Anisotropy Of Diamagnetic Susceptibility Research Articles

Determination of the bend elastic constant and the anisotropy of diamagnetic susceptibility of lyotropic nematic calamitic liquid crystals

The bend elastic constant ${K}_{3}$ and the anisotropy of diamagnetic susceptibility ${\ensuremath{\chi}}_{a}$ of two lyotropic mixtures: potassium laurate, decanol, and water and sodium decylsulfate, decanol, and water in the nematic calamitic phase are determined. The experimental method by the action of magnetic fields in samples with and without ferrofluid. These distortions are observed in a polarizing microscope. The results of ${K}_{3}$ and ${\ensuremath{\chi}}_{a}$ are compared with the measurements of these parameters for a different lyotropic mixture.

Magnetic susceptibility tensors and deuterium quadrupole coupling constants of the methyl halides

High-resolution 95.2 MHz deuteron NMR spectra were obtained from dilute ether solutions of the monodeuteromethyl halides (X = Cl, Br, I). Small quadrupole splittings were resolved, resulting from molecular alignment by the 14.57 T field. These splittings relate the deuterium quadrupole coupling constants (qcc) to the anisotropy in the molecular diamagnetic susceptibility (Δξ). The results are consistent with literature qcc and Δξ values for CH 3CI and CH 3 Br, but suggest that Δξ of CH 3I determined by microwave spectroscopy is high by about 14%. A survey of literature qcc data for the complete series of halomethanes is given. The data indicate a good agreement between qcc's found from gas-phase studies (microwave), low-temperature solid-state NMR and NQR experiments, and investigations using certain well-chosen liquid crystal mixtures. The values judged to be most accurate were combined with quadrupole splitting data to obtain molecular and bond susceptibilities for this series of compounds.

Some N.M.R. experiments using lyotropic nematic liquid crystals

Abstract A series of lyotropic nematic liquid crystals based on aniso-dimensional micelles can be subdivided according to micelle shape and diamagnetic susceptibility anisotropy. The proton magnetic resonance spectra of benzene dissolved in dilute solution in several of these phases have been analysed. The signs of the partially averaged dipole–dipole coupling give information about the relative alignment of the six-fold symmetry axis of benzene, the director and the magnetic field. The system caesium perfluorooctanoate/water is unusual in two respects. The system forms a nematic phase with only a binary mixture and the disk micelles align in a perpendicular plane to the magnetic field. As a result the binary system has been studied previously in some detail (see N. Boden, S. A. Come and K. W. Jolley, 1987, J. phys. Chem. 91, 4092). New measurements on the interesting phase diagram have been accomplished using caesium-133 and deuterium magnetic resonance measurements at various temperatures. Some dilatometric measurements have been made to complement the N.M.R. studies.

Inertial effects in rotating lyotropic liquid crystals

Abstract The orientation of the director of a lyotropic liquid crystal with negative anisotropy of the diamagnetic susceptibility is studied when the sample is spun at various speeds and angles from the magnetic field. Different behaviours is observed at low and high spinning speeds where the orientation is governed by the magnetic and the inertial torques, respectively. At intermediate frequencies it is possible to reach an equilibrium between these two torques. The evolution of this equilibrium with the average magnetic torque allows one to form a conclusion on the inertial nature of this effect.

Determination of the carbon-13 shielding tensors for methyl halides and haloforms by N.M.R. in mixtures of liquid crystals

The carbon-13 isotropic chemical shifts and anisotropies of the shielding tensors of methyl halides (CH3F, CD3Br, CH3I) and haloforms (CHF3, CDCl3, CDBr3, CHI3) have been determined from carbon N.M.R. spectra in mixtures of liquid crystals with opposite anisotropy of diamagnetic susceptibility. By using these results the components of the chemical shielding tensors were determined. The tensor element σ zz (z axis parallel to the C 3 symmetry axis) was found to be approximately constant (ca. 200 ppm) for methyl halides. The element σ xx (= σ yy ) takes values between 84 ppm (for CH3F) and 226 ppm (for CH3I). On the contrary, the change of σ zz for haloforms was about three times larger than the change of σ xx . The experimental shielding tensors of methyl halides have been compared with the theoretical ones calculated by the Coupled Hartree-Fock method. There is a good agreement between the experimental and theoretical values for σ zz , but not for σ xx . The difference of these values increases when the electronegativity of the substituent decreases, i.e. from F to I.

