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排序方式: 共有124条查询结果,搜索用时 265 毫秒
1.
W. D. Hutchison A. V. J. Edge N. Yazidjoglou D. H. Chaplin 《Hyperfine Interactions》1992,75(1-4):291-298
The high resolution hyperfine spectroscopy, modulated adiabatic passage of oriented nuclei (MAPON), has been applied for the
first time to high purity, elemental systems. Detailed comparisons between the electric quadrupole hyperfine interactions
(EQI’s) and, in particular, their distributions, are obtained for60CoCo where the hosts are a single crystal of hcp cobalt and a polycrystalline cobalt foil of predominantly fcc character. For
hcp Co, with the electronic magnetization, M, parallel to the c-axis, the mode value P/h=3e2qQ/4I(2I−1)h=−48.5(5) kHz. This fractional distribution implies the sharpest electric field gradient (efg) measured in a metal
to date, using MAPON spectroscopy, in excess of two times sharper than that of the most dilute impurity efg in a crystallographically
cubic ferromagnetic host. The mode efg is Vzz=−27.3(32)×1019 Vm−2. For the polycrystalline, predominantly fcc foil, prepared by quenching, the EQI mode value is P/h=−6.2(4) kHz with a FWHM
of 12.0(7) kHz yielding a mode efg of Vzz=−3.5(5)×1019 Vm−2. 相似文献
2.
W. D. Hutchison N. Yazidjoglou D. J. Isbister D. H. Chaplin L. N. Shakhmuratova 《Hyperfine Interactions》1993,80(1-4):1173-1178
Results for an inhomogeneously broadened60CoFe NMRON sample have been obtained for gamma detected single pulse (nutation) and two pulse (free induction decay) experiments in the region of intermediate-to-lowR(=1/). Here 1 is the angular frequency measure of the strength of the ferromagnetically enhanced RF field at the nucleus and is the HWHM of the inhomogeneously broadened line. Comparisons of the oscillatory FID signals obtained are made with the theoretical predictions of the following paper [6]. 相似文献
3.
High resolution conventional AFP NMR/ON studies provide evidence for very small efg's of predominantly negative sign at the nucleus of the nominal S-state ion54Mn in single crystal nickel when the applied field is paralled to the 111 direction. The form of the mid passage signals for opposing sweep directions indicates a unique efg super-imposed upon a random component of comparable magnitude. The advantages of a second analyzing sweep performed during spin lattice relaxation of a conventional post-passage AFP NMR/ON sweep are demonstrated. 相似文献
4.
The reactions of the polymethylmethacrylate anion have been investigated at 200 K and 250 K in both THF and 9/1 toluene/THF. Sodium-α methyl styrene tetramer and fluorenyl sodium were used as initiators. Only ion pair reactions were investigated. The rate constant of monomer addition to the ion pair at 200 K was determined to be 80 ± 6 M?1 sec?1. At 250 K in the presence of excess monomer, the poly MMA anion reacts with the monomer vinyl function and the monomer ester function at comparable rates. Once fully reacted, the poly MMA anion terminates very slowly in THF at 250 K by an intermolecular mechanism. This rate of termination is enhanced in the toluene/THF system. No evidence was found for different reaction mechanisms for the two initiators. 相似文献
5.
