19F and 1H NMR spectra of halocarbons

19F NMR chemical shifts and coupling constants are reported for 215 compounds. For 77 of these compounds, 1H NMR spectral data are also given. Long-range couplings, including 8J(F,F) and 5J(F,H), are reported. The complexity of halocarbon spectra owing to the presence of rotational isomers, asymmetric centers, long-range couplings, and chlorine isotope effects are illustrated, and the methods used for analyzing such complex spectra are briefly discussed.     

Refined cell parameters of the Ln2WO6-type rare earth tungstates

Rare earth tungstates of the stoichiometry Ln₂WO₆ where Ln = Ce to Lu have been prepared and their lattice parameters were refined by a least-squares method. They have two structures: the $\text{C}\text{2}\text{c}$ symmetry for the compositions Ce₂WO₆ through Ho₂WO₆, and a monoclinic group from Er₂WO₆ to Lu₂WO₆ crystallizing most likely in the $\text{P}\text{2}\text{m}$ or $\text{P2}{}_1\text{m}$ structure. High pressure modifications are described for Dy₂WO₆ and Ho₂WO₆.     

Structure and luminescence of the α-LnNb3O9-type rare earth niobates

α-LnNb₃O₉ (Ln = La, Pr, Nd) compounds have been prepared hydrothermally from acidic solutions. In comparison to the previously reported orthorhombic β modifications, α-LnNb₃O₉ compounds are monoclinic. The structure of α-PrNb₃O₉ was determined with a = 5.3784(6), b = 7.602(2), c = 16.344(2) Å, and β = 92.21(1)°, space group $\text{P2}{}_1\text{c}$. It is built of double and single chains of corner-shared NbO₆ octahedra extended along the b axis. Praseodymium atoms reside in tunnels along the b axis and are in eight-coordination with oxygen. All α-LnNb₃O₉ compounds can be irreversibly converted to the β modification by heating in air to 1200°C. The X-ray excited luminescence of Sm-, Eu-, Tb-, and Dy-doped α-LaNb₃O₉ is also reported.     

Narrow band quantitative and multivariate electroencephalogram analysis of peri-adolescent period

ABSTRACT: BACKGROUND: The peri-adolescent period is a crucial developmental moment of transition from childhood to emergent adulthood. The present report analyses the differences in Power Spectrum (PS) of the Electroencephalogram (EEG) between late childhood (24 children between 8 and 13 years old) and young adulthood (24 young adults between 18 and 23 years old). RESULTS: The narrow band analysis of the Electroencephalogram was computed in the frequency range of 0--20 Hz. The analysis of mean and variance suggested that six frequency ranges presented a different rate of maturation at these ages, namely: low delta, delta-theta, low alpha, high alpha, low beta and high beta. For most of these bands the maturation seems to occur later in anterior sites than posterior sites. Correlational analysis showed a lower pattern of correlation between different frequencies in children than in young adults, suggesting a certain asynchrony in the maturation of different rhythms. The topographical analysis revealed similar topographies of the different rhythms in children and young adults. Principal Component Analysis (PCA) demonstrated the same internal structure for the Electroencephalogram of both age groups. Principal Component Analysis allowed to separate four subcomponents in the alpha range. All these subcomponents peaked at a lower frequency in children than in young adults. CONCLUSIONS: The present approaches complement and solve some of the incertitudes when the classical brain broad rhythm analysis is applied. Children have a higher absolute power than young adults for frequency ranges between 0-20 Hz, the correlation of Power Spectrum (PS) with age and the variance age comparison showed that there are six ranges of frequencies that can distinguish the level of EEG maturation in children and adults. The establishment of maturational order of different frequencies and its possible maturational interdependence would require a complete series including all the different ages.     

Integral equation formulation for buoyancy-driven convection problems

An integral equation formulation for buoyancy-driven convection problems is developed and illustrated. Buoyancy-driven convection in a bounded cylindrical geometry with a free surface is studied for a range of aspect ratios and Nusselt numbers. The critical Rayleigh number, the nature of the cellular motion, and the heat transfer enhancement are computed using linear theory. Green's functions are used to convert the linear problem into linear Fredholm integral equations. Theorems are proved which establish the properties of the eigenvalues and eigenfunctions of the linear integral operator which appears in these equations.     

Effect of Pr-Ca substitution on the transport and magnetic behavior of LaMnO3 perovskite

The effect of simultaneous substitution of a fluctuating cation and a divalent cation in LaMnO₃ perovskite modifies the properties of the material to exhibit large valence colossal magnetoresistance (CMR) effect. A good example of these properties is (La_1−2x Pr _x Ca _x )MnO₃ (LPCMO) type CMR material. In this communication it is reported that, with the increase in x (for x=0.1, 0.15, 0.2), the T _c varies between 100 and 120 K with improvisation in metal-insulator transition. Interestingly, resistance increases with x from few hundred ohms to few kilo ohms with corresponding decrease in the unit cell volume. The results of the studies using X-ray diffraction (XRD), electrical resistivity, magnetoresistance and ac susceptibility measurements on LPCMO samples for understanding the structural, transport and magnetic properties are discussed in detail.     

Structure and luminescence of the orthorhombic LnWO4Cl-type rare earth halo tungstates

LaWO₄Cl is representative of a series of LnWO₄Cl compounds in which Ln can be La to Sm. LaWO₄Cl crystallizes in orthorhombic Pbcm symmetry, and its structure was refined using a disordered model to an R value of 4.0%. The most unusual structural feature is the 5-coordination of the W atom in the form of a trigonal bipyramid. The La atom is 9-coordinated. Luminescing ions, such as Sm, Tb, and Eu, were introduced into LaWO₄Cl as a host, and their excitation and emission spectra are reported. The Raman spectrum of LaWO₄Cl is compared to those of compounds containing W in more common tetrahedral and octahedral environments. X-Ray powder diffraction data are also reported.     

Copper and iron interactions with angiotensin-converting enzyme inhibitors. A study by fast-atom bombardment tandem mass spectrometry

Fast atom bombardment, combined with high-energy collision-induced tandem mass spectrometry, has been used to investigate gas-phase metal-ion interactions with captopril, enalaprilat and lisinopril, all angiotensin-converting enzyme inhibitors.Suggestions for the location of metal-binding sites are presented. For captopril, metal binding occurs most likely at both the sulphur and the nitrogen atom. For enalaprilat and lisinopril, binding preferably occurs at the amine nitrogen. Copyright 1999 John Wiley & Sons, Ltd.