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21.
It is known that the deformations of the electronic charge density induced by lattice displacements play an important role in lattice dynamics. The multipole expansion of these deformations around the positions of both the cation and the anion leads to a rather general type of deformable shell model. This cluster deformation model can be applied not only to ionic crystals but also to a large variety of other materials like mixed-valence compounds. Taking TiN as an example, it is shown that the model is able to describe the very pronounced phonon anomalies in superconducting transitionmetal compounds and to reproduce the measured phonon dispersion curves within the experimental errors. 相似文献
22.
An extendeds–f model is used for the study of intermediate-valence 4f-systems, which have antiferromagnetic ground states in the normal-valence phase and fluctuate between a magnetic and a nonmagnetic configuration in the intermediate-valence phase. The local magnetic 4f-moment, the sublattice-magnetization as well as the Néel-temperatureT
N
are discussed in terms of thes–f hybridizationV and thef-level positionE
f
. Reduction of the energy gap betweenE
f
and conduction band simulates the influence of external pressure or of alloying with suitable impurities. IncreasingV or decreasing4f-(5d, 6s)-gap intensifies the electronic fluctuations, which have two competitive consequences. They tend to quench the local moments, but simultaneously enhance the coupling between them. This results in a typical behaviour of the Néel-temperature. Conditions for the coexistence of antiferro-magnetism and intermediate valence are derived. Comparison with experimental data for systems like Eu-metal and Eu(Pd1–x
Au
x
)2Si2 shows qualitative agreement. 相似文献
23.
The excited state dynamics of protonated adenine in the gas phase were investigated by femtosecond pump-probe transient mass spectroscopy. Adenine was protonated in an electrospray ionization source and transferred to a Paul trap. Two femtosecond laser pulses at 266 nm and 800 nm excited the lowest electronic pipi* state and probed the excited-state dynamics by monitoring ion fragment formation. The measured excited state decay is monoexponential with a lifetime shorter than 161 fs. This agrees with a theoretical prediction of very fast internal conversion via a conical intersection with the ground state. 相似文献
24.
25.
We investigate the influence of electron correlations on the temperature-dependence of the electronic structure of ferromagnetic bcc iron by use of a manybody evaluation of a generalized model of magnetism. The single-particle part of the model-Hamiltonian is taken from an LDA band structure calculation. The manybody interactions are described by only two parameters, an intraband Coulomb interactionU and an interband exchangeJ. WithU=1.8 eV andJ=0.2 eV the self-consistent model solution yields aT=0 moment of about 2.04 µB and a Curie-temperature of 1044K. Details of the magnetic behaviour of Fe can be traced back to a striking temperature variation of the quasiparticle density of states. A novel explanation for the experimentally-observed non-collapsing exchange splitting is demonstrated in terms of the temperature-dependent spectral density for wave-vectors near the -point. Typical differences in the magnetic behaviour of Fe and Ni are worked out. 相似文献
26.
Kubik S Kirchner R Nolting D Seidel J 《Journal of the American Chemical Society》2002,124(43):12752-12760
An artificial anion receptor is presented, in which two cyclohexapeptide subunits containing l-proline and 6-aminopicolinic acid subunits in an alternating sequence are connected via an adipinic acid spacer. This compound was devised to stabilize the 2:1 sandwich-type anion complexes that are observed when the two cyclopeptide moieties are not covalently connected and to obtain a 1:1 stoichiometry for these aggregates. Electrospray ionization mass spectrometry and NMR spectroscopic investigations showed that the bridged bis(cyclopeptide) does indeed form defined 1:1 complexes with halides, sulfate, and nitrate. ROESY NMR spectroscopy and molecular modeling allowed a structural assignment of the sulfate complex in solution. The stabilities of various anion complexes were determined by means of NMR titrations and isothermal titration microcalorimetry in 50% water/methanol. Both methods gave essentially the same quantitative results, namely stability constants that varied in the range 105-102 M-1 and decreased in the order SO42- > I- > Br- > Cl- > NO3-. This order was rationalized in terms of the size of the anions with the larger anions forming the more stable complexes because they better fit into the cavity of the host. The ability of sulfate to form stronger hydrogen bonds to the NH groups of the receptor, in addition to its slightly larger ionic radius with respect to iodide, causes the higher stability of the sulfate complex. No significant effect of the countercation on complex stability was observed. Furthermore, complex stability is enthalpically as well as entropically favored. A comparison of the iodide and sulfate complex stabilities of the ditopic receptor with those of a cyclopeptide that forms 1:1 anion complexes in solution showed that the presence of a second binding site increases complex stability by a factor of 100-350. 相似文献
27.
