 4322-4328. 

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. 

L. C. Snyder and S. Meiboom, Molecular structure of cyclopropane from its proton NMR in a nematic solvent.. Chem. Phys., 1967, 47(4), 1480-1487. 

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... 

The H nmr spectrum of thietane 1,1-dioxide in a nematic solvent indicates the ring is nearly planar with a very low barrier to planarity, a result that agrees with variable temperature investigations at 300 and 100MHz. An early nmr study compared the chemical shifts of thietane, thietane 1,1-dioxide, oxetane, and cyclobutane. The chemical shift (6) for the a-protons is 4.09 and for the /3-protons, 2.14. The geminal coupling constant for the a-protons is — 14.0 Hz and for the /3-protons, —12.6 Hz. Other coupling constants are /cis 10-3 Hz, Jtmns ... 

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