Dr. Caroline M. Storey Dr. Matthew R. Gyton Dr. Rhiann E. Andrew Dr. Adrian B. Chaplin 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(64):14715-14723
Terminal alkyne coupling reactions promoted by rhodium(I) complexes of macrocyclic NHC-based pincer ligands—which feature dodecamethylene, tetradecamethylene or hexadecamethylene wingtip linkers viz. [Rh(CNC-n)(C2H4)][BArF4] (n=12, 14, 16; ArF=3,5-(CF3)2C6H3)—have been investigated, using the bulky alkynes HC≡CtBu and HC≡CAr’ (Ar’=3,5-tBu2C6H3) as substrates. These stoichiometric reactions proceed with formation of rhodium(III) alkynyl alkenyl derivatives and produce rhodium(I) complexes of conjugated 1,3-enynes by C−C bond reductive elimination through the annulus of the ancillary ligand. The intermediates are formed with orthogonal regioselectivity, with E-alkenyl complexes derived from HC≡CtBu and gem-alkenyl complexes derived from HC≡CAr’, and the reductive elimination step is appreciably affected by the ring size of the macrocycle. For the homocoupling of HC≡CtBu, E-tBuC≡CCH=CHtBu is produced via direct reductive elimination from the corresponding rhodium(III) alkynyl E-alkenyl derivatives with increasing efficacy as the ring is expanded. In contrast, direct reductive elimination of Ar'C≡CC(=CH2)Ar’ is encumbered relative to head-to-head coupling of HC≡CAr’ and it is only with the largest macrocyclic ligand studied that the two processes are competitive. These results showcase how macrocyclic ligands can be used to interrogate the mechanism and tune the outcome of terminal alkyne coupling reactions, and are discussed with reference to catalytic reactions mediated by the acyclic homologue [Rh(CNC-Me)(C2H4)][BArF4] and solvent effects. 相似文献
6.
D. H. Chaplin W. D. Hutchison M. P. Kopp N. Yazidjoglou 《Hyperfine Interactions》1988,43(1-4):241-247
Electric quadrupole interactions of impurity nuclei in single crystals of54MnNi,125SbNi and125SbFe have been studied as a function of crystal field direction and applied magnetic field using MAPON. Distributions are in all
cases broad compared with the mode values of the EQI's. For54MnNi the mode efg is isotropic to better than 5% between the easy <111> axis and a hard <100> axis. The efg is +0.88 (15)x 1019 Vm−2. The mode efg for125SbFe along its easy <100> axis is one half of that along a hard <112> axis, and one third of that measured along the easy <111>
axis of125SbNi. The much larger efg mode and distribution seen in125SbNi, for four to eight times greater dilution than for125SbFe, suggests intrinsic contributions due to valence screening effects in the more itinerant nickel host. 相似文献
7.
8.
Nuclear spin-spin relaxation of60Co and56Co in iron single crystals has been studied, using the three-pulse NMRON spin echo. A previously reported rapidT
2 in60CoFe is shown to have arisen from a modulation of the echo amplitude, caused by variations in the phase of the Larmor precession
relative to the applied rf field. A lower limit ofT
2∼0.2s is found in56Co56
Fe. Extension of this result to other CoFe samples is discussed. 相似文献
9.
The influence of the skin effect on single and triple (spin echo) gamma detected pulsed NMRON signals is calculated using
a density matrix approach within a pure Zeeman manifold. For single pulse NMRON the turn angle dependences of the signals
for uniform and exponential profiles of the resonant nuclei are presented for a typical inhomogeneous broadening applicable
to intermediate mass impurities in ferromagnetic hosts. For triple pulse NMRON the baseline and principal spin echo amplitudes
for equal resonant rf pulses are presented for the same inhomogeneous broadening. It is found that the skin effect leads to
the form of pulsed NMRON signals that are in accord with experiment. 相似文献
10.
Under supersonic flow conditions, slender bodies in close proximity induce aerodynamic interference effects. This paper aims
to quantify the magnitude of the resulting interference loads and to understand the underlying flow-physics mechanisms that
cause them. A pair of identical slender bodies are investigated through a series of wind-tunnel experiments and supporting
computational fluid dynamics (CFD) predictions. The bodies induce a complex interference flowfield, which tends to be bespoke
to each configuration. The flow features include impinging shock and expansion waves, conical shock reflections, strong skewing
of the boundary-layer flows and shock diffraction. The effects of axial stagger, lateral separation and the strength of the
primary disturbance flow field are evaluated. The interference loads are found to be most sensitive to the initial location
of the primary disturbance but are also affected by its strength. In addition, maximum interference loads which equate to
an effective incidence of up to 6° are observed. Finally, very good agreement is found between the measurements and the CFD
predicted normal force and pitching moment. 相似文献