Diamagnetic Shielding Research Articles

Observation of buttressing effect and hindered rotation about C(sp2)—C(sp2) single bond in styrenes coordinated to a ruthenium cation, Cp(diphosphine)Ru+

Complexes of (η5-cyclopentadienyl)(bis(di-p-tolylphosphino)ethane)ruthenium(II) cation with some styrenes containing meta or para substituants were prepared and their NMR spectra examined in detail. Variable-temperature NMR studies on the unsubstituted and para-substituted styrene analogues demonstrated occurrence of a restricted rotation about the C(sp2)—C(sp2) single bond of the styrenes where one of the ortho hydrogens of the styrene phenyl group receives a very large diamagnetic shielding effect by one of the phosphine tolyl groups. Similar studies on the meta-substituted styrene complexes showed existence of two unequally populated conformational isomers arising from the similar restricted rotation where the meta substituent in the dominant isomer was placed further away from the C=C group. The origin of such conformational isomerism was deduced to be the buttressing effect of the meta substituent transmitted via the ortho-hydrogen atom. Key words: buttressing effect, hindered rotation, Ru–styrene complex.

New Superconducting Phase in Potassium Amalgam Doped Fullerenes

Structural and magnetic studies have been performed on potassium amalgam doped fullerenes ( C60). New superconducting phases of K3Hgx C60 (x=1, 2, 3) have been discovered by the measurement of magnetization, the transition temperatures of which are 2.7 K, 2.8 K and 3.3 K, respectively. The diamagnetic shielding fractions range from 2 to 17%. The crystal structures of the new phases are found to be face centered cubic and body centered tetragonal ones. In contrast, in the K3-x Hgx C60 (x=0.3, 0.75, 1.5, 2) samples, Hg atoms are not substitutionally intercalated with K atoms as we expected, and thus Hg atoms have little effect on the formation of a new superconducting phase.

Meissner and diamagnetic shielding effects of α-EDT-TTF[Ni(dmit) 2

Meissner and diamagnetic shielding effects were investigated for an organic superconductor, α-EDT-TTF[Ni(dmit) 2] (dmit = 4,5-dimercapto-1,3-dithiole-2-thione, EDT-TTF=ethylenedithio-tetrathiafulvalene), under ambient pressure. The onset of the diamagnetic transition occurred at 1.39 K. From the measurements of the Meissner effect, the volume fraction of the superconducting phase was estimated to be more than 62%. From the measurements of the diamagnetic shielding effect, this fraction was estimated to be about 93%. This high level of volume fraction shows that the superconducting phase is the main phase in α-EDT-TTF[Ni(dmit) 2].

NMR of Terminal Oxygen—17—Ab initio IGLO Study of the High Shielding of O (and C, N) in Linear Heteronuclear π-Systems

Linear heteronuclear pi-systems of sp character, when compared with corresponding sp(2) systems, show increased shift to higher field for all atoms concerned (O, C, N); acetylene compared with ethylene is a model case. In O-17 NMR, the molecules NO+, PhCO(+), RC=NO, N=C=O-, RN=C=O and NO,+ were examined (in addition to known CO, CO2, COS and N2O). For these molecules, C-13 and N-15 shieldings were also analysed. As had been shown before for C-13 and N-15, the increase in shielding of O-17 in linear molecules is mainly attributable to the vanishing of the deshielding component along the linear axis of the molecule (manifest also in the increase in the anisotropy of shielding Delta sigma). Ab initio IGLO calculations were executed to evaluate Delta sigma (=sigma(parallel to)-sigma(perpendicular to)). For each of the atoms concerned, the resulting calculated average values agreed well with the experimental values of isotropic shielding, sigma(iso). As in the case of C and N, the values of sigma(parallel to) Of O-17 in linear compounds are close to that of the diamagnetic shielding of the isolated O atom, sigma(d) (atom). The particular case of organic isocyanates, RNCO, which, although not completely linear, show related behaviour, is discussed

NMR of Terminal Oxygen—17—Ab initio IGLO Study of the High Shielding of O (and C, N) in Linear Heteronuclear π-Systems

