Nuclear magnetic resonance studies of the molecular structures of phosphabenzene and arsabenzene partially oriented in a nematic solvent

The molecular structures of phosphabenzene and arsabenzene partially oriented in the nematic liquid crystal crystal EBBA were studied by NMR. The structures were in excellent agreement with the combined electron diffraction and microwave spectroscopic studies except the angle relating to the β hydrogens. Improved values for this parameter were suggested. The absolute signs of the J_PH of phosphabenzene and some information on the ³¹P chemical shift anisotropies were also obtained.     

A nuclear magnetic resonance study of 2,5-dihydrofuran partially oriented in a nematic phase

From the study of the proton magnetic resonance spectra of dihydrofuran dissolved in a nematic phase the ratios of interproton distances have been derived.     

Proton nuclear magnetic resonance study of phthalide and 2-coumaranone partially oriented in nematic phases

The ¹H NMR spectra of phthalide in Merck Nematic Phase IV and in N-(p-ethoxybenzylidene)-p-n-butylaniline (EBBA) and of 2-coumaranone in Merck Nematic Phase IV were analyzed. Structural parameters were obtained. The planarity of the 5-membered ring and the molecular orientations were discussed.     

A nuclear magnetic resonance investigation of tetraselenafulvalene partially oriented in a nematic liquid crystal

The molecular structure of tetraselenafulvalene (TSeF) dissolved in a nematic crystal solvent is determined from the PMR spectra including ⁷⁷Se satellites. The molecule is found to be either puckered in a boat form or possessing a broad potential function for the in-phase puckering motion. The dihedral angle between the SeHCCHSe and Se₂CCSe₂ plane is 15.5 ± 0.6°. The structure is in good agreement with that of the dimerized TSeF in the solid state. Some ⁷⁷Se indirect coupling constants are also determined.     

The molecular structure of tetrathiofulvalene-13C (TTF) from proton magnetic resonance studies using nematic solvents

The molecular structure of TTF dissolved in nematic liquid crystalline solvents has been determined from the proton magnetic resonance including couplings due to ¹³C in natural abundance. The molecule is puckered in a boat conformation with the SCHCHS planes making a dihedral angle of 13 ± 2° with the S₂C  CS₂ plane. The other structural parameters obtained are r_CH = 1.085 ± 0.014 Å and the angel CCH = 123.7 ± 1.5°.     

Nuclear magnetic resonance spectra of methyl fluoride—13C partially oriented in various nematic phases: A reinvestigation and an explanation of anomalous results

New experimental NMR work on ¹³CH₃F dissolved in various nematic phases is reported. These as well as previous results cannot be interpreted solely as the effect of J anisotropy, molecular vibrations or solvent dependence of J. The results demonstrate that CH₃F is rapidly exchanging between at least two sites which have slightly different geometries and orientation parameters of opposite sign.     

Nuclear magnetic resonance study of low molecular weight polychlorotrifluoroethylene

The molecular motion of low molecular weight polychlorotrifluroethylene of different molecular weights between 500 and 1300 was investigated by means of broad-line nuclear magnetic resonance measurements. The line width and second moment of the resonant absorption lines were obtained at temperatures from 77 to 300°K. The line narrowing for low molecular weight samples takes place in one step. In samples higher than 900 in molecular weight, however, there appears to be a glass transition process, and the line narrowing tends to occur in three steps. The line narrowing due to local molecular motion becomes observable with increasing molecular weight.     

The NMR-spectrum of benzaldehyde partially oriented in the nematic phase

The NMR spectrum of benzaldehyde oriented in the nematic phase of a mixture of p-(p-ethoxyphenylazo)phenylheptanoate and p-(p-ethoxyphenylazo)phenylundecylenate is analysed. It is demonstrated that the data are inconsistent with models in which the aldehyde group freely rotates or has minimum energy when it is perpendicular to the ring plane. The barrier height to rotation, however, cannot be obtained, nor is it possible to discriminate between the two possible solutions in which the averaging planar forms do or do not reorient in the liquid crystal between successive internal rotations.     

An NMR study of cis-2-butene partially oriented in a nematic phase

The NMR spectrum of cis-2-butene partially oriented in a nematic phase has been analysed, and information about the geometry and the indirect spin-spin coupling constants has been obtained. Assuming the barrier of rotation of the methyl groups and other geometrical information from microwave studies, we obtained = 126 ± 2° and = 116.8 ± 0·6°. The angle that the CH₃ rotation axis makes with the double bond was calculated to be 127·5 ± 0.4°; this value is closely linked with the assumed value of = 109°28′.     

