Two-dimensional Powder Pattern Research Articles

Xenon NMR of phase biaxiality in liquid crystals

Biaxial thermotropic nematic liquid crystals would be of great importance in liquid crystal display technology. Less than a decade ago, such liquid crystals were suggested. The biaxiality of the phases was confirmed using (2)H NMR spectroscopy of deuterated probe molecules. The spectra were collected from a sample rotating around an axis perpendicular to the external magnetic field, resulting in a two-dimensional powder pattern. We have proposed an alternate technique that is based on the second order quadrupole shift detectable in (131)Xe NMR spectra of dissolved xenon. The method has many advantages, such as the NMR spectra are taken from a static sample and the (131)Xe quadrupole coupling tensor is extremely sensitive to the symmetry of the phase. In the present study, we report results obtained on a 600-MHz NMR spectrometer. Together with the data of our earlier study, they confirm that the asymmetry parameter of the (131)Xe quadrupole coupling tensor in the nematic phase of a ferroelectric liquid crystal is 0.85 and in the smectic A phase ca 0.62, indicating significant phase biaxiality.

Shiftless nuclear magnetic resonance spectroscopy

The acquisition and analysis of high resolution one- and two-dimensional solid-state nuclear magnetic resonance (NMR) spectra without chemical shift frequencies are described. Many variations of shiftless NMR spectroscopy are feasible. A two-dimensional experiment that correlates the dipole-dipole and dipole-dipole couplings in the model peptide , (15)N labeled N-acetyl-leucine is demonstrated. In addition to the resolution of resonances from individual sites in a single crystal sample, the bond lengths and angles are characterized by the two-dimensional powder pattern obtained from a polycrystalline sample.

Possible spin triplet superconductivity inNaxCoO2·yH2O —59Co NMR studies

We report 59Co NMR studies on magnetically oriented powder samples of Co oxide superconductors,NaxCoO2·yH2O,with Tc∼4.7 K. From the two-dimensional powder pattern in the NMR spectrum, theab-plane Knight shift in the normal state was estimated from the magnetic field dependence ofthe second-order quadrupole shifts at various temperatures. Below 50 K, the Knight shift showsa Curie–Weiss-like temperature dependence, similarly to the bulk magnetic susceptibilityχ. From the analysisof the so-called K–χ plot, the spin and the orbital components ofK and the positive hyperfine coupling constant were estimated. The onset temperature of thesuperconducting transition in the Knight shift does not change much in an appliedmagnetic field up to 7 T, which is consistent with the reported high upper critical fieldHc2. The Knight shift at 7 T shows an invariant behaviour belowTc. No coherencepeak just below Tc was observed in the temperature dependence of the nuclear spin–lattice relaxation rate1/T1 in either case (NMR, nuclear quadrupole resonance). Weconclude that the invariant behaviour of the Knight shift belowTc and unconventionalbehaviours of 1/T1 may indicate spin triplet superconductivity with p- or f-wave symmetry.

Analysis of correlation patterns in hyperfine sublevel correlation spectroscopy ofS=1/2,I=3/2 systems

The properties of two-dimensional four-pulse electron spin echo envelope modulation spectra of nuclei with nuclear spinI=3/2 in disordered systems are discussed by means of calculated lithium hyperfine sublevel correlation spectra. Since a case of small coupling is treated, the multifrequency transition components appear only in the first quadrant of the two-dimensional spectra. The influence of the hyperfine and nuclear quadrupole interaction parameters on the cross peak ridges in the two-dimensional powder pattern is analyzed. Furthermore the possibility to determine the magnitude of the dipolar hyperfine and the nuclear quadrupole coupling from the cross peak line shapes is investigated. In this context the potential of those spectral features representing the so-called multiquantum transitions for the interpretation of the spectra is demonstrated.

Magnitudes and Orientations of the 15N Chemical Shift Tensor of [1-15N]-2′-Deoxyguanosine Determined on a Polycrystalline Sample by Two-Dimensional Solid-State NMR Spectroscopy

The magnitudes and orientations of the 15N chemical shift tensor of [1-15N]-2′-deoxyguanosine were determined from a polycrystalline sample using the two-dimensional PISEMA experiment. The magnitudes of the principal values of the 15N chemical shift tensor of the N1 nitrogen of [1-15N]-2′-deoxyguanosine were found to be ς11 = 54 ppm, ς22 = 148 ppm, and ς33 = 201 ppm with respect to (15NH4)2SO4 in aqueous solution. Comparisons of experimental and simulated two-dimensional powder pattern spectra show that ς33N is approximately collinear with the N–H bond. The tensor orientation of ς33N for N1 of [1-15N]-2′-deoxyguanosine is similar to the values obtained for the side chain residues of 15Nϵ1-tryptophan and 15Nπ-histidine even though the magnitudes differ significantly.

Relayed anisotropy correlation NMR: determination of dihedral angles in solids

A two-dimensional solid-state NMR method is proposed for determining the mutual orientation of the two interaction tensors in each of which a different chemical group participates, from which information on the dihedral angle can be extracted. The two-dimensional powder pattern was observed for 1,2- 13C-labeled DL-alanine as a demonstration, correlating the 13C 1 chemical shift anisotropy and the 13C 2 1H dipolar coupling via polarization transfer. The OC 1C 2H dihedral angle as well as the orientation of the 13C 1 chemical shift tensor were determined from the spectrum.