Linear heteronuclear π-systems of sp character, when compared with corresponding sp2 systems, show increased shift to higher field for all atoms concerned (O, C, N); acetylene compared with ethylene is a model case. In 17O NMR, the molecules NO+, PhCO+, RCNO, NξCξO−, RNξCξO and NO2+ were examined (in addition to known CO, CO2, COS and N2O). For these molecules, 13C and 15N shieldings were also analysed. As had been shown before for 13C and 15N, the increase in shielding of 17O in linear molecules is mainly attributable to the vanishing of the deshielding component along the linear axis of the molecule (manifest also in the increase in the anisotropy of shielding Δσ). Ab initio IGLO calculations were executed to evaluate Δσ ( = σ‖-σ⊥). For each of the atoms concerned, the resulting calculated average values agreed well with the experimental values of isotropic shielding, σiso. As in the case of C and N, the values of σ‖ of 17O in linear compounds are close to that of the diamagnetic shielding of the isolated O atom, σd (atom). The particular case of organic isocyanates, RNCO, which, although not completely linear, show related behaviour, is discussed.

Influence of epitaxial properties on the mutual inductance response of high-quality YBCO thin films

High-quality YBa 2Cu 3O 7− x films have been produced on SrTiO 3 (001) and NdGaO 3 (001) substrates. X-ray Φ scans show a good in-plane texture but do not exclude the presence of domains at 90°. Diamagnetic shielding properties obtained from mutual-inductance measurements show a double peak structure which may be ascribed to the presence of those domains. This point is further supported with an analysis of the normal-state resistivity. Films with an interdomain resistivity of 10 −5 Ω cm show a double peak structure in the mutual-inductance measurements and relatively low values of the critical current densities (1.5 × 10 5 A cm −2 at 77 K), while films with interdomain resistivities lower than 10 −6 Ω cm show a narrow single peak and higher values of the critical current (10 6 A cm −2 at 77 K).

Small Azaparacyclophanes as Potential Selective Scavengers of Mercury. Crystal Structure of the Complex Hg2(L1)Cl4 (L1 = 16,17,19,20-Tetramethyl-2,6,9,13-tetraaza[14]paracyclophane)

The ligational properties of the macrocycles 16,17,19,20-tetramethyl-2,6,9,13-tetraaza[14]paracyclophane (L1) and 2,6,9,13-tetraaza[14]paracyclophane (L2) and of its dibenzylated open-chain counterpart 1,12-dibenzyl-1,5,8,12-tetraazadodecane (L3) toward Hg2+ are reported. The equilibrium constants have been determined by potentiometry at 298.1 K in 0.15 mol dm-3 NaCl. Competition of the ligands with chloride anions for the coordination of Hg2+ was required for the determination of the stability constants due to the high stability of the complexes formed. L1 and L2 selectively complex Hg2+ over first transition series metal ions and particularly over Cu2+. NMR measurements prove that just three of the four nitrogen donors in ligands L1 and L2 are involved in the coordination to Hg2+; the remaining benzylic nitrogen atom is not coordinated. NMR spectra also show a strong diamagnetic shielding (ca. 2 ppm) for one of the protons in the central methylene groups of the propylenic chain linked to the uncoordinated benzylic nitrogen. These signals appear for [HgL1]2+ at 0.3 ppm and for [HgL2]2+ at −0.4 ppm. The crystal structure of [Hg2L1Cl4] agrees with the NMR studies. Crystals of [Hg2L1Cl4] (C20H36Cl4Hg2N4) are monoclinic space group P21/c, with a = 10.360(2) Å, b = 17.535(2) Å, c = 14.918(1) Å, β = 105.98(1)°, Z = 4, R1 = 0.0471, wR2 = 0.1064. The tetrahedral coordination polyhedra around the Hg2+ ions are formed in one case by the benzylic nitrogen atom and the two consecutive nitrogen atoms of the central part of the bridge and one chloride anion and, in the other case, by the remaining benzylic nitrogen and three chloride anions. Both coordination polyhedra are strongly distorted, particularly the one involving three nitrogen atoms of the bridge.

