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991.
Baltisberger JH Walder BJ Keeler EG Kaseman DC Sanders KJ Grandinetti PJ 《The Journal of chemical physics》2012,136(21):211104
We present an improved and general approach for implementing echo train acquisition (ETA) in magnetic resonance spectroscopy, particularly where the conventional approach of Carr-Purcell-Meiboom-Gill (CPMG) acquisition would produce numerous artifacts. Generally, adding ETA to any N-dimensional experiment creates an N + 1 dimensional experiment, with an additional dimension associated with the echo count, n, or an evolution time that is an integer multiple of the spacing between echo maxima. Here we present a modified approach, called phase incremented echo train acquisition (PIETA), where the phase of the mixing pulse and every other refocusing pulse, φ(P), is incremented as a single variable, creating an additional phase dimension in what becomes an N + 2 dimensional experiment. A Fourier transform with respect to the PIETA phase, φ(P), converts the φ(P) dimension into a Δp dimension where desired signals can be easily separated from undesired coherence transfer pathway signals, thereby avoiding cumbersome or intractable phase cycling schemes where the receiver phase must follow a master equation. This simple modification eliminates numerous artifacts present in NMR experiments employing CPMG acquisition and allows "single-scan" measurements of transverse relaxation and J-couplings. Additionally, unlike CPMG, we show how PIETA can be appended to experiments with phase modulated signals after the mixing pulse. 相似文献
992.
The complexes M(II){N(H)Ar(Pr(i)(6))}(2) (M = Co, 1 or Ni, 2; Ar(Pr(i)(6)) = C(6)H(3)-2,6(C(6)H(2)-2,4,6-Pr(i)(3))(2)), which have rigorously linear, N-M-N = 180°, metal coordination, and M(II){N(H)Ar(Me(6))}(2) (M = Co, 3 or Ni, 4; Ar(Me(6)) = C(6)H(3)-2,6(C(6)H(2)-2,4,6-Me(3))(2)), which have bent, N-Co-N = 144.1(4)°, and N-Ni-N = 154.60(14)°, metal coordination, were synthesized and characterized to study the effects of the metal coordination geometries on their magnetic properties. The magnetometry studies show that the linear cobalt(II) species 1 has a very high ambient temperature moment of about 6.2 μ(B) (cf. spin only value = 3.87 μ(B)) whereas the bent cobalt species 3 had a lower μ(B) value of about 4.7 μ(B). In contrast, both the linear and the bent nickel complexes 2 and 4 have magnetic moments near 3.0 μ(B) at ambient temperatures, which is close to the spin only value of 2.83 μ(B). The studies suggest that in the linear cobalt species 1 there is a very strong enhanced spin orbital coupling which leads to magnetic moments that broach the free ion value of 6.63 μ(B) probably as a result of the relatively weak ligand field and its rigorously linear coordination. For the linear nickel species 2, however, the expected strong first order orbital angular momentum contribution does not occur (cf. free ion value 5.6 μ(B)) possibly because of π bonding effects involving the nitrogen p orbitals and the d(xz) and d(yz) orbitals (whose degeneracy is lifted in the C(2h) local symmetry of the Ni{N(H)C(ipso)}(2) array) which quench the orbital angular momentum. 相似文献
993.
Reduction of [(3,5-(i)Pr(2)-Ar*)Co(μ-Cl)](2) (3,5-(i)Pr(2)-Ar* = -C(6)H-2,6-(C(6)H(2)-2,4,6-(i)Pr(3))(2)-3,5-(i)Pr(2)) with KC(8) in the presence of various arene molecules resulted in the formation of a series of terphenyl stabilized Co(I) half-sandwich complexes (3,5-(i)Pr(2)-Ar*)Co(η(6)-arene) (arene = toluene (1), benzene (2), C(6)H(5)F (3)). X-ray crystallographic studies revealed that the three compounds adopt similar bonding schemes but that the fluorine-substituted derivative 3 shows the strongest cobalt-η(6)-arene interaction. In contrast, C-F bond cleavage occurred when the analogous reduction was conducted in the presence of C(6)F(6), affording the salt K[(3,5-(i)Pr(2)-Ar*)Co(F)(C(6)F(5))] (4), in which there is a three-coordinate cobalt complexed by a fluorine atom, a C(6)F(5) group, and the terphenyl ligand Ar*-3,5-(i)Pr(2). This salt resulted from the formal insertion of a putative 3,5-(i)Pr(2)-Ar*Co species as a neutral or anionic moiety into one of the C-F bonds of C(6)F(6). Reduction of [(3,5-(i)Pr(2)-Ar*)Co(μ-Cl)](2) in the presence of bulkier substituted benzene derivatives such as mesitylene, hexamethylbenzene, tert-butylbenzene, or 1,3,5-triisopropylbenzene did not afford characterizable products. 相似文献
994.
