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51.
Azulen-1-yl-dicarbonyl (Az) is a novel, coloured, protecting group used to mask primary and secondary alcohols. In particular, the use of Az in the construction of carbohydrates, where a number of orthogonal protecting groups are typically required, is of merit. Introduction and removal of the Az-group is performed easily and in near quantitative yields in the presence of other commonly used protecting groups, and the Az-group is compatible with glycosylation reactions using trichloroacetimidate donors. The deep-red colour of the Az-group greatly facilitates the purification of carbohydrate building blocks and the monitoring of coupling reactions. Of particular note is that the Az-group can be selectively introduced on diol systems and removed in the presence of esters, such as acetates. 相似文献
52.
Scheurer A Gieb K Alam MS Heinemann FW Saalfrank RW Kroener W Petukhov K Stocker M Müller P 《Dalton transactions (Cambridge, England : 2003)》2012,41(12):3553-3561
Reaction of nickel(II) chloride hexahydrate with N-n-butyldiethanolamine H(2)L (3) in the presence of LiH in anhydrous THF leads to the formation of the unique octanuclear chloro-bridged nickel(II) double cubane [({Ni(II)(4)(μ(3)-OH)Cl(3)(HL)(3)}μ(2)-Cl)(2)] (4) in 57% yield. According to single crystal X-ray structure analysis, complex 4·4CH(2)Cl(2) possesses a [({Ni(4)(μ(3)-OH)(μ(3)-O)(3)(OH)(3)(N)(3)(Cl)(3)}μ(2)-Cl)(2)] core and crystallizes in the monoclinic space group P2(1)/c with a = 18.292(2), b = 19.8972(5), c = 23.295(2) ?, β = 98.408(6)°, V = 8387.3(8) ?(3), and four molecules in the unit cell. The analysis of the SQUID magnetic susceptibility data identified 4 as a weakly coupled dimer (J(1) = 14.5 K, J(2) = -0.6 K) with a ground state of S = 0, resulting from two S = 4 states of each {Ni(4)} subunits. Although complex 4 does not show an ac out-of-phase signal in a zero dc field at temperatures of 1.8 K and higher, low-temperature magnetization measurements revealed that complex 4 is a single-molecule magnet and shows hysteretic magnetization characteristics with typical temperature and sweep-rate dependencies. The eye-catching feature of complex 4 is the presence of two different blocking temperatures (0.9 K around zero field and 1.3 K at higher fields). The origin of this highly unusual behavior can be assigned to the dimer-nature of the interaction between the two S = 4 units. Furthermore STM and current imaging tunnelling spectroscopy (CITS) were performed on aggregates of 4 drop-coated on highly oriented pyrolytic graphite (HOPG) surfaces. CITS measurements show a strong contrast in the area of the nickel centers and a HOMO-LUMO gap of approximately 0.8 V. 相似文献
53.
The products formed by the hydroxyl radical initiated photo-oxidation of diethyl ether have been investigated by irradiating synthetic air mixtures containing diethyl ether and nitrous acid in amounts approaching ppbv levels in a smog chamber. The decay of reactants and formation of products were monitored by gas chromatography, HPLC, and by chemiluminescent analysis. The major products are ethyl formate and formaldehyde, minor products include ethyl acetate, acetaldehyde, peroxyacetyl nitrate, methyl nitrate, and ethyl nitrate. The products observed arise from the decomposition reactions of the 1-ethoxyethoxy radical and its reaction with oxygen. One molecule of nitric oxide appears to be oxidized per molecule of diethyl ether reacted. This is lower than would be expected on the basis of the proposed mechanism and possible reasons for the discrepancy are discussed. © 1993 John Wiley & Sons, Inc. 相似文献
54.