Ab initio studies of the electronic structure of Be93, Be105, Be111, and Be123 clusters

AbstractAb initio self‐consistent‐field calculations are reported for electronic states of beryllium clusters comprised of 93, 105, 111, and 123 atoms. The respective clusters correspond to coordination shells 12‐15 of a central Be atom with internuclear separations derived from the lattice constants of the bulk metal. Ab initio effective core potentials have been employed to replace the 1 s electrons, thereby reducing the complexity of the calculations. In addition, use of the full D3h point group symmetry of the clusters results in a substantial reduction of the numbers of two‐electron integrals that must be computed and processed. Binding energies, orbital energies, electric field gradient, nuclear‐electrostatic potential, diamagnetic shielding constant, second moments, and Mulliken populations are calculated for selected electronic states. Calculated binding energies when compared among the different clusters as well as to smaller and larger fragments from earlier studies provide evidence for the onset of convergence to the Hartree–Fock limit of the bulk. Lowest‐state ionization potentials are consistently above and agree to within 14% of the experimental workfunction. The net charge on the central beryllium atom decreases toward zero. The variability of observed bulklike behavior for the different properties indicates that the transition between cluster and bulklike behavior is not sharp and depends on the quantity of interest. © 1995 John Wiley & Sons, Inc.

Electric and magnetic properties of the BrCl molecule

Self-consistent-field calculations on BrCl in a basis of 212 uncontracted Gaussian functions reproduce the measured dipole moment, nuclear quadrupole and spin—rotation coupling constants to within a few percent. The calculated mean polarisability of 36.8 au is 7% smaller than the average of the measured values for Br 2 and Cl 2. Combination of the calculated diamagnetic nuclear shielding with the measured spin—rotation constant gives a value of 1627 ppm for the total shielding at bromine in BrCl.

Paramagnetic Meissner effect in a niobium disk

The diamagnetic shielding and flux expulsion in a Nb disk have been studied as a function of temperature and magnetic field. Both the zero-field-cooled (ZFCM) and the field-cooled (FCM) magnetizations show diamagnetic responses for fields parallel to the surface of the disk and indicate two different transition temperatures, 9.26 K and 9.06 K. Whereas for fields perpendicular to the surface, the ZFCM shows an additional diamagnetic transition at 9.17 K which is strongly field dependent while the FCM shows a positive magnetization for temperatures below 9.17 K. These features are qualitatively similar to the so-called paramagnetic Meissner effect observed in granular high-T c superconductors.

Enhancement of critical current intensities in single crystals of the LiTi 2O 4 spinel

Single crystals of LiTi 2O 4 grown by two methods and anisotropically refined by single crystal X-ray diffraction show critical temperatures of 11.5 K, transition widths of 1.5 K and diamagnetic shieldings of 65–70%. Magnetization measurements up to 2 T at 6 K show lower critical fields H Cl of 250 Oe and critical currents of 10 6 Acm −2, four orders of magnitude higher than those previously reported for polycrystalline samples.

Two-zone annealing of Tl0.5Pb0.5(Sr0.8Ba0.2)2Ca2Cu3Oy

Stable conditions have been identified for two-zone processing of the superconducting thallium cuprate Tl0.5Pb0.5(Sr0.8Ba0.2)2Ca2Cu3Oy. With P(O2) of 0.8 atm, P(Tl2O) of 4.4 × 10−3 atm, and a sample temperature of 920 °C, single-phase Tl0.5Pb0.5(Sr0.8Ba0.2)2Ca2Cu3Oy, is produced with a Tc of 115 K, complete diamagnetic shielding, and Meissner fraction greater than 70%. Although a small amount of melting occurs under these conditions, a comparison of the low-field diamagnetic shielding for these samples with samples of Pb- and Sr-free TlBa2Ca2Cu3Oy and Tl2Ba2Ca2Cu3Oy, suggests that such melting is not necessary to produce the triple-CuO2-layer superconductors.