“Near magic angle” sample spinning in NMR spectra of molecules oriented in individual and mixed liquid crystals of opposite diamagnetic anisotropies

The dynamics of the liquid crystal director in “near magic angle” sample spinning has been investigated in mesophases with positive and negative diamagnetic susceptibility anisotropies and their mixtures with near-zero macroscopic diamagnetic anisotropy. To this end the deuteron quadrupolar splitting of deuterochloroform dissolved in liquid crystals has been monitored. In systems with weakly positive diamagnetic susceptibility anisotropy (Δ χ), the director has been observed to switch from an orientation parallel to the spinning axis at low rotational speeds to one perpendicular to the spinning axis at high rotational speeds when the angle (θ) the axis of rotation makes with the magnetic field is smaller than the magic angle (θ m). For systems with a small negative Δχ similar director behaviour has been observed for θ greater than θ m.

Magnetic susceptibility investigations on EBBA

Abstract The temperature variation of the magnetic susceptibility (χg) of the liquid crystal N-(p-n-ethoxybenzylidene)p-n-butylaniline (EBBA) in the isotropic liquid and nematic phases is reported. Abrupt changes in Xg and S. ΔXg (S: order parameter, ΔXg : diamagnetic susceptibility anisotropy of the perfectly aligned phase) are observed at the isotropic-nematic phase transition. The order parameter is calculated using the estimated value of the molar susceptibility anisotropy.

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

The 1H 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°.

Magnetic birefringence studies of dilute purple membrane suspensions

We have observed the magnetically induced orientation of purple membrane suspensions by measuring the birefringence as a function of concentration and temperature at fields up to 10.5 Tesla (T). At these fields, the orientation approaches saturation even in dilute solutions; therefore, the birefringence data, together with an estimate of the membrane size distribution obtained from electron microscopy, permits one to determine the diamagnetic susceptibility anisotropy. We find delta chi mole = 1.2 +/- 0.3 X 10(-3) erg G-2mol-1 of bacteriorhodopsin. If delta chi were due only to the oriented peptide bonds of the transmembrane alpha helices, this experimental value would indicate that delta K, the anisotropy per mole of peptide bonds, is considerably larger than previously suggested. On the other hand, the large value for delta chi mole of bacteriorhodopsin can also be explained by a net orientation of the aromatic amino acid side chains of bacteriorhodopsin with their planes perpendicular to the membrane surface. In addition, the present data analysis demonstrates the critical dependence of the calculated delta chi value on the values for the membrane size distribution.

Cotton-mouton constant and anisotropy of magnetic susceptibility of haloalkanes

1. The principal semiaxes of the ellipsoid of diamagnetic susceptibility have been determined for the C-Cl, C-Br, and C-I bonds in haloalkanes. 2. The anisotropy of diamagnetic susceptibility of C-Hal bonds is negative, and it changes regularly in homologous series of compounds. 3. An empirical relationship has been established between the anisotropy of diamagnetic susceptibility and the electronic polarizability of C-Hal bonds in haloalkanes and halobenzenes.

Analysis of magnetic-field-induced distribution using ESR parallel-edge lines of a slow tumbling spin probe in a rigid liquid crystal

The ESR spectra of a slowly tumbling nitroxide radical in rigid p,p-methoxy benzylidene n-butylaniline (MBBA) exhibit three step-shoulders at resonant magnetic fields of the molecules with the symmetric axes oriented nearly parallel to the applied static magnetic field. The linewidth differences in two parallel-edge fines for hyperfine components in the second derivative spectra were observed and analyzed as the sum of a term dependent on rotational correlation time and a term which contains the effects of magnetic field B q quenching the samples at 77 K and the degree of orientation distribution. The first term has been predicted by Kivelson and Lee to vary as τ c −1 2 at a limit of τ c → ∞, where τ c is the rotational correlation time. The second term was determined to be inversely proportional to 1 θ 0 2 + ΔχB q 2/2kT eq ) 2 where θ 0/√2 means the standard deviation from the mean of the Gaussian distribution, Δχ (= X | − χ ⊥) is the anisotropy in diamagnetic susceptibility, and T eq is the temperature of liquid crystal before quenching.