A new analytical Liouville-space representation of the time-propagator under magic angle spinning (MAS) is introduced using the formalized quantum Floquet theory. This approach has the advantage that it is applicable to the analysis of any type of NMR experiment where MAS is combined with multiple-pulse excitation. General relationships describing the spectral parameters in multiple-quantum (MQ) MAS spectra are derived in this representation. Their use is illustrated with an application to double-quantum (DQ) NMR spectra of dipolar-coupled multi-spin systems. Corresponding to the separation of the MAS time-propagator into a rotor modulated and a dephasing component, two distinct mechanisms for DQ excitation are identified. One of them exploits the rotor-modulated component to excite DQ coherences through dipolar-recoupling techniques, which are familiar for spin pairs. Analytical expressions of the integral intensities and linewidths in the resulting DQ sideband pattern are derived in the form of power series expansions of the inverse rotor frequency, of which coefficients depend on structural parameters. In a multi-spin system they can most reliably be extracted in the fast spinning regime. The other mechanism exploits the dephasing component, which is characteristic to multi-spin systems only. This is shown to give rise to DQ coherences by free evolution at full rotor periods. The possibility to exploit it for selective excitation of higher order MQ coherences is discussed. In either case, the dephasing component also leads to residual broadening. The main results of the theoretical developments are demonstrated experimentally on adamantane. 相似文献
55.
Mark T McAuley Rose Anne Kenny Thomas BL Kirkwood Darren J Wilkinson Janette JL Jones Veronica M Miller 《BMC neuroscience》2009,10(1):26
Background
The hippocampus is essential for declarative memory synthesis and is a core pathological substrate for Alzheimer's disease (AD), the most common aging-related dementing disease. Acute increases in plasma cortisol are associated with transient hippocampal inhibition and retrograde amnesia, while chronic cortisol elevation is associated with hippocampal atrophy. Thus, cortisol levels could be monitored and managed in older people, to decrease their risk of AD type hippocampal dysfunction. We generated an in silicomodel of the chronic effects of elevated plasma cortisol on hippocampal activity and atrophy, using the systems biology mark-up language (SBML). We further challenged the model with biologically based interventions to ascertain if cortisol associated hippocampal dysfunction could be abrogated. 相似文献56.
W. Stocker 《Zeitschrift für Physik A Hadrons and Nuclei》1966,193(2):310-317
In order to obtain a regular but energy-dependent nuclear potential, the following modification of the separation method ofMoszkowski andScott is used: we replace the nuclear potentialv c (r) by a long-range potentialv l (r)=v c (r) Θ(r? d 0) together with a short-range energy dependent repulsionv s =h(k) Θ(r c ?r), whered 0 is the separation distance for vanishing energy andr c is the hard-core radius. The potentialv=v s +v l (r) is fitted to theS-wave scattering data and the binding energy of the deuteron.h(k) turns out to be almost proportional to the scattering energyE rel for energiesE rel<150 MeV. 相似文献
57.
The mean free path of nucleons in heavy ion collisions is most essential for the development of nuclear collective phenomena. We discuss the effect of the nucleon Fermi motion in nuclei for shortening the mean free path. This Fermi motion together with the prior Pauli effect makes nuclei nontransparent for heavy ions in the medium energy domain. 相似文献
58.
The angular distributions of the elastic scattering of 17, 18O on 12, 13C were measured at c.m. energies between 12.6 to 14.0 MeV. A rise of the cross section at backward angles was observed. Standard optical-model fits were found to reproduce reasonably well the forward part of the cross section, but fail at backward angles. Possible contributions of first-order cluster exchange and compound-nucleus reactions are discussed. Excitation functions of various exit channels in the 18O + 12C system were measured at backward angles in the energy range of 12.0 to 14.8 MeV c.m. No significant correlation was found between any of these cross sections. 相似文献
59.
YS Moroz S Demeshko M Haukka A Mokhir U Mitra M Stocker P Müller F Meyer IO Fritsky 《Inorganic chemistry》2012,51(14):7445-7447
A dodecanuclear 3 × [2 × 2] nickel(II) complex has been obtained via the self-association of three tetranuclear [2 × 2] molecular grids. X-ray diffraction shows a "propeller-like" structure of the [Ni(4)](3) scaffold with a central μ(3)-hydroxide. The dodecanuclear species remains stable in solution and can be deposited without decomposition on highly ordered pyrolytic graphite surfaces. 相似文献
60.