Magnetic-field-induced quadrupole splitting in gaseous and liquid 131Xe NMR: quadratic and quartic field dependence

A theory for the magnetic-field-dependent quadrupole splitting in the 131Xe NMR spectra in isotropic media is presented and tested by ab initio electronic structure calculations. Evidence exists only for even-power magnetic field dependence. The dominant mechanism is verified to be the electric field gradient caused by the diamagnetic distortion of the atomic electron cloud, quadratic in the magnetic field. The computed results are in excellent agreement with the recent experiment by Meersmann and Haake.     

Weak quadrupole interactions in orientationally modulated NMR spectra of rotating crystals

We discuss orientational modulation (OM) of the weak nuclear quadrupole interaction energy by means of adiabatic rotation of the crystal about an axis fixed in space with a constant external static magnetic field. Subsequent standard synchronous detection of the NMR signals at twice the rotational frequency produces unusual orientationally modulated NMR spectra (unlike the ordinary spectra) reflecting only the characteristic features of the anisotropic nuclear quadrupole interaction. We substantiate, present, and analyze the basic relations describing the structure of the observed OM NMR signals, with arbitrary symmetry (and in a special case, axial symmetry) of the EFG tensor. An important feature of the orientational modulation method under discussion is the possibility (with appropriate choice of the phase of the reference signal) of complete suppression of all the fine structure components belonging to a certain system of structurally or magnetically equivalent nuclei in the complex superposition NMR spectra. The basic theoretical results are confirmed experimentally.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 24–29.     

Electro-optical phase shift in polymer dispersed liquid crystals

An anisotropic version of the Maxwell Garnett approximation is applied for studying the electro-optical phase modulation by polymer dispersed liquid crystals (PDLC). The PDLC contain bipolar liquid crystal droplets that can be reoriented by an external field causing a change in the optical birefringence. This approach provides an explicit link between the droplet orientation distribution and the electro-optical phase shift. For aligned droplets we find that the sharpness of the change in the birefringence may be controlled by selecting the initial orientation. For a planar distribution we find sharp transitions with a hysteresis loop whose width depends on the droplet concentration. For a random distribution, the droplet orientation and the optical phase shift change more gradually with the applied field. These results demonstrate that PDLC may be suitable for a wide range of electro-optic applications based on their field-induced phase modulation properties. In addition, it is apparent that the optical phase shift is quite sensitive to changes in droplet orientation. It should therefore be useful for studying reorientation phenomena in PDLC, overcoming the problems due to light scattering in these materials. Received 25 November 1999 and Received in final form 20 January 2000     

Search for optical biaxiality in discotic liquid crystals

Using a high-finesses Fabry-Perot structure, we examine the behaviour of discotic liquid crystals for possible biaxiality. This is done by confining the discotic liquid crystals in the Fabry-Perot cavity and examining the mode structure in presence and absence of an applied electric field. It is concluded that at least in the discotic liquid crystal that we have examined, there is no evidence of biaxiality.     

13C NMR spectral assignments for nematic liquid crystals by 2D chemical shift gamma-encoding NMR

A novel method to accomplish 13C NMR spectral assignments for nematic liquid crystals is proposed. The two-dimensional (2D) isotropic/anisotropic chemical shift correlation spectrum is observed in which the anisotropic shift parameters are represented as sharp lines by gamma-encoding. The 13C spectral assignments can be made from the 2D spectrum with the aid of the 13C isotropic shift assignments for the same compound in the isotropic liquid state. The experiments were performed on p-methoxybenzilidene-p-n-butylaniline.     

Toward automated analysis of dipole-coupled NMR spectra of solutes in liquid crystals

Dipolar-coupled spectra of molecules dissolved in liquid crystalline solvents are analyzed by applying pattern recognition procedures to two-dimensional z-filtered correlation spectra (z-COSY). Transitions that share common common energy levels can be recognized as such by recording spectra with different flip angles. This allows one to obtain a picture of the energy-level diagram without knowledge of the Hamiltonian, although in the current preliminary version of the program, the number of spins and their symmetry must be known prior to analysis. By adapting the programs NMREN and NMRIT, described bySwalen and Reilly (J Chem. Phys. 37, 21 (1962)) and Ferguson and Marquardt (J. Chem. Phys. 41, 2087 (1964)), the energies of the eigenstates derived from the spectra are assigned to the diagonal elements of the Hamiltonian in the product base. These assignments can be made without guesswork, except for states with the same quantum numbers, for which it is necessary to permute assignments by trial and error. The algorithm is iterated for different tentative assignments of the eigenstates to yield the shifts and couplings. The best Hamiltonian (best assignment) is chosen on the grounds of a comparison between simulated and experimental spectra. Examples of entirely automated analysis are shown for A₃B and A₂BC systems.     