A. Koinecke R. Kreuzig M. Bahadir J. Siebers H. G. Nolting 《Fresenius' Journal of Analytical Chemistry》1994,349(4):301-305
According to the multiresidue analysis method DFG S 19, different approaches for the substitution of dichloromethane in the liquid/liquid partition step were investigated. Due to maximum residue limits (MRL), fortification experiments with 17 GC/ECD and GC/NPD detectable pesticides were carried out in concentrations of 1 and 0.1 mg/kg apples, 1 and 0.05 mg/kg tomatoes, 1 and 0.1 mg/kg lettuce, respectively. In supplementary greenhouse experiments, lettuce was cultivated and a mixture of 10 plant protection products was applied in commonly used amounts. In both cases, the comparison of pesticide recoveries shows that several less toxic solvents like cyclohexane, light petroleum and tertiary butyl methyl ether are suitable substitutes for the more toxic and persistent dichloromethane. 相似文献
28.
We investigate the coexistence of ferromagnetism and intermediate valence in 4f-systems, which have ferromagnetic ground states in the normal valence phase and fluctuate between a magnetic (J0) and a nonmagnetic (J=0) configuration in the intermediate valence phase. We use the well-knowns-f model, which is extended by a hybridization term and an intraatomicf-Coulomb-interaction, for systems with ferromagnetics-f exchange. The alloy analogy of this model is solved within a CPA-scheme. We inspect the stability of the localf-momentm
f
, the valencen
f
and relevant quasiparticle densities of states in terms of the Bloch bandwidthW (closely related to external pressure!), thef-level positionE
f
(alloying!), and the hybridization matrix elementV. 相似文献
29.
Sven Kramer Birte Nolting Andrej-Jakob Ott Christian Vogel 《Journal of carbohydrate chemistry》2013,32(7):891-921
ABSTRACT Glycosylation of the D-galacturonic acid ester derivatives 15 and 17, which are prepared directly from D-galacturonic acid, with the thioglycosides 28 and 32, derived from the same sugar, provides α(1→4)-linked dimers. The formation of the glycosidic linkage between the galacturonic acid moieties is best achieved by iodonium di-sym-collidine perchlorate promotion. Thus, the 4'-O-p-methoxybenzyl dimer 38 can be obtained in 64% yield. Partial deprotection of the 4'-O-position provided the glycosyl acceptor 36, which was coupled with the donor 32 to yield the α(1”→4')-linked trimer 39 (48%). Approximately 8% of the β(1”→4')-coupled isomer was observed in the 13C NMR spectrum of the reaction mixture. 相似文献
30.
ABSTRACT The partially deprotected trisaccharide 17 has been synthesized as an analogue of the repeating unit of the backbone of rhamnogalacturonan I. The trisaccharide 17 was obtained after prior selective derivatization of HO-3 and HO-4 of a rhamnopyranose cyanoethylidene glycosyl donor, followed by coupling with a tritylated galactopyranosyluronic acceptor (11), selective removal of the acetyl group at the O-2' position of the formed disaccharide 12, and glycosylation of the HO-2' position with methyl (ethyl 2,3-di-O-benzyl-4-O-p-methoxybenzyl-1-thio-β-D-galactopyranosid)uronate (14) providing methyl (methyl 2,3-di-O-benzyl-4-O-p-methoxybenzyl-α-D-galactopyranosyluronate)-(1→2)-(4-O-benzoyl-3-O-benzyl-α-L-rhamnopyranosyl)-(1→4)-(allyl 2,3-di-O-benzyl-β-D-galactopyranosid)uronate (15). Finally, palladium chloride catalyzed deallylation (16) and hydrogenolysis over Pd-C resulted in methyl (methyl α-D-galactopyranosyluronate)-(1→2)-(4-O-benzoyl-α-L-rhamnopyranosyl)-(1→4)-α/β D-galactopyranuronate (17). 相似文献