Additives atomares punktdipol-(APUDI)-modell zur berechnung der 1h-NMR-werte von benzoiden kohlenwasserstoffen

Our empirical atomic point dipole (APUDI) model simply allows the calculation of ring-current produced chemical shifts of the ring protons of 17 planar benzenoid hydrocarbons with high accuracy. It simulates the effect of the anisotropy of diamagnetic susceptibility of the π-systems by assumption of two effective atomic magnetic point dipoles, located perpendicular above and below each carbon atom of the π-system at 70 pm, the distance of the maximum of π-electron density. Application of the McConnell equation on each of these point dipoles leads after summation for each ring proton j to a corresponding geometry factor GF j which was correlated by linear least squares method with 160 literature values of experimental chemical shifts leading to a value of (- 12.5± 0.7) × 10 -36m 3, the effective anisotropy of diamagnetic susceptibility per atomic point dipole with a correlation coefficient of 0.943. The chemical shift values may be obtained in the APUDI-model by the least squares equation δ calc j p me>= - 12.5 × 10 -36 GF j - 5.16. This empirically determined slope may be interpreted as the average value of the z-component of the experimental diamagnetic susceptibility per atomic point dipole of these benzenoid aromatic compounds. Besides the simple applicability the APUDI-model yields superior results for the chemical shifts of sterically crowded protons in comparison to many known ring current models.

Field-Induced Order in Nematic Liquid Crystals

Abstract The effect of an external magnetic field on the orientational order of a nematic liquid crystal has been examined using both Landau-de Gennes and Maier-Saupe theories. In the Maier-Saupe approach a rotationally invariant form of the pseudo-potential is introduced, which in the absence of an external field leads to three degenerate isomorphic solutions for the order parameter, corresponding to alignment along three principal axes; a similar result is obtainable from the Landau-de Gennes theory. Application of a magnetic field lifts the degeneracy of these solutions, and for materials having a positive diamagnetic susceptibility anisotropy, the uniaxial solution with alignment along the field direction is always energetically favorable. For materials with a negative susceptibility anisotropy, a biaxial solution minimizes the free energy at low temperatures, but on increasing the temperature there is a transition from a biaxial phase to an uniaxial phase. The field dependence of the transition tempe...

Anisotropy and asymmetry of diamagnetic susceptibility, quadrupole coupling constants and molecular motion of fluorene and the fluorenyl carbanion

Fluorene and the fluorenyl carbanion, both perdeuterated, have been studied by 2H N.M.R. at 11·7 Tesla. From deuteron splittings and spin-spin relaxation times, information is obtained about the anisotropy and asymmetry of the diamagnetic susceptibility, the quadrupole coupling constants of the different deuterium nuclei and the rotational diffusion constants of the two species.

Solvent dependence of deuteron quadrupole coupling constants and of methyl angles of some CD 3X compounds determined by NMR of partially oriented molecules

The deuteron quadrupole coupling constants, Q D, and methyl angles of the solute molecules CD 3CN, CD 3Br, CD 3I, and CD 3NO 2 were determined in various liquid-crystal solvents by use of NMR spectroscopy of oriented molecules. Carbon-13 satellites at natural abundance in the 2H or 1H spectra were used for the measurement of the degree of order of the CD bonds. The results reveal a strong solvent dependence of Q D (maximum variation of 26% in CD 3NO 2) as well as of the DCD angle (maximum variation of 2.3 degrees in CD 3I). These large apparent effects are attributed to much smaller real variations occurring in a two-site interaction. A general correlation between increasing anisotropy of the solvent diamagnetic susceptibility, decreasing Q D, and increasing DCD angle is observed.

Diamagnetic anisotropy as a probe of electron delocalisation in polymers: application to polydiacetylenes, polyethylene, and polyethylene terepthalate

A quartz fiber torsion balance has been used to measure the highly anisotropic diamagnetic susceptibility of a number of polymer materials, including several polydiacetylenes, polyethylene and polyethylene terepthalate. A large diamagnetic anisotropy was observed to be associated with the conjugated backbone of the polydiacetylenes. Using a semiclassical model, the anisotropy is related to an effective electronic delocalisation along the polymer backbone. The polydiacetylene, TCDU, exhibited a large change in its anisotropy upon radiation-induced polymerisation. The anisotropy of a polyethylene terepthalate film was used to characterise the chain disorder in the sample. These results suggest that diamagnetic anisotropy measurements provide a simple and useful means of probing molecular structure in disordered materials which are difficult to characterise by X-ray diffraction. Specific applications are suggested toward the study of thin film conducting polymers and the polymerisation process in plastic films.