Iterative deconvolution of quadrupole split NMR spectra

We propose a simple method to deconvolute NMR spectra of quadrupolar nuclei in order to separate the distribution of local magnetic hyperfine field from the quadrupole splitting. It is based on an iterative procedure which allows to express the intensity of a single NMR line directly as a linear combination of the intensities of the total experimental spectrum at a few related frequencies. This procedure is argued to be an interesting complement to Fourier transformation since it can lead to a significant noise reduction in some frequency ranges. This is demonstrated in the case of the ¹¹B-NMR spectrum in SrCu₂(BO₃)₂ at a field of 31.7 T, where a magnetization plateau at 1/6 of the saturation has been observed.     

129Xe and131Xe NMR of xenon on silica surfaces at 77 K

Little is known about¹²⁹Xe NMR spectral features and spin-lattice relaxation behavior, and the dynamics of xenon atoms, for xenon adsorbed on solid surfaces at cryogenic temperatures (≤77 K), where exchange with gas-phase atoms is not a significant complication. We report¹²⁹Xe NMR experiments at 9,4 T that provide such information for xenon adsorbed onto the hydroxylated surface of a number of microporous silica samples at 77 K. A convenient design for these cryogenic experiments is described. Dynamics of surface-adsorbed xenon atoms on the time scale of seconds can be observed by¹²⁹Xe NMR hole-burning experiments; much slower dynamics occurring over hours and days are evidenced from changes with time of the¹²⁹Xe NMR chemical shifts. The peak maxima occur in the region ca. 180–316 ppm, considerably downfield of¹²⁹Xe shifts previously reported on surfaces at higher temperatures, and closer to the shift of xenon bulk solid (316.4±1 ppm). The¹²⁹Xe spin-lattice relaxation timesT ₁ range over five orders of magnitude; possible explanations for both nonexponential relaxation behavior and extremely shortT ₁ values (35 ms) are discussed. Preliminary¹³¹Xe and¹H NMR results are presented, as well as a method for greatly increasing the sensitivity of¹²⁹Xe NMR detection at low temperatures by using closely-spaced trains of rf pulses.     

NMR in the quadrupole regime: Measurement of quadrupole and chemical shift parameters in single-crystal paradibromobenzene

The exact theory for the frequency of transition between the two lowest levels of a spin $\text{I =}\text{3}\text{2}$ nucleus experiencing a large asymmetric electric field gradient, an applied magnetic field, and an anistropic chemical interaction was presented in an earlier paper. Using the assumption that the quadrupolar and chemical shift tensors have the same principal axis system, the Hamiltonian was solved exactly — analytically for the applied field aligned along each of the three axes of the quadrupolar principal axis system, and numerically for arbitrary orientations.This theory is reviewed here and applied to our room-temperature experiments in single-crystal paradibromobenzene. The self-consistent least-squares fit to the field-dependencies and simultaneously the angular dependence (rotational pattern) of the resonance frequency was performed using the literature value for the pure quadrupole frequency $\text{ν}{}_{\mathrm{Q}}\text{(1 +}\text{η}{}^2\text{3}\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 223·8}\text{MHz}$. The fit yielded values for the quadrupolar asymmetry η = 0·0461 ± 0·0004 and the chemical shift components σ_x = ?0·001 ± 0·001, σ_v = σ_z = 0·000 ± 0·001. Our value for η is in good agreement with values determined by other methods; it and our shift values are consistent with the information obtained by this method using a powdered specimen.The process of using the NMR signal itself to align the specimen yielded sufficient information for an unambiguous determination of the Euler angles of orientation of the crystal in its mounting within ± 0.6°.     

NMR spin echoes and quadrupole effects in nickel single crystals

NMR spin echo measurements of naturally abundant⁶¹Ni have been performed with Néel-type nickel single crystals at 4.2 K. The NMR excitation condition could properly be chosen in order to get signals either from nuclei situated within magnetic domains or within domain walls. In both cases a quadrupole splitting of the NMR line could be observed. By applying an external magnetic field, the direction of the domain magnetization could be varied with respect to the fcc crystal lattice. From the variation of the quadrupole splitting with the crystallographic direction, the tensor of the electric field gradient (EFG) has been derived. The quadrupole splittings and the corresponding field gradients in the principal axis system of the EFG are: Δv_Q kHz, Δv_Q kHz, Δv_Q kHz, V_<111>=(6.6±0.5)·10¹⁸ V/m², V_<110>=4.6±0.5)·10¹⁸ V/m², V_<112>=(2.0±0.5)·10¹⁸ V/m².     

Spectral assignments based on the 13C NMR spectra of mixed liquid crystals of opposite diamagnetic anisotropies

The proton-decoupled ¹³C NMR spectra of mixtures of liquid crystals with opposite diamagnetic anisotropies have been studied in the natural abundance of ¹³C. A new method to assign the spectral lines to specific carbons in the liquid crystalline phase has been developed. For this purpose, the assignments of lines in the isotropic media are required, and they were obtained from two-dimensional hetero-COSY experiments. From the spectra in the “critical” mixtures where both the orientations of the liquid crystal directors, with the alignments along and perpendicular to the direction of the magnetic field, “coexist,” the ¹³C chemical-shift anisotropies have been determined, assuming uniaxial symmetry.     