Crystal growth of superconducting LiTi 2O 4

Crystals of LiTi 2O 4, a member ( s = 1) of the hypothetical Li s Ti(III) 4-3 s Ti(IV) 2 s-1 O 4 (0.50≤ s≤1.33) spi nel series, were grown by two methods: electrolysis (30 mA, 100 mV) of a bath containing a suspension of TiO 2 (anatase) in molten LiBO 2, and chemical reduction by addition of a metal, Zr, more electropositive than Ti, on the indicated suspension. LiTi 2O 4 crystals were characterized by energy dispersive spectroscopy (EDS), X-ray powder diffraction and scanning electron microscopy. Electrolysis led to crystal aggregates of about 1 mm 3, grown on the cathode and to skeletal crystals, 0.15 mm 3, of cubic morphology related to the development of growth spirals in the three directions. The reduction using Zr plates gave rise to rows of twinned crystals as well as to isolated crystals showing planar faces unequally developed. The crystal structure of LiTi 2O 4 was anisotropically refined by single-crystal X-ray diffraction in space group Fd 3 m(No. 227), with a = 8.372(1) Å, V = 586.8(1) Å 3, Li and Ti at tetrahedral and octahedral sites, respectively, an oxygen parameter u = 0.2628(3) and, as a consequence, practically equal Li-O and Ti-O distances. As is shown by AC susceptibility measurements, electrotically grown crystals as well as those grown on Zr plates are very similar in their superconducting transitions, with T c (onset) of 11.5 K, transition widths of 1.5 K, and diamagnetic shieldings of 65–70%. Magnetization measurements up to 2 T at 6 K on electrolytically grown crystals show a lower critical field H c1 of 250 Oe and a critical current of 10 6 A cm -2.

Production of single-crystal K3C60

A simplified technique for synthesizing high-quality millimeter-sized single-crystals of K3C60 is reported. The preparation method consists of a two-step process where pure C60 crystals are first grown and later doped with potassium vapor. The undoped crystals are grown using an open-tube vapor transport with a flowing helium gas carrier. Doping occurs in a highvacuum environment inside a specialized quartz tube which allows phase purity to be monitored via magnetization measurements of the superconducting state. Utilizing the aforementioned procedure, one can attain samples with magnetically determined superconducting transition widths (10–90% diamagnetic shielding) of less than 1 K.

Type-II superconductivity in a dilute magnetic system: La1-xTmxRu3Si2.

Superconducting and magnetic properties of ${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Tm}}_{\mathit{x}}$${\mathrm{Ru}}_{3}$${\mathrm{Si}}_{2}$ (x=0, 0.08, and 0.16) have been investigated through measurements of both electric and magnetic properties. Tm is in a trivalent state and carries a magnetic moment \ensuremath{\approxeq}8${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$. We find that the magnetic measurements are not suited to the determination of the critical line ${\mathit{H}}_{\mathit{c}}$(T) for the onset of type-II superconductivity in materials where one has at the same time superconducting diamagnetic shielding and paramagnetic contribution due to the localized spins. On the other hand, transport experiments are much less ambiguous, and allow us to determine accurately the critical temperature in a given applied magnetic field. In particular, they allow us to suggest a more appropriate interpretation of the maximum in the magnetic-susceptibility curve already observed in materials of the same family: this maximum results from a competition between the incomplete diamagnetic shielding and the paramagnetic contribution from the magnetic ions in the mixed phase, rather than from a spin-glass freezing of the paramagnetic ions. In zero field, the rate of depression of the critical temperature upon substituting nonmagnetic La ions by magnetic Tm ions is dT/dx=-8\ifmmode\pm\else\textpm\fi{}1 K/at. % Tm.