A simple method for determining anisotropies of diamagnetic susceptibilities of liquid crystals: use of 1H NMR spectroscopy

In mixtures of oriented liquid crystals with opposite signs of diamagnetic susceptibility anisotropies, the point of macroscopic zero anisotropy can be detected by 1H NMR from the degree of order of a dissolved molecule. On the basis of additivity and of the liquid-crystal molar concentrations relative anisotropies can be determined.

Magneto-Orientation of Phospholipids

Abstract Magneto-orientation effects were observed for platelets of thin layered single crystals of L- and DL-dipalmitoylphosphatidylcholine (DPPC), and for single crystals of polyethylene (PE) suspended in xylene. For both L- and DL-DPPC crystals, a biaxial magneto-orientation was observed, in which both directions of hydrocarbon chain and phosphorylcholine group were directed perpendicular to the applied field direction. For PE, the hydrocarbon chain direction was oriented perpendicular to the field. From the orientational behaviour under the magnetic field, the anisotropy of diamagnetic susceptibility, the difference between susceptibilities parallel and perpendicular to the hydrocarbon chain direction, is evaluated as − 9 × 10−8 and − 1 × 10−8 emu/cm3 for L-DPPC and PE, respectively. The origin of the diamagnetic anisotropy of DPPC is briefly discussed in terms of the molecular and crystal structures.

Proton relaxation measurements on lipid membranes oriented at the magic angle

Smectic lipid-water systems were macroscopically oriented between glass slides. Lipids used were dimyristoyllecithin, potassium oleate, and potassium linolate. Proton spectra of hitherto unattained resolution were observed when these lipid model membranes were oriented at the magic angle. The T 1 value of resolved resonances was measured as a function of temperature at 100 and 360 MHz. The residual linewidth at the magic angle orientation is determined by the inherent anisotropy of diamagnetic susceptibility in lipid bilayers. The T 2 value of the methylene resonance in the potassium oleate-water system was evaluated from a saturation study. The conclusion from these experiments is that the model membrane is a two-dimensional fluid; the molecular motion is a plane perpendicular to the bilayer normal is characterized by a single correlation time of the order of 10 −10 sec. Dimyristoyllecithin was also investigated in vesicle form; T, values and activation energies differ from the multilayer results but most of the discrepancy can be ascribed to the different water contents of the two systems.

Magneto-orientation of lecithin crystals.

For the purpose of clarifying the magneto-orientation mechanism of several biological membrane systems, xylene-suspended thin-layered single crystals of lecithin, which is a major component of membranes, were subjected to magnetic fields of various strengths, and the orientation behavior of the crystals was analyzed. Lecithin crystals oriented in such a way that the directions of both the hydrocarbon chains and the phosphorylcholine group of the lecithin molecule were perpendicular to the magnetic field, suggesting that the crystals were diamagnetically biaxial. In contrast, hydrocarbon chains of phospholipids in many biological membranes are known to orient parallel to magnetic fields. The value of the volume diamagnetic susceptibility anisotropy, delta chi (i.e. the difference between susceptibilities parallel and perpendicular to the direction of the hydrocarbon chain in the lecithin crystals), estimated from the magneto-orientation behavior was about -9 X 10(-8) cgs. This is distinctly larger than that of vesicles of egg yolk lecithin measured by Boroske and Helfrich [Boroske, E. & Helfrich, W. (1978) Biophys. J. 24, 863-868] and also larger than that of single crystals of polyethylene, which was measured for comparison by the same technique as that used for lecithin in this study. The differences between these values of delta chi seem to result from the contribution of the polar head (i.e., the part of the lecithin molecule other than the two hydrocarbon chains), which is ordered, less ordered, or deleted in the respective cases above, although mainly from the contribution of the hydrocarbon chains, which are ordered in crystals and disordered in vesicles.