T. Väänänen

The r ∞-structure of thiophene in various liquid crystals with 13C-methane as an internal reference

The r ∞-structure of thiophene was studied in thermotropic liquid crystals and in their mixtures. In each case, 13C-enriched methane was added to the sample to serve as an internal ‘deformation reference’. The results indicate that the structure of thiophene is very sensitive to the liquid crystal environment. The structure obtained for thiophene in such liquid crystal mixtures in which the D CH of methane is vanishingly small is in agreement with the microwave structure. The best agreement, however, is obtained in the mixture of MBBA and ZLI 1167 where the D CH of methane is c. 2 Hz.

N.M.R. study of monofluorobenzene partially oriented in thermotropic liquid crystals

The 1H and 19F N.M.R. spectra, with natural abundance carbon-13 satellites, of monofluorobenzene dissolved in the nematic ZLI 1167 and ZLI 1132 liquid crystals were recorded and analysed. The r α-structure of monofluorobenzene was determined in both solutions. The proton structure was also determined in the nematic Phase IV liquid crystal; it was found to be independent of the liquid crystal solvent. However, significant deformations in the carbon skeleton are observable. The indirect contributions to the 13C-19F dipole-dipole coupling constants were found to be negligible. The analysis of the 19F spectra and 19F-13C satellite spectra yielded the 13C(12C) isotope effects on the 19F chemical shifts. These shifts are comparable with those observed in normal solvents or in the isotropic phase of liquid crystal solutions.

The r α-structure and orientational behaviour of monoiodobenzene in nematic liquid crystals

The 1H N.M.R. spectrum with carbon-13 satellites of monoiodobenzene dissolved in a nematic liquid crystal was recorded and analysed. The r α-structure was obtained by using the force field of monobromobenzene in the calculation of the harmonic vibrational corrections to the dipole-dipole coupling constants. The H-C and C-C bond lengths and the CCC and CCH bond angles are reported. The nature of the orienting forces was studied by calculating the theoretical relationships between the two independent ordering matrix elements for monoidobenzene using simple models based on dispersion and repulsion forces. The results strongly suggest that dispersion forces are decisive in thermotropic ZLI 1132 and ZLI 1167 liquid crystals and significant also in some lyotropic liquid crystals.

Variable-angle spinning 1H NMR spectra of molecules in liquid-crystalline phases with positive and negative diamagnetic anisotropy

Abstract The 1 H NMR spectrum of thiophene dissolved in the liquid crystals ZLI 1167 (with negative anisotropy of diamagnetic susceptibility, Δ χ ) and ZLI 1132 (with positive Δ χ ) was studied by applying the so-called variable-angle spinning (VAS) method. In this method, the liquid-crystal sample is spun with relatively high speed around an axis forming a certain adjustable angle with the external magnetic Held. It is shown that a normal high-resolution spectrum with a varying scaling factor in the anisotropic parameters is obtained at any spin angle between 0 and 90°.

The structure and orientational behavior of ortho-difluorobenzene in nematic liquid crystals

The 1H and 19F spectra with natural-abundance carbon-13 satellites of ortho-difluorobenzene dissolved in the nematic ZLI 1167 and ZLI 1132 liquid crystals were recorded and analyzed. The rα structure of ortho-difluorobenzene was determined in both solutions. The analyses of the 19F spectra and 19F13C satellite spectra yielded the 13C(12C) isotope effects on the 19F chemical shifts and provided evidence of the indirect contributions to some 19C19F dipole-dipole coupling constants. The nature of the orienting forces was studied by calculating theoretical relationships between the two independent ordering matrix elements of ortho-difluorobenzene by simple models based on dispersion and repulsion forces. For the thermotropic ZLI 1132 and ZLI 1167 liquid crystals, the results are well fitted by a curve based on the dispersion forces, but for other liquid crystals specific interactions also seem to affect the orientation.

Determination of the 13C13C and 13C15N coupling constant anisotropies of acetonitrile partially oriented by nematic liquid crystals

Abstract The anisotropies of the 13C13C and 13C15N spin-spin coupling constants of acetonitrile were determined by application of the method of partially oriented molecules. The relative anisotropy,, R = ΔJ J , of the 13C13C coupling constant was found to be 2.0 ± 0.4. Because of large errors in the 13C15N internuclear distances, the anisotropies of the corresponding coupling constants are less reliable. (R = 11 ± 6 for the one-bond and −5 ± 3 for the two-bond coupling). The observed one-bond coupling tensors are compared with extended Huckel level calculations.

A variable-angle spinning system for the determination of NMR parameters of liquid-crystalline samples

Abstract A variable-angle spinning (VAS) system for the regulation of the liquid crystal orientation with respect to the external magnetic field has been constructed. The construction, the stability of the rotation, the achievable spectral resolution, and the possible applications of the system will be discussed. As an example, the measurement of 1JCH and the proton-shielding anisotropy in methyl iodide dissolved in liquid crystals is demonstrated.

The rα structure of monobromobenzene partially oriented in a nematic liquid crystal solvent. The use of 13C satellites in the 1H NMR spectrum

Abstract The proton magnetic resonance spectrum with carbon-13 satellites of monobromobenzene in a nematic liquid crystal solvent was recorded and analyzed. The HH and CH dipolar coupling constants corrected for harmonic vibrations were used to determine the rα structures of the proton and carbon skeletons. The proton-proton, proton-carbon, and carbon-carbon internuclear distance ratios as well as the bond angles are reported. The proton structure was found to be hexagonal within the experimental errors, while the carbon structure turned out to be clearly deformed. The effect on the molecular structure of small deviations in force fields was tested using the force field of bromobenzene on the one hand and the modified force field of chlorobenzene on the other to calculate the harmonic corrections for the dipolar coupling constants. The resulting structures are identical within the error limits.

1Hand 19F NMR study with carbon-13 satellites of para-difluorobezene in various nematic liquid crystals: structure and J-coupling anisotropy

Abstract The proton and fluorine NMR spectra, with natural-abundance carbon-13 satellites, of para-difluorobenzene dissolved in various nematic liquid crystals are analysed. The complete rα-structure was determined in five liquid crystals solvents and found to be practically independent of the solvent. Evidence has been found of an anisotropy in the indirect fluorine—fluorine and in the three- and four-bond fluorine—carbon couplings whereas the anisotropy of the other fluorine—carbon couplings appears negligible. The isotope effects on fluorine chemical shielding due to the replacement of carbon-12 by carbon-13 are also reported. The substituent effects of fluorine in difluorobenzenes are summarized.

An experimental 1H NMR study of the structural deformation of benzene and the dipolar CH splitting of methane in various nematic liquid crystals and in smectic HAB

Abstract The structure of benzene dissolved in various thermotropic nematic liquid crystals has been studied. 13 C-methane was used as an internal “deformation reference”. The results indicate that the HH distance ratios obtained front the direct dipole-dipole coupling constants correspond to the hexagon values in cases where benzene is dissolved in mixtures of nematic liquid crystals which produce negligible dipolar CH splittings for methane. The ratio r CH/ r CC for benzene is in good accord with the value obtained by other spectroscopic means. The proton structure of benzene in the smectic and nematic phases of p , p ′-di- n -heptylazoxybenzene (HAB), however, does not follow the general trend observed in the purely nematic phases. But the dipolar CH splitting of methane also behaves inconsistently.