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571.
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574.
M. Steyer H. -U. Krebs H. C. Freyhardt 《Zeitschrift für Physik B Condensed Matter》1987,66(3):317-323
For melt-spun metallic CoxZr100–x glasses (22x53) the total pair correlation functionG(r) has been derived from X-ray diffraction measurements. In most samples, the first maximum ofG(r) can be resolved into two single maxima belonging to the Co–Zr and Zr–Zr nearest-neighbor distance, respectively. Partial correlation numbers are estimated in the whole concentration range. The density of the samples has been measured with a buoyancy method. It is compared to those of otherM–Zr glasses (M=Fe, Ni, Cu) and to predictions deduced from Miedema's model of alloy formation. 相似文献
575.
LiBaBS3 and LiBaB3S6: Two New Quaternary Thioborates with Trigonally Coordinated Boron LiBaBS3 (P21/c; a = 7.577(2) Å, b = 8.713(2) Å, c = 8.687(2) Å, β = 116.22(2)°; Z = 4) und LiBaB3S6 (Cc; a = 15.116(3) Å, b = 8.824(2) Å, c = 8.179(2) Å, β = 117.46(3)°; Z = 4) were prepared by reaction of stoichiometric amounts of the metal sulfides, boron, and sulfur at 750°C. The anionic part of the structure of the orthothioborate LiBaBS3 consists of isolated planar [BS3]3? anions. The crystal structure of the metathioborate LiBaB3S6 contains [B3S6]3? anions formed by six-membered B3S3 rings with three exocyclic sulfur atoms. The metal cations are situated between the anion units leading to a ninefold sulfur coordination of the barium atoms and to a fivefold (LiBaBS3) and fourfold (LiBaB3S6) coordination of the lithium atoms. 相似文献
576.
Norbert Strter William N. Lipscomb Thomas Klabunde Bernt Krebs 《Angewandte Chemie (International ed. in English)》1996,35(18):2024-2055
Numerous studies, both in enzymatic and nonenzymatic catalysis, have been undertaken to understand the way by which metal ions, especially zinc ions, promote the hydrolysis of phosphate ester and amide bonds. Hydrolases containing one metal ion in the active site, termed mononuclear metallohydrolases, such as carboxypeptidase. A and thermolysin were among the first enzymes to have their structures unraveled by X-ray crystallography. In recent years an increasing number of metalloenzymes have been identified that use two or more adjacent metal ions in the catalysis of phosphoryl-transfer reactions (R-OPO3 + R′-OH → R′-OPO3 + R-OH; in the case of the phosphatase reaction R′-OH is a water molecule) and carbonyl-transfer reactions, for example, in peptidases or other amidases. These dinuclear metalloenzymes catalyze a great variety of these reactions, including hydrolytic cleavage of phosphomono-, -di- and -triester bonds, phosphoanhydride bonds as well as of peptide bonds or urea. In addition, the formation of the phosphodiester bond of RNA and DNA by polymerases is catalyzed by a two-metal ion mechanism. A remarkable diversity is also seen in the structures of the active sites of these di- and trinuclear metalloenzymes, even for enzymes that catalyze very similar reactions. The determination of the structure of a substrate, product, stable intermediate, or a reaction coordinate analogue compound bound to an active or inactivated enzyme is a powerful approach to investigate mechanistic details of enzyme action. Such studies have been applied to several of the metalloenzymes reviewed in this article; together with many other biochemical studies they provide a growing body of information on how the two (or more) metal ions cooperate to achieve efficient catalysis. 相似文献
577.
Joachim Krebs Volkhard Helms Christian Griesinger Ernesto Carafoli 《Helvetica chimica acta》2003,86(11):3875-3888
Calcium may have a static, structure‐stabilizing role in biological organs like the bones and the teeth, or may fulfill a dynamic function in cells as a regulator of signal‐transduction pathways. This is made possible by the properties of the Ca2+ ion (e.g., high dehydration rate, great flexibility in coordinating ligands, largely irregular geometry of the coordination sphere). Since Ca2+ is a universal carrier of signals, the control of its homeostasis is of central importance for the organism. It involves exchanges between the skeleton (which is the major calcium reservoir) and the extracellular and intracellular fluids. It also involves the intestine and the kidney, the organs of Ca absorption and release, respectively. The highly integrated homeostasis process consists of a number of hormonally controlled feedback loops, and an elaborate system of membrane channels, exchangers, and pumps that control the Ca2+ flux into and out of cells. 相似文献
578.
