Irreducible tensor operator methods in intermediate-field coupling

The algebra of irreducible tensor operators is developed in the intermediate-field coupling case. The Wigner-Eckart theorem is formulated for a simple irreducible tensor operator as well as for the Kronecker and scalar products of these operators. The expressions required for the calculation of Coulomb repulsion, crystal field splitting, spin-orbit interaction, and Zeeman effect are given in detail. Recent applications to various problems in spectroscopy and magnetism of transition metal compounds are referred to.     

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.     

Two-dimensional analysis of the SAXS of partially oriented polymers*)

The Small-angle X-ray scattering (SAXS) patterns of oriented LDPE (λ=1...5.7) were fitted by calculating the small-angle intensity for a structure consisting of a linear paracrystalline lattice built up by finite lamellar or cylindrical crystallites. The resulting data of the superstructure were compared with corresponding values from wideangle X-ray scattering. It is shown that it is impossible to get quantitative information about the superstructure of oriented polymers by using SAXS alone. Nevertheless, the qualitative transition of the crystals during orientation can be explained very well.     

Absolute structural constraints on amyloid fibrils from solid-state NMR spectroscopy of partially oriented samples

We demonstrate that absolute, molecular-level structural information can be obtained from solid-state NMR measurements on partially oriented amyloid fibrils. Specifically, we show that the direction of the fibril axis relative to a carbonyl 13C chemical shift anisotropy (CSA) tensor can be determined from magic-angle spinning (MAS) sideband patterns in 13C NMR spectra of fibrils deposited on planar substrates. Deposition of fibrils on a planar substrate creates a highly anisotropic distribution of fibril orientations (hence, CSA tensor orientations) with most fibrils lying in the substrate plane. The anisotropic orientational distribution gives rise to distorted spinning sideband patterns in MAS spectra from which the fibril axis direction can be inferred. The experimentally determined fibril axis direction relative to the carbonyl CSA tensor of Val12 in fibrils formed by the 40-residue beta-amyloid peptide associated with Alzheimer's disease (Abeta1-40) agrees well with the predictions of a recent structural model (Petkova et al. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 16742-16747) in which Val12 is contained in a parallel beta-sheet in the cross-beta motif characteristic of amyloid fibrils.     

Interatomic correlation in partially oriented polymer fibers

The atomic ordering in materials possessing cylindrical symmetry can be investigated by an approximation of the cylindrical distribution function in a given direction. The approximation gives correctly the extent of the interatomic correlation for a wide class of polymer fibers and reduces considerably the amount of the experimental and computational times.     

Small angle scattering of partially oriented polymers

Summary A general method for the calculation of the small angle scattering pattern of partially oriented polymers with rotational symmetry about the fiber axis is explained. The method can be used with widely varying models for the basic scattering unit.

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Zusammenfassung Es wird eine allgemeine Methode zur Berechnung des Kleinwinkelstreudiagramms von partiell orientierten Polymeren mit Rotationssymmetrie bezüglich der Faserachse angegeben. Als Modell für die streuende Grundeinheit können die verschiedensten Strukturen verwendet werden.

     

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.     

Small-angle scattering of partially oriented polymers: model calculations

Three-dimensional small-angle intensity plots are calculated and discussed for a structure consisting of a linear paracrystalline lattice built up by finite lamellar or cylindrical crystallites, tilted with respect to the lattice direction. The lattice directions are distributed with respect to the fiber axis. The effects of different parameters (orientation, tilt angle, crystallite size, etc.) on the diffraction pattern are studied.     

1H NMR study of partially oriented 1-phenylpropyne

The proton magnetic resonance spectrum of partially oriented 1-phenylpropyne has been studied and the values of the chemical shifts, the direct and the indirect coupling constants determined from the analysis of the spectrum. The ratios of the interproton distances have been derived. No significant deviations have been observed between the relative proton positions on the aromatic ring in 1-phenylpropyne and those expected for the regular hexagonal geometry of the benzene ring. The results are insensitive to the mode of internal rotation of the methyl group about the C? C bond.     

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.     

Automatic generation of ultra-pure hydroxide eluent for carbohydrate analysis of environmental samples

High-performance anion-exchange chromatography coupled with pulsed amperometric detection under alkaline conditions has significantly improved our understanding of the function of carbohydrates in cycling of terrestrial carbon. The use of NaOH as an eluent has many advantages for anion analysis, but NaOH solutions are also an excellent trap for atmospheric CO2 resulting in eluent carbonate ion contamination. To minimize the carbonate contamination, eluent generation technology has been marketed that produces ultra-pure hydroxide (OH) eluents on demand. Application of this technology significantly reduced the amounts of OH present in the waste stream and provided faster chromatographic runs with greater sensitivity and precision for determining the monosaccharide composition from environmental samples. Minimizing the salt concentration to < or = 50 mm from acid hydrolysis extractions injected (25 microl injection volume) on column improved reproducibility of analyte retention times and separations. Eluent generation technology eliminated inconsistent OH eluent preparation that is often due to carbonate contamination. The contamination increases retention time variability for monosaccharides released by hydrolysis of biological samples.     

