An NMR study of some low-dimensional magnetically correlated materials

A "7Li NMR study of the antiferromagnetic cuprate compound Li_2CuO_2 is presented. The hyperfine-shift and quadrupolar-splitting interaction tensors were determined for the paramagnetic state, using orientation dependence measurements on a single crystal. The hyperfine-coupling interaction was calculated, and found to be anisotropic. Spectra were obtained between 6 and 296 K, with the sample's major quadrupolar axis parallel to the applied magnetic field. At temperatures above 20 K, where the quadrupolar satellites were resolved, their splitting was found to be constant. The hyperfine shift showed a Curie- Weiss-like temperature dependence above 25 K. This shift exhibited a sharp transition at the Neel temperature (T_N), below which it showed an easy-axis behaviour, confirming that this axis is the a-axis. The spin-lattice-relaxation rate exhibited a decrease by a factor of #approx# 20,000 at T_N suggesting that the fluctuation rate of the Cu spins rapidly reduces as the antiferromagnetic order is established. The behaviour of this relaxation rate in the paramagnetic state shows a critical temperature dependence which suggests that the exchange interaction is anisotropic. An "8"9Y NMR study of the Haldane-gap antiferromagnet Y_2BaNi_1_-_xZn_xO_5, where x = 0.02, and 0.04, revealed local disturbances to the magnetic behaviour caused by the doping-induced Ni-O chain ends. The hyperfine shifts of "8"9Y nuclei were found to vary with their distance from these chain ends. Room temperature MAS NMR spectra showed sites up to #approx# 10 lattice constants from a chain end, which had hyperfine shifts that varied sign incommensurately with distance. The magnitude of this variation showed a roughly- exponential decay, which is related to the electronic-spin correlations. Static spectra showed that these resonances have comparatively strong, paramagnetic temperature dependences. Two NaV_2O_5 samples with different Na deficiencies were studied with "2"3Na and "5"1V NMR. This system exhibits a spin-Peierls-like behaviour for Na deficiencies of less than 4 %. Na deficiencies create non-magnetic V"5"+ ions in the magnetic V-O chains. One sample studied exhibited a spin-Peierls transition, and the other did not. "2"3Na and "5"1V MAS NMR studies suggested that the Na deficiencies were mainly caused by unreacted NaVO_3. The "2"3Na hyperfine shift, for the spin-Peierls sample, showed a temperature dependence that is characteristic of this transition. The temperature dependence of this shift for the normal state of the spin-Peierls sample, and that for all temperatures studied for the other sample, showed the expected 1-dimensional linear Heisenberg chain behaviour. The line- broadening related to the Na impurities showed a temperature dependence that was enhanced in the spin-Peierls state. "5"1V NMR spectra, obtained using frequency sweeps and MAS, showed no non-magnetic V"5"+ sites in either sample, suggesting that this system does not have both magnetic and non-magnetic chains as originally suggested, but is actually a quarter-filled ladder compound. (author)Available from British Library Document Supply Centre-DSC:DXN043708 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo

Thermometers for low temperature Magic Angle Spinning NMR

The measurement of temperature in a Magic Angle Spinning NMR probe in the temperature range 85-300 K is discussed. It is shown that the shift of the Sn-119 resonance of Sm2Sn2O7 makes a good thermometer with shift being given by delta = 223 - 9.54 x 10(4)/T ppm and a potential precision of better than 0.5 K over the entire temperature range. The sensitivity is such (e.g. 4.2 ppm/K at 150 K) that small temperature gradients across the sample can readily be measured. Furthermore, since the spin-lattice relaxation time is very short, measurements can be made in similar to 1 s enabling relatively rapid temperature changes to be followed. Values for the chemical shift of Pb-207 in Pb(NO3)(2) down to similar to 85 K are also presented. Although the Pb-207 shift variation is approximately linear near room temperature (we find a slope 0.725 +/- 0.002 ppm/K over the range 293-153 K), it clearly deviates from linearity below similar to 130 K

