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81.
E. I. Golovenchits V. A. Sanina A. A. Levin Yu. I. Smolin Yu. F. Shepelev 《Physics of the Solid State》1997,39(9):1425-1432
Local ion displacements and their anharmonicity in Eu2CuO4 and La2CuO4 crystals are studied by high-precision x-ray diffractometry. A symmetry analysis and the temperature behavior of the probability
density function of Cu2+ ions reveal a degenerate ground orbital state in the Eu2CuO4 crystal (tetragonal doublet d
xz, d
yz). The pattern and anharmonicity of the local displacements are found to be determined not only by lattice vibrations but
by 2D antiferromagnetic spin fluctuations in the CuO2 sheet as well. By contrast, in the La2CuO4 crystal the orbital ground state of the Cu2+ ion is a singlet (d
z
2). The pattern of Eu3+ local displacements and their effect on the properties of the Cu subsystem in the Eu2CuO4 crystal are also studied.
Fiz. Tverd. Tela (St. Petersburg) 39, 1600–1608 (September 1997) 相似文献
82.
Jonathan M. Levin Lawrence L. Wald Marc J. Kaufman Marjorie H. Ross Luis C. Maas Perry F. Renshaw 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1998,130(2):292-295
Residual effects of an initial bolus of gadolinium contrast agent have been previously demonstrated in sequential dynamic susceptibility contrast MR experiments. While these residual effects quickly reach a saturation steady state, their etiology is uncertain, and they can lead to spurious estimates of hemodynamic parameters in activation experiments. The possible influence ofT1effects is now investigated with experiments in whichT1weighting is varied as well as with serial regionalT1measurements. Little evidence for significant residualT1effects is found, suggesting instead that susceptibility effects underlie these observations. An initial saturation dose of contrast agent minimizes this effect. 相似文献
83.
V. D. Blank V. M. Levin V. M. Prokhorov S. G. Buga G. A. Dubitskii N. R. Serebryanaya 《Journal of Experimental and Theoretical Physics》1998,87(4):741-746
The elastic properties of C60 fullerite samples synthesized under pressure P=13.0 GPa at high temperatures were investigated using acoustic microscopy. The velocities of longitudinal (c
L=17–26 km/s) and transverse (c
T=7.2–9.6 km/s) elastic waves in the samples were measured. It was established that the longitudinal sound velocity of ultrahard
fullerites is higher than that of any other known solid. The bulk modulus of these ultrahard samples is higher than that of
diamond and reaches a value greater than 1 TPa. The high bulk modulus, the relatively large shear moduli, and the substantial
Poisson ratio indicate that the structure of the ultrahard fullerites is fundamentally different from that of diamond.
Zh. éksp. Teor. Fiz. 114, 1365–1374 (October 1998) 相似文献
84.
Hirshfield J.L. Levin L.A. Danziger O. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1989,17(5):695-700
Requirements for economic plasma centrifuge isotope enrichment are reviewed. A distributed-discharge vacuum arc in chromium, as reported by A.I. Vasin et al. (1979), is described and a qualitative model for its operation is given. Experiments were performed on a modified distributed-discharge vacuum arc in which the parameters appear to be well suited for plasma centrifuge operation. A 2-m-long, 10-cm-diameter fully ionized DC chromium arc was produced with ion density and temperature in the range of 1013 cm-3 and 0.16 eV. Chromium mass throughput rates in the range of 1 g/min are reported, with continuous running times of up to 40 min., limited only by chromium supply 相似文献
85.
86.
Iain J. W. McKean Joanna C. Sadler Anibal Cuetos Amina Frese Luke D. Humphreys Gideon Grogan Paul A. Hoskisson Glenn A. Burley 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(49):17747-17752
A tandem enzymatic strategy to enhance the scope of C‐alkylation of small molecules via the in situ formation of S‐adenosyl methionine (SAM) cofactor analogues is described. A solvent‐exposed channel present in the SAM‐forming enzyme SalL tolerates 5′‐chloro‐5′‐deoxyadenosine (ClDA) analogues modified at the 2‐position of the adenine nucleobase. Coupling SalL‐catalyzed cofactor production with C‐(m)ethyl transfer to coumarin substrates catalyzed by the methyltransferase (MTase) NovO forms C‐(m)ethylated coumarins in superior yield and greater substrate scope relative to that obtained using cofactors lacking nucleobase modifications. Establishing the molecular determinants that influence C‐alkylation provides the basis to develop a late‐stage enzymatic platform for the preparation of high value small molecules. 相似文献
87.
