C. L. Khetrapal

Nmr Spectra Of Isotopically Enriched N-Methyl Formamide In Isotropic And Nematic Media

Abstract NMR spectra of 15N-methyl (13C) formamide (13C) in isotropic as well as liquid crystalline nematic phases have been studied. The NMR and the structural parametes are derived.

The structure and conformation of 3-methylphosphacymantrene

Abstract The proton NMR spectrum of 3-methylphosphacymantrene in a nematic solvent has been interpreted in terms of the molecular structure and conformation. The preferred conformation of the molecule appears to be the one where a CH bond of the methyl group is perpendicular to the ring plane. The results are in accord with the bond polarisation hypothesis.

NMR spectra of molecules oriented in a lyotropic mesophase. II. Spectra of pyridine and pyridine N-oxide

Abstract PMR spectra of pyridine and pyridine N -oxide are studied in a lyotropic mesophase formed by a 14:1:1:20 mixture (by weight) of sodium dodecyl sulphate, decanol, sodium sulphate, and heavy water. The geometry information and the order parameters are derived. The structural information for pyridine thus obtained is compared with that obtained from the studies in a thermotropic phase. Differences up to 2 % in the interproton distance ratios for pyridine in the lyotropic and thermotropic phases are observed. They may arise partly due to the hydrogen bonding between the nonbonded electrons of nitrogen in pyridine and the water molecules of the lyotropic mesophase. No structural data for pyridine N -oxide are available for comparison. It is found that although the arrangement of protons in pyridine is very similar to that in pyridine N -oxide, their molecular orientations in the lyotropic mesophase are quite different. A comparison of the order parameters of the various axes under similar conditions in both the cases is made.

Some Partially Resolved Problems in NMR of Mixed Liquid Crystals of Opposite Diamagnetic Anisotropies

Abstract NMR spectra of molecules dissolved in mixed liquid crystals of opposite diamagnetic anisotropies have been extensively used in the determination of chemical shift anisotropy and other useful information which is normally difficult to derive without the help of such ekxperiments. However, studies on systems such as methanol, tetramethylsilane and the observation of two spectra at the critical concentration and temperature are only partially understood. Details of such investigations are presented with critical up-to-date evaluation of the experiments.

A Theory of Concentration Dependent Switching Transition in Mixed Liquid Crystals of Opposite Diamagnetic Anisotropies

A macroscopic theory of switching transition in a liquid crystal mixture of two thermotropic nematic systems of opposite diamagnetic anisotropies is presented. This is based on Landau theory and shows that temperature being fixed, a transition occurs at a critical relative concentration of the two liquid crystals under consideration. The theoretical dependence of the relevant order parameters is in good agreement with recently observed experimental results on such mixtures.

NMR Spectra of π-Cyclopentadienyl Manganese Tricarbonyl in Nematic and Isotropic Solvents

Abstract 1H-NMR spectra including 13C-H satellites in the natural abundance of 13C in π-cyclopentadienyl manganese tricarbonyl dissolved in a nematic solvent are reported. In addition, the 13C-NMR spectrum of the compound in the natural abundance of 13C, in an isotropic medium is studied. Values of the direct and the indirect HH and 13CH couplings are given. Information on the molecular geometry of the ring skeleton thus obtained is discussed. Contributions of the molecular vibrations to the observed dipolar couplings are estimated. The direct dipolar coupling between the carbonyl 13C and the ring protons are used to derive the distance between the plants containing these carbons and the cyclopentadienyl ring.

Solvent effects on the structure of N-methylpyrrole as studied by nmr in nematic solvents

The proton NMR spectra of N-methylpyrrole oriented in the nematic phases of liquid crystals with positive and negative diamagnetic anisotropies and their mixtures are reported. Geometrical parameters derived from the spectra at the critical point in the mixture of liquid crystals with positive and negative diamagnetic anisotropies, where macroscopic diamagnetic anisotropy vanishes, are similar to those obtained in the solvent with negative diamagnetic anisotropy. However, significant distortions in the molecular structure attributed to solvent effects have been observed in liquid crystals with positive diamagnetic anisotropy. The minimum energy conformation has one C---H of the methyl perpendicular to the ring.

The Amide Planarity as Studied by NMR of Oriented Molecules - The Spectrum of N-Methylacetamide

Abstract The use of the technique of NMR spectroscopy of molecules oriented in the nematic phase of liquid crystals for the study of the ‘planarity’ of the three bonds meeting at the nitrogen atom in a peptide unit has been suggested1 and applied to the simplest member, namely, N-methyl formamide. Such studies are of fundamental importance since in most of the theoretical calculations, a peptide unit is assumed to be completely planar whereas the necessity of considering non-planar distortions in such a unit has been pointed out.2. For N-methyl formamide, using reasonable values of the bond lengths and the bond angles, it was found that the results are consistent with the molecule having a plane of symmetry which may arise, either from the H.CO.NH.C moiety of the molecule being rigidly planar or from a ‘rapid inversion’ through the nitrogen atom. The results support the fromer if there is a ‘free’ rotation of the CH3 group about the N-CH3 bond. However, if the methyl group rotation is hindered in a 3-fold...

The conformation of (μ-butatriene)hexacarbonyldiiron complex from proton NMR studies in three nematic phases

Proton NMR spectroscopy in three different liquid crystals has been used to determine two conformational angles of (μ-butatriene)hexacarbonyldiiron complex, namely the angle between the two CH2 planes and the dihedral angle between the two planes containing four carbon atoms of the butatriene moiety. The values are 44 and 46°, respectively. The direct and the indirect geminal HH couplings are shown to be of the same sign in the liquid crystals with positive diamagnetic anisotropy.

The influence of bond-polarisation on the molecular structures of halo-fluorobenzenes

Abstract From the proton NMR spectra of oriented m -chloro- and m -iodo-fluorobenzenes, relative interproton and proton—fluorine distances, have been obtained. A study of both the systems in two different liquid crystal solvents indicates negligible anisotropic contributions to the indirect spin—spin couplings and negligible solvent—solute effects on the internuclear distances. The geometrical data derived have been interpreted in terms of polarisation of the C H and the C halogen bonds.