Orientational dependence of the tails of dipole-broadened NMR spectra in crystals

This paper describes experimental and theoretical studies of the tails of the dipole-broadened nuclear magnetic resonance (NMR) absorption spectra of ¹⁹F in isomorphic single crystals of BaF₂ and CaF₂ with the magnetic field directed along three crystallographic axes. The results obtained by directly measuring the derivative of the tail of the NMR absorption spectrum and the falloffs of the Engelsberg-Lowe free precession after Fourier transformation qualitatively agree. It is shown that the shape of the tail is well described by an exponential function in which the orientational dependence of the exponent does not reduce to variation of the second moment. The observed shape of the tail and the orientational dependence of its parameters are explained on the basis of a self-consistent fluctuating-local-field theory. Nonlinear integral equations are derived for the correlation functions, taking into account the changes of the actual number of nearest neighbors caused by the anisotropy of the dipole-dipole interaction and the contribution of lattice sums with loops. The equations are solved numerically. Good agreement is obtained for the computed dropoffs of the free precession, the NMR spectra, and the cross-polarization rates with the experimental results. Zh. éksp. Teor. Fiz. 115, 285–305 (January 1999)     

Reentrant phase transitions in liquid crystals

A comprehensive review of the recent developments regarding the phenomenon of reentrant phase transitions (RPT) in liquid crystals is presented. In addition to liquid crystals this phenomenon has been observed in amazingly diverse systems. A critical assessment of the experimental investigations concerning single and multiple reentrances is given. A brief account of the theoretical efforts is also given. The article ends with the identification of the factors which impede the proper understanding of the phenomenon.     

Diffusion studies in confined nematic liquid crystals by MAS PFG NMR

Pulsed field gradient (PFG) NMR and magic-angle spinning (MAS) NMR have been combined in order to measure the diffusion coefficients of liquid crystals in confined geometry. Combination of MAS NMR with PFG NMR has a higher spectroscopic resolution in comparison with conventional PFG NMR and improves the application of NMR diffusometry to liquid crystals. It is found that the confinement of the liquid crystal 5CB in porous glasses with mean pore diameters of 30 and 200 nm does not notably change its diffusion behavior in comparison with the bulk state.     

Detection of homonuclear decoupled in vivo proton NMR spectra using constant time chemical shift encoding: CT-PRESS

A new pulse sequence, termed CT-PRESS, is presented, which allows the detection of in vivo ¹H NMR spectra with effective homonuclear decoupling. A PRESS sequence with a short echo-time TE, used for spatial localization, is supplemented by an additional 180° pulse. The temporal position of this 180° pulse is shifted within a series of experiments, while the time interval between signal excitation and detection is kept constant. CT-PRESS is a two-dimensional (2D) spectroscopic experiment as far as data acquisition and processing are concerned, although only diagonal signals are generated in the 2D spectrum. However, since the principle of constant time chemical shift encoding is used in the t₁ domain, effective homonuclear decoupling is obtained by projecting the 2D spectrum onto the corresponding f₁ axis. Thus, good spectral resolution and high signal-to-noise ratio are obtained. The main advantage, as compared to localized 2D J-resolved MRS, is that optimized experiments can be performed for coupled resonances of interest by choosing the sequence parameters dependent on the type of multiplets, the J-coupling constants and T₂. Major fields of application will be parametric studies on coupled resonances, (e.g., T₁, diffusion behavior or magnetization transfer) and/or the detection of spatial and temporal changes of metabolites with coupled spin systes.     

Pseudocritical NMR frequency shift above the normal-incommensurate phase transition

The NMR satellite frequencies were measured as a function of temperature in the normal high-temperature phase for ⁸⁷Rb in Rb₂ZnBr₄ and Rb₂ZnCl₄ and for ³⁵Cl in betaine calciumchloride dihydrate. Approaching the respective normal-incommensurate phase transition an anomalous shift of the NMR frequency is observed for the first two cases. This effect is ascribed to the increasing order parameter fluctuations. The experimental data are compared to calculations which relate the observed behaviour of the NMR frequencies to the non-classical critical behaviour of the substances under investigation. Received 6 August 1998     

Birefringence and phase transitions in liquid crystals

The birefringence of liquid-crystalline phases is the result of the parallel order of molecules exhibiting a polarizability anisotropy. The magnitude and sign of the birefringence are determined by the structure and order of the liquid-crystalline phase types as well as by the polarizability properties of the constituent molecules. The characteristic change of the birefringence at phase transitions between liquid-crystalline phases indicates more or less pronounced structural changes. The temperature dependence of the birefringence is due to the temperature change of the molecular order.

It is shown that the structural variety of the liquid crystalline state is reflected by a big variety of their optical anisotropy properties.