Makal A Benedict J Trzop E Sokolow J Fournier B Chen Y Kalinowski JA Graber T Henning R Coppens P 《The journal of physical chemistry. A》2012,116(13):3359-3365
The excited-state structure of [Cu(I)[(1,10-phenanthroline-N,N') bis(triphenylphosphine)] cations in their crystalline [BF(4)] salt has been determined at both 180 and 90 K by single-pulse time-resolved synchrotron experiments with the modified polychromatic Laue method. The two independent molecules in the crystal show distortions on MLCT excitation that differ in magnitude and direction, a difference attributed to a pronounced difference in the molecular environment of the two complexes. As the excited states differ, the decay of the emission is biexponential with two strongly different lifetimes, the longer lifetime, assigned to the more restricted molecule, becoming more prevalent as the temperature increases. Standard deviations in the current Laue study are very much lower than those achieved in a previous monochromatic study of a Cu(I) 2,9-dimethylphenanthroline substituted complex ( J. Am. Chem. Soc. 2009 , 131 , 6566 ), but the magnitudes of the shifts on excitation are similar, indicating that lattice restrictions dominate over the steric effect of the methyl substitution. Above all, the study illustrates emphatically that molecules in solids have physical properties different from those of isolated molecules and that their properties depend on the specific molecular environment. This conclusion is relevant for the understanding of the properties of molecular solid-state devices, which are increasingly used in current technology. 相似文献
995.
The surface structure and properties of the HfB2(0 0 0 1) (Hafnium diboride, HfB2) surface have been investigated with X-ray photoelectron spectroscopy, low energy electron diffraction (LEED), and scanning tunneling microscopy (STM). Annealing temperatures above 1900°C produce a sharp (1×1) LEED pattern, which corresponds to STM images showing flat (0 0 0 1) terraces with a very low contamination level separated by steps 3.4 Å in height, corresponding to the separation of adjacent Hf planes in the HfB2 bulk structure. For lower annealing temperatures, extra p(2×2) spots were observed with LEED, which correspond to intermediate terraces of a p(2×1) missing row structure as observed with STM. 相似文献
996.
The model considered here is the “jellium” model in which there is a uniform, fixed background with charge density −eρ in a large volume V and in which N=ρV particles of electric charge +e and mass m move – the whole system being neutral. In 1961 Foldy used Bogolubov's 1947 method to investigate the ground state energy
of this system for bosonic particles in the large ρ limit. He found that the energy per particle is −0.402 in this limit, where . Here we prove that this formula is correct, thereby validating, for the first time, at least one aspect of Bogolubov's pairing
theory of the Bose gas.
Received: 23 August 2000 / Accepted: 5 October 2000 相似文献
997.
Philip Hall 《Studies in Applied Mathematics》2001,106(1):69-83
Small-amplitude wave systems interacting nonlinearly can produce 0(1)amplitude streamwise vortex structures through the vortex–wave interaction mechanism described, for example, by [1–3]. The key feature of the interaction is that the spanwise velocity component of a vortex is small as compared to the streamwise component so that a nonlinear wave system driving the spanwise velocity component through Reynolds stresses can provoke a 0(1) response of the vortex. The wave system can correspond to either a Rayleigh or Tollmien–Schlichting wave disturbance, but previous work on the initiation of the process has been confined to Rayleigh waves (see, for example, [5, 6]). Here, we address the nonlinear initial value problem for Tollmien–Schlichting wave–vortex interactions in channel flows. The evolution of the disturbances is accounted for using the phase equation approach of [7]. We determine the circumstances, if any, under which the finite amplitude vortex–wave equilibrium states of [4] are generated. Our discussion of the nonlinear evolution of a wave system points toward a possible mechanism for the experimentally observed breakup of three-dimensional instabilities into shorter streamwise scales. 相似文献
998.
N. P. Boley Paul De Bièvre Philip D. P. Taylor Adam Uldall 《Accreditation and quality assurance》2001,6(6):244-251
Many laboratories take part in proficiency testing schemes, external quality assessment programmes and other interlaboratory
comparisons. These have many similarities but also important differences in their modus operandi and evaluation of performance of participating laboratories. This paper attempts to highlight both the similarities and differences. It also puts particular emphasis on requirements
called ”target values for uncertainty” and their meaning.
Received: 24 January 2001 Accepted: 25 January 2001 相似文献
999.
A model describing electrochemical reactivity at nanoelectrode ensembles consisting of redox-molecule-based active sites immobilized on otherwise passivated electrode surfaces is presented. A mathematical treatment in terms of hemispherical diffusion of redox-active solutes to a layer of independent molecule-based nanoelectrode sites is shown to be equivalent to one in terms of a bimolecular diffusion-limited reaction between a layer of immobilized redox molecules and a reservoir of redox-active solutes. This equivalence derives from the fact that in both cases the mass-transfer problem is essentially that of hemispherical diffusion. The model is further developed to consider rate limitation by both the bimolecular redox reaction between the active-site molecule and redox molecules in solution and the heterogeneous redox reaction between the electrode and the active-site molecule. Analytical expressions are derived for the current–voltage relation corresponding to catalyzed electron transfer at an ensemble of redox-molecule-based nanoelectrode sites, and the expressions are used to interpret preliminary data for ultrasensitive electrochemical detection in flow streams via an electrochemical amplification process that is thought to involve redox mediation by individual analyte molecules adsorbed onto monolayer-coated electrodes. 相似文献
1000.
Annamma John Daizy Philip Keith R. Morgan S. Devanarayanan 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2000,56(14):386
The FT IR and FT Raman spectra of Co(en)3Al3P4O16 · 3H2O (compound I) and [NH4]3[Co(NH3)6]3[Al2(PO4)4]2 · 2H2O (compound II) are recorded and analysed based on the vibrations of Co(en)33+, Co(NH3)63+, NH4+, Al---O---P, PO3, PO2 and H2O. The observed splitting of bands indicate that the site symmetry and correlation field effects are appreciable in both the compounds. In compound I, the overtone of CH2 deformation Fermi resonates with its symmetric stretching vibration. The NH4 ion in compound II is not free to rotate in the crystalline lattice. Hydrogen bonding of different groups is also discussed. 相似文献