Scanning Tunneling Microscopy Study of Granular Intercalated Fullerenes

Abstract We have studied by scanning tunneling microscopy (STM) the surface of polycrystalline powders of Rb3C60, (KTI1.5)3C60 and (RbTl1.5)3C60 (nominal compositions) in a high purity argon environment at 295 K. Large scale STM images reveal effective grain sizes of some hundred nanometers. This small grain size strongly reduces the superconductivity fraction of the diamagnetic shielding. On a molecular scale, the STM images show ball-shaped features which can be identified with individual fullerene molecules in case of Rb3C60(111) and (311) facets, but in case of (KTl1.5)3C60 and (RbTl1.5)3C60 their apparent diameter (1.7 nm) is significantly larger. Since Ac-susceptibility and X-ray diffraction have traced metallic Tl in the (KTl1.5)3C60 and (RbTl1.5)3C60 samples, we interpret the larger apparent diameter of the ball-shaped units as a complex of Tl and Rb3C60.

CIDNP study of the aromatic side chain interactions in myotoxin alpha.

CIDNP and COSY measurements were applied to study aromatic side chain interactions and conformations in myotoxin alpha, a Crotalus venom toxin which acts as blocker of the Ca2+ influx in the sarcoplasmic reticulum calcium pump. New evidence for the existence of a hydrophobic aromatic cluster at the amino terminus was obtained. This cluster consists of Tyr1, His5, His10, and (possibly) F12. The CIDNP data clearly establish that the usual order of the tyrosine 2, 6 and 3, 5 proton signals of Tyr1 is inverted, because of the large diamagnetic shielding effects of one ring on the other. The lines of the 2, 6 ring protons of Tyr1 and proton 4 in each of His5 and His10 are significantly broadened, an outcome of the side-chain hydrophobic interaction. The aromatic cluster could possibly present a hydrophobic sticky patch for binding of toxin by Ca2+ ATPase.

STRONG FLUX PINNING EFFECTS IN BULK Y1Ba2Cu3O7−δ

A modification of the quench-melt-growth (QMG) method of Sawano et al. has been used to produce highly textured superconducting samples of YBa 2 Cu 3 O 7−δ (123) with a high density of inclusions, a high density of parallel microcracks, and strong flux pinning persisting up to temperatures close to Tc. A selected sample has: 1) a very sharp transition width, ≃0.3 K as determined from ac susceptibility, and <0.5 K as determined from diamagnetic shielding or trapped flux measurements; 2) a zero Meissner effect in applied fields as low as 5 Oe; 3) a high magnetic critical current sustained over a broad temperature range; and 4) wide hysteresis loops at temperatures up to 0.5 K below Tc.

Semiempirical calculation of 29Si NMR chemical shifts and 29Si- 13C spin-spin coupling constants in some substituted bridgehead polycycloalkanes

NMR parameters pertaining to the 29Si- 13C b bond between the bridgehead carbon atom C b of the polycyclic hydrocarbon skeleton and the silicon atom belonging to the substituent group R = Si(CH 3) 3 are considered by using the semiempirical AM1 method. It is found that the J( 29Si- 13C b) coupling constants are very well correlated with the product of the corresponding s-characters ( s%)Si( s%)C b. The same holds for the J( 13C b-H) couplings in the parent hydrocarbons, where the C b s-content is linearly related to the one-bond spin-spin constants. The NMR chemical shifts of the 29Si and 13C b nuclei are bilinear functions of the total valence electron populations Q A v (A = Si, C b) and the product of the s-contents of the hybrids forming the Si-C b bond. Correlations are of fair quality with δ Si shifts being better described. A diamagnetic contribution to the total NMR shift of the 29Si and 13C b nuclei is discussed briefly. It is shown that a trend in the changes in δ A (A = 29Si, 13C b) is roughly determined by variation in the diamagnetic shielding.

Diamagnetic shielding and Meissner effect in the high T c superconductor Sr 0.2La 1.8CuO 4

SQUID susceptibility measurements of the high T c superconductor Sr 0.2La 1.8CuO 4, with a 40 K resistive onset, are reported for fields from 1 μT to 4.5 T (10mG to 45 kG). Data show an average lower critical field of 30 mT at 5 K, but with nonlinearities appearing as low as 100 μT. Zero field cooling and subsequent application of a field shows a diamagnetic shielding effect reaching 100% in the lowest fields with a 37 K onset and 31 K midpoint. The observed Meissner effect in single phase material exceeds 40% but decreases with oxygen annealing even though T c increases.