Madsen GK Krebs FC Lebech B Larsen FK 《Chemistry (Weinheim an der Bergstrasse, Germany)》2000,6(10):1797-1804
The electron density distribution of the molecular pyroelectric material phosphangulene has been studied by multipolar modeling of X-ray diffraction data. The "in-crystal" molecular dipole moment has been evaluated to 4.7 D corresponding to a 42% dipole moment enhancement compared with the dipole moment measured in a chloroform solution. It is substantiated that the estimated standard deviation of the dipole moment is about 0.8 D. The standard uncertainty (s.u.) of the derived dipole moment has been derived by splitting the dataset into three independent data-sets. A novel method for obtaining pyroelectric coefficients has been introduced by combining the derived dipole moment with temperature-dependent measurements of the unit cell volume. The derived pyroelectric coefficient of 3.8(7) x 10-6 Cm 2K-1 is in very good agreement with the measured pyroelectric coefficient of p = 3 +/- 1 x 10-6 Cm-2 K-1. This method for obtaining the pyroelectric coefficient uses information from the X-ray diffraction experiment alone and can be applied to much smaller crystals than traditional methods. 相似文献
579.
Adam W Degen HG Krebs O Saha-Möller CR 《Journal of the American Chemical Society》2002,124(44):12938-12939
The ene reaction of 4-nitronitrosobenzene (ArNO), N-phenyl-1,2,4-triazoline-3,5-dione (PTAD), and singlet oxygen (1O2) with the optically active tiglic-acid derivatives of Oppolzer's bornane-derived sultam affords the respective ene products regioselectively in excellent diastereoselectivity (de up to 99%) and in good yield (55-90%). The enophiles ArNO and PTAD give with the methyl-substituted substrate exclusively the like-configured ene adduct, while 1O2 leads to an 83:17 diastereomeric mixture. With the sterically more demanding isopropyl-substituted derivative even the smallest enophile 1O2 forms exclusively the like diastereomer. The high diastereoselectivity is rationalized in terms of the proper conformational alignment of the substrate and a preferred enophilic attack from the C(beta)-re face of the double bond. This concept offers an efficient synthetic route to enantiomerically pure nitrogen- and oxygen-functionalized acrylic acid derivatives. 相似文献
580.
The synthesis of the novel asymmetric ligand 1-[bis(2-pyridylmethyl)amino]-3-[2-(2-pyridyl)ethoxy]-2-hydroxypropane (ppepOH) is reported. The ligand is suitable to form asymmetric dinuclear complexes with various transition metal ions. As an example, the synthesis and X-ray structure analysis of the dinickel(II) complex [Ni(2)(ppepO)(C(6)H(5)COO)(2)(CH(3)COOH)]ClO(4).C(4)H(10)O are described. The complex crystallizes in the monoclinic space group P2(1)/n with the following unit cell parameters: a = 13.704(10) ?, b = 14.849(10) ?, c = 22.697(14) ?, beta = 96.80(5) degrees, Z = 4. The nickel(II) ions are bridged by the alkoxy donor of the ligand and two benzoate anions. The hexadentate ligand leaves a free coordination site at one of the nickel(II) ions, which is occupied by a monodentate coordinated acetic acid molecule. The coordination of the neutral acetic acid molecule is selectively stabilized by a strong intramolecular hydrogen bond of the acidic proton to the &mgr;-alkoxo bridge of the dinuclear complex. The asymmetric complex was prepared in order to mimic the substrate uptake in the dinuclear active site of ureases. The magnetic and spectroscopic properties of the complex were determined and related to those of the urease enzymes. 相似文献