Two-dimensional satellite-transition MAS NMR of quadrupolar nuclei: shifted echoes, high-spin nuclei and resolution

Modifications to the satellite-transition magic angle spinning (STMAS) experiment for obtaining high-resolution NMR spectra of quadrupolar nuclei are discussed. A phase-modulated `shifted-echo' STMAS experiment that yields pure absorptive lineshapes is presented and shown to be compatible with the `split-t1' technique used in multiple-quantum (MQ) MAS NMR to reduce the duration of t2 acquisition and avoid shearing the final two-dimensional spectrum. The application of STMAS to nuclei with spin greater than I=3/2 is also considered, the dispersion of isotropic shifts achieved by STMAS and MQMAS are compared, and the effects of anisotropic `cross-term' broadening mechanisms on linewidths in `isotropic' STMAS spectra are discussed

Modulation-aided signal enhancement in the magic angle spinning NMR of spin-5/2 nuclei

We report signal enhancement schemes using fast amplitude modulated pulses for the one-dimensional (I D) nuclear magnetic resonance (NMR) of spin-5/2 nuclei under magic-angle spinning. Signal enhancement by a factor of around 2.5 is observed when amplitude modulated pulses precede selective excitation of the central transition. This enhancement is a result of the redistribution of energy level populations through partial saturation of the satellite transitions. Results are shown for Al-27 and O-17. The gain in signal intensity is very useful for the observation of weak signals from low abundance quadrupolar nuclei. The scheme works for wide ranges of quadrupole interactions and rf powers

Silica-supported imine palladacycles - recyclable catalysts for the Suzuki reaction?

Silica-supported, imine-based palladacyclic catalysts have been synthesised and the crystal structure of complex 9, the triphenylphosphine adduct of the pre-supported precursor complex 8, has been determined. The solid-supported catalysts show considerably lower activity in the Suzuki reaction than their homogeneous counterparts. Poor recyclability of the silica-immobilised catalysts and the presence of active catalysts in solution indicate that imine-based palladacyclic catalysts are unstable with respect to liberation of zero-valent palladium species. Whilst the solid-supported complexes are not useful as catalysts, they do function as excellent mechanistic probes. Studies on model complexes give further information on the processes that cause the liberation of zero-valent species not only from the solid-supported catalysts, but also from homogeneous systems. In all cases it appears that a reductive-elimination event occurs to generate the active catalyst

ADHD PADA ANAK DENGAN RETARDASI MENTAL

The diagnosis of mental retardation with ADHD in children, statistically record 8% - 39%. Children with mental retardation, requires certain ways to maximize their potential and quality of life. The condition ADHD are likely brought certain challenges to the handling of these children. This article presents the description of mentally retarded children with ADHD in their natural setting, school and home. It’s a qualitative research, descriptive case study, with holistic multi-case design. Purposive sampling technique found two (2) children as research subjects. Pattern matching technique is used for data analysis. The result of this study shows challenges for school teacher while working with mentally retarded with ADHD children. Accommodate their ADHD in positive and productive activities, are likely helps the handling of their mental retardation conditio

New insights into the bonding arrangements of L- and D-glutamates from solid state 17 O NMR

Magic angle spinning (MAS) from L- and D-glutamic acid-HCl at 14.1 T produces highly structured and very similar NMR spectra. Lines from all 4 oxygen sites are readily distinguished and assigned. These 17O NMR spectra are very different from the previously reported 17O spectrum of the D,L-form presumably because that was a racemic crystal. 17O NMR from L-monosodium glutamate-HCl is very different again requiring the application of double angle rotation and 3 quantum MAS NMR to provide resolution of 5 different sites. Hence high resolution 17O solid state NMR techniques offer possible new insight into biochemical bonding processes. © 2003 Elsevier Science B.V. All rights reserved