An NMR study of the conformation of glutaric anhydride partially oriented in a nematic solvent

The structural information derived from the dipolar proton-proton couplings in glutaric anhydride is consistent with the sofa-model: only the β-carbon atom is out of the plane of the other ring atoms. With reasonable values of the other internal coordinates the value of the torsional angle between the vicinal cis-protons is between 50 and 53°, which agrees with the value previously determined by Koer et al. The orientation of glutaric anhydride cannot be described by two orientation parameters, which indicates that the reorientational motion is faster than the interconversion between the two sofa-conformations.     

NMR analysis of benzotrifluoride partially oriented in a nematic phase

The NMR spectrum of benzotrifluoride dissolved in the nematic phase of n-(p-ethoxybenzylidene)-p'-n-butylaniline has been analysed, and the structure of the proton and fluorine skeleton has been determined. The height of the hindering potential to trifluoromethyl rotation cannot be measured. Due to the low accuracy of the determined F? F distance, no precise information about the anisotropy of the indirect coupling can be given.     

Nuclear magnetic resonance study of speciation of water at low concentrations in hydrogen bond acceptor solvents

Equilibrium constants for the formation of 1:1 and 1:2 complexes of monomeric water with twelve hydrogen bond acceptor solvents were evaluated from the chemical shifts of the water protons in ternary mixtures containing a low concentration of water and varying proportions of the acceptor solvent and carbon tetrachloride. Formation constants of the 1:1 complexes (with the exception of those of dioxane and dimethoxyethane) correlate well with Kamlet-Taft -parameters. A good correlation is also obtained with Gutmann donor numbers if nitromethane and pyridine are excluded. Stepwise formation constants of the 1:2 complexes vary relatively little; this is attributed to a leveling effect of first hydrogen bond formation.     

The structure and magnetic anisotropies of hexafluorobenzene partially oriented in liquid crystalline solutions

¹⁹F NMR spectra and ¹³C satellites of hexafluorobenzene partially oriented in nematic liquid crystals have been recorded and analysed. Spin-spin couplings, corrected for vibrations, are used to investigate the anisotropic contribution of the indirect ¹⁹F¹⁹F and ¹³C¹⁹F couplings. The chemical shielding anisotropy is investigated using nematic and smectic solutions and compared with results from solid-state NMR.     

Nuclear magnetic resonance study of NbBex compounds

Nuclear magnetic resonance techniques have been employed to study powder samples of the compounds NbBe₂, NbBe₃, Nb₂Be₁₇ and NbBe₁₂. Nuclear quadrupole coupling constants and Knight shifts were determined for the Nb sites in these compounds. The ${}^{93}\text{Nb NMR}$ from NbBe₂ is a single unsplit line with zero quadrupole coupling constant and a Knight shift of 0.70 ± 0.03%. The ⁹³Nb NMR from a NbBe₃ sample shows the presence of three resonance lines having values of $\text{e}{}^2\text{qQ}\text{h}\text{= 0, 24.2 ± 0.2, 30.1 ± 0.3}\text{MHz}$ and corresponding Knight shifts of 0.69 ± 0.03, 0.72 ± 0.03, 0.53 ± 0.03%. One of these lines was identified as a NbBe₂ impurity. The ${}^{93}\text{Nb NMR}$ of Nb₂Be₁₇ exhibits a quadrupole coupling, $\text{e}{}^2\text{qQ}\text{h}\text{= 5.93 ± 0.15}\text{MHz}$, and a Knight shift, K_axiat = 0.014% and K_isotropic = 0.34 ± 0.02%. The ${}^{93}\text{Nb}$ resonance from NbBe₁₂ was split by a quadrupole interaction of $\text{e}{}^2\text{qQ}\text{h}\text{= 21.7 ± 0.3}\text{MHz}$ and a Knight shift of 0.55% ± 0.02%. The beryllium NMR was examined, yielding little information, since no measurable quadrupole interaction or Knight shift were present.     

Nuclear magnetic resonance of oriented molecules in lyotropic liquid crystalline media: p-cresol

Proton-proton distance ratios and order parameters for partially oriented p-cresol in a lyotropic mesophase derived from potassium laurate are obtained. The order parameters are the largest yet found for an ordered solute in a nematic-like lyotropic system. The dipolar couplings and structural data were independent of the rotational potential from 0–200 cal mol^?1.