High-resolution waveguide terahertz spectroscopy of partially oriented organic polycrystalline films

We have characterized the terahertz (THz) vibrational spectroscopy of organic polycrystalline thin films using the new experimental technique of waveguide terahertz time domain spectroscopy (waveguide THz-TDS). The organic materials used in this study are tetracyanoquinodimethane (TCNQ) and 1,3-dicyanobenzene (13DCB). For each material, a thin film is cast onto one of the inner surfaces of a metal parallel plate waveguide (PPWG), followed by measurement of the low-frequency vibrational spectrum using waveguide THz-TDS. The vibrational spectra of the waveguide films are compared to corresponding vibrational spectra of standard pellet samples made by dispersing the organic solid in transparent polyethylene. We show how the waveguide films produce significantly narrower THz vibrational line shapes and reveal additional spectral lines that are obscured by inhomogeneous broadening effects in the pellet samples. When TCNQ waveguide films are cooled to 77 K, vibrational line widths as sharp as 25-30 gigahertz (0.83-1.0 cm(-1)) at the full width at half-maximum are observed, which are among the narrowest far-infrared line widths measured for this material. The origin of the line-narrowing effect for the waveguide films is the suppression of inhomogeneous broadening due to the planar ordering of the film on the waveguide surface. The TCNQ waveguide films are further characterized using optical microscopic evaluation to understand how film morphology affects the THz vibrational spectrum. X-ray diffraction is used to determine the orientation of the polycrystalline TCNQ films on the PPWG surface and to qualitatively explain the different vibrational line strengths observed for the ordered waveguide film relative to the random pellet.     

Structure and dynamics of the triplet state of oligothiophenes in isotropic and partially oriented matrices

The photoexcited triplet states of 4,4'-dipentoxy-2,2'-dithiophene (4-T2), 3,3'-dipentoxy-2,2'-dithiophene (3-T2), and 4,4'-dipentoxy-2,2':5',2':5',2'-tetrathiophene (4-T4) have been investigated by time-resolved electron paramagnetic resonance in glassy toluene and in a frozen oriented liquid crystal, which provides a partially ordered medium. The preferential orientation of the rod-like 4-T2 and 4-T4 is compared to that of the disk-like 3-T2. The use of an oriented matrix coupled to simple semiempirical calculations allowed us to determine the orientation of the principal axes of the fine interaction with respect to the molecular axes. The motional behavior of the molecules in the isotropic and anisotropic matrices has been studied by comparing the spectral profiles of the Echo detected EPR (Echo-EPR) spectra with those of the continuous wave time resolved EPR (TR-EPR). A model considering the modulation of the zero-field splitting (ZFS) by molecular libration accounts for the Echo-EPR lineshape, on the basis of motions around preferential axes depending on the embedding matrix. The differences in the ZFS parameters of the two isomers 4-T2 and 3-T2 are attributed to a mesomeric effect due to the substituents.     

Approximate vibrational corrections for NMR spectral parameters of partially oriented molecules

The NMR spectral parameters of partially oriented molecules, such as indirect couplings, chemical shifts and quadrupolar couplings, should be corrected for effects due to vibrational motions. A practicable method of correction, applicable to cases where the parameters are determined approximately by axially symmetric bonds, is described.     

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′.     

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.     

The NMR spectra of 1,4-dioxin and 1,4-dithiin partially oriented in a nematic phase

The PMR spectra of 1,4-dioxin and 1,4-dithiin have been studied in nematic phase liquid crystalline solvents. The ratio of the interproton distances r₁₂ and r₁₃ and the CCH angles have been determined.     

Preliminary FTIR study of swelling of oriented polyamide 11 samples

The MIR spectra of oriented films of polyamide 11, swollen with various organic liquids, have been investigated by Fourier transform infrared spectroscopy. The main modifications result from the interaction of the amide groups with polar organic molecules. The vibration modes involving the NH and CO stretchings, i.e. amide A and B and amide I bands, are deeply modified by swelling.     

Characterization of oriented low-density polyethylene samples by differential scanning calorimetry

The melting behavior of samples of oriented low-density polyethylene is examined as a function of annealing temperature and time, subsequent heat treatment, and irradiation dose. Three types of endotherm are identified which are attributed to primary melting of the lamellae, to melting of the products of reorganization during the scan, and to melting of material crystallized during cooling from the original annealing temperature. Irradiation was found to suppress the refolding which normally occurs when a sample is heated above its annealing temperature, but also produces an increase in melting point. The behavior of similar samples prepared from high-density polyethylene is briefly considered.