V. S. Reznik Ya. A. Levin V. D. Akamsin I. V. Galyametdinova R. I. Pyrkin 《Russian Chemical Bulletin》2000,49(3):495-498
Primary alkylamines, β-aryloxyethylamines, orN-[β-(2-methoxyphenoxy)ethyl]-α,ω-diaminoalkanes react with alkyl divinyl phosphinates or phenyl(divinyl)phosphine oxide to give the corresponding 4-alkoxy(4-phenyl)-1-alkyl-4-oxo-1,4-azaphosphorinanes. Reactions of the latter with mono- or dihaloalkanes afford 4-phosphapiperidinium halides. 1,4-Azaphosphorinanes containing a β-aryloxyethyl fragment exhibit hypotensive activity. 相似文献
88.
Jochanan Blum Yaron Cohen Shulamit Levin Andrea Katschak Hansruedi Glatt 《Journal of heterocyclic chemistry》1998,35(1):39-43
Racemic K-region imines of benz[a]anthracene, 7-methylbenz[a]anthracene, chrysene and benzo[a]pyrene ( 1–4 , respectively) have been resolved by high performance liquid chromatography on a column packed with amylose tris(3,5-dimethylphenylcarbamate) on silica gel. The absolute configuration of the resolved aziridines was assigned by comparison of their circular dichromism spectra to those of the corresponding enantiomerically pure arene oxides. The mutagenicity of the enantiomers of the arene imines was investigated using Salmonella typhimurium strains TA98 and TA100. Although all arene imines investigated were potent mutagens, quantitative and qualitative differences in the mutagenic activity were observed between enantiomers. 相似文献
89.
Heterogeneous grafting on polyvinylchloride suspended in water was carried out using N-butyl-3-mercaptopropionamide as nucleophile. Over 50% graft was obtained by using a small amount of solvent as a swelling agent and tricapryl methyl ammonium chloride as a phase transfer catalyst. Elemental analysis of the grafted polymer shows that the chlorine displaced from the polymers is replaced by the thio-amide group. The above conclusion is supported by NMR and IR analysis. The kinetics of the chlorine displacement from PVC by the thio amide group obeys the Shell progressive mechanism. The rate at which an individual spherical particle reacts depends on the diffusion through the reacted layer. The grafted polymer is soluble in tetrahydrofuran or nitrobenzene. The films obtained from the grafted material are brittle due to excessive internal hydrogen bonding. The electrostatic charge which is a characteristic surface phenomena in PVC is diminished in the grafted polymer which may be due to the existence of the amide group near the surface. The amide groups attached to the side chains on the polymer may participate in various reactions, e.g., with epoxy resins. IR analysis of the cured film indicates the disappearance of the oxiran band at 913 cm?1 and an increase in the hydroxyl band around 3300 cm?1. Thus, grafting of amide groups on PVC enables us to further modify PVC by epoxy resins. 相似文献
90.
Proton, 13C, 6Li, and 15N NMR line-shape studies of exo,exo-1-trimethylsilyl-3-(dimethylethylsilyl)allyllithium-6Li complexed to [14N,15N]-N,N,N',N'-tetramethylethylenediamine (TMEDA) 2 as a function of temperature and of added diamine reveal the dynamics of three fast equilibrium reorganization processes. These are (with DeltaH values in kilocalories per mole and DeltaS values in entropic units): mutual exchange of lithium between two 2 molecules (6.3, -21), exchange of TMEDA between its free and complexed states (5.0 and -22), and first-order transfer of complexed ligand between the allyl faces (7.0 and -20). Intermediates that are dimeric in TMEDA are proposed for the first two of these reorganization processes. 相似文献