Observation and Modelling of Preferred Orientation in Two-Dimensional Powder Patterns

A novel approach to the detection and modelling of preferred orientation is presented, based on the interpretation of two-dimensional powder patterns. A simple graphical construction is introduced to aid interpretation, and the application of this construction to some standard diffraction geometries is discussed. It is also shown in outline how a standard preferred-orientation model can be adapted to describe two-dimensional data.

Conduction ESR of vapor grown carbon fibers

Conduction electron spin resonance (CESR) spectra have been obtained for vapor-grown carbon fibers. Large diameter fibers annealed at temperatures between 2300 and 3025 K exhibit broad “two-dimensional” powder patterns when the fiber axis is perpendicular to the magnetic field. The CESR of fibers heat treated at 3175 K and having diameters less than 40 μm show a collapse of the two-dimensional powder pattern into a relatively sharp line at the isotropic value. This narrowing results from the diffusion of the electron in the basal planes. The r.m.s. migration of the electron on the ESR timescale must be on the order of the fiber diameter for motional narrowing to occur. Electron migration is limited by electron-phonon scattering at room temperature. Products of the electron mean free paths and Fermi velocities are determined from the narrowing of the ESR signals. They are found to increase with increasing annealing temperatures. For all fibers this product is less than that estimated for pure graphite.

Motional narrowing of the conduction ESR of vapour-grown carbon fibres

Conduction electron spin resonance (CESR) spectra have been measured for pristine and boron-doped vapour-grown carbon fibres. Pristine fibres annealed at temperatures between 2300 and 3025 K exhibit broad 'two-dimensional' powder patterns when the fibres axis is perpendicular to the magnetic field. The full g-anisotropy of single crystal graphite is not established until the fibres are annealed above 2800 K. The CESR spectra of fibres heat treated above 2750 K and having diameters of less than 20 mu m show a collapse of the two-dimensional powder pattern into a relatively sharp ESR line at the isotropic value of the pattern, (gc+ga)/2. This narrowing results from the large-scale motion of the electrons in the basal planes during the experiment. Due to the annular morphology of the vapour-grown fibres, the RMS migration of the electrons during the ESR experiment must be of the order of the fibre diameter for motional narrowing to occur. Electron migration is limited by electron-phonon scattering at room temperature. Products of the electron mean free paths and Fermi velocities determined from the narrowing of the ESR signals increase with increasing annealing temperatures. For fibres annealed at all temperatures this product is still slightly less than that estimated for pure graphite. Fibres doped with boron at 2975 K display a reduced anisotropy due to a shift in gc. Boron-doped fibres exhibit motional narrowing behaviour similar to pristine fibres.

Determination of 13C chemical shift tensor orientations in cis- and trans-polyacetylene by measurements of two-dimensional NMR powder patterns in rotating solids

The orientations of the 13C chemical shift tensors in undoped cis- and trans-polyacetylene have been determined from the chemical shift-dipolar two-dimensional powder patterns observed in rotating samples.

Elucidation of the two-dimensional structure of an α-amino acid surfactant monolayer on water using synchrotron X-ray diffraction

Recently it has been demonstrated that a compressed monolayer of palmitoyl-(R)-lysine (Fig. 1) on the surface of water induces the orientated growth of α-glyeine crystals1. This effect was inter-preted to be a structural match between the monolayer and the α-glycine. To test this hypothesis we undertook to determine the packing arrangement of the monolayer by grazing incidence X-ray diffraction and reflection. These techniques2 use the unique proper-ties of synchrotron radiation: high intensity within a small natural collimation3. In the diffraction experiment two peaks were detect-able in the two-dimensional powder pattern from a monolayer of palmitoyl-(R)-lysine. The positions and intensities of these peaks allowed us to choose between various models and determine the monolayer structure. This is the first time that the crystal structure of a compressed surfactant monolayer at the air–water interface has been determined. The same techniques could be used for structural characterization of other monolayers of interest in fields as diverse as biological membranes4 and optical second harmonic generation5. The packing arrangement of the α-amino acid head groups in the model proved to be very similar to that found in the crystal structures of α-glycine and several hydrophobic α-amino acids.

51V and133Cs NMR Study of a Quasi-One-Dimensional Triangular Lattice Antiferromagnet CsVCl3

The 51 V and 133 Cs NMR study has been performed in order to obtain the information on the spin state of a single crystal of CsVCl 3 . In the ordered state the 51 V NMR signal was observed around 106.5 MHz at 4.2 K and at zero external field. From this the spin reduction associated with the V 2+ ions was estimated to be 71%. The observed external field dependence of the 51 V spectrum could not be explained from the simple 120° Neel model proposed by the neutron diffraction experiment. In the paramagnetic state, the quadrupole split resonance lines of the 133 Cs signals were observed, which were broadened into a spectrum characteristic of the two-dimensional powder pattern in the ordered state. The spectrum could be reproduced by a modified 120° Neel structure.