全文获取类型
收费全文 | 1555篇 |
免费 | 29篇 |
国内免费 | 8篇 |
专业分类
化学 | 812篇 |
晶体学 | 10篇 |
力学 | 11篇 |
数学 | 205篇 |
物理学 | 554篇 |
出版年
2020年 | 21篇 |
2019年 | 15篇 |
2016年 | 30篇 |
2015年 | 23篇 |
2014年 | 24篇 |
2013年 | 45篇 |
2012年 | 36篇 |
2011年 | 56篇 |
2010年 | 42篇 |
2009年 | 30篇 |
2008年 | 43篇 |
2007年 | 61篇 |
2006年 | 52篇 |
2005年 | 38篇 |
2004年 | 41篇 |
2003年 | 39篇 |
2002年 | 34篇 |
2001年 | 30篇 |
2000年 | 31篇 |
1999年 | 17篇 |
1998年 | 18篇 |
1997年 | 21篇 |
1996年 | 25篇 |
1995年 | 34篇 |
1994年 | 36篇 |
1993年 | 27篇 |
1992年 | 24篇 |
1991年 | 21篇 |
1990年 | 22篇 |
1989年 | 22篇 |
1988年 | 20篇 |
1987年 | 19篇 |
1986年 | 24篇 |
1985年 | 32篇 |
1984年 | 25篇 |
1983年 | 12篇 |
1982年 | 21篇 |
1981年 | 27篇 |
1980年 | 23篇 |
1979年 | 20篇 |
1978年 | 24篇 |
1977年 | 28篇 |
1976年 | 14篇 |
1975年 | 18篇 |
1974年 | 19篇 |
1973年 | 11篇 |
1972年 | 12篇 |
1969年 | 13篇 |
1967年 | 12篇 |
1930年 | 12篇 |
排序方式: 共有1592条查询结果,搜索用时 31 毫秒
51.
K. Hofmann 《Fresenius' Journal of Analytical Chemistry》1970,250(4):256-259
Zusammenfassung Im Hinblick auf die Bestimmung von Thiolgruppen in Muskelgewebe und Fleischerzeugnissen mit Hilfe der amperometrischen Titration mit Silbernitrat wurde der Einfluß folgender Salze, die in dem Untersuchungsmaterial enthalten sein können, geprüft: CaCl2, MgCl2, ZnCl2, MnSO4, CuSO4, FeCl3, NaCl, KCl, NaNO2, Na3PO4, ATP und Lactat. Diese Salze wurden vor der Titration zu SH-Glutathion in 0,14 M Tris-Puffer pH 7,4 zugegeben. CuSO4 verursachte eine starke Erniedrigung des SH-Gehaltes der Lösungen, die — bei nicht zu hoher Konzentration an CuSO4 — durch Zusatz von 0,5% ÄDTA verhindert werden konnte. In Gegenwart von MnSO4 trat ebenfalls eine Abnahme der vorgelegten SH-Menge ein. NaCl verursachte in Konzentrationen von 0,1 M und höher abnorm verlaufende Titrationskurven, wobei die graphische Ermittlung des Äquivalenzpunktes Schwierigkeiten bereitete. Die Titration von SH-Gruppen in 0,6 M KCl-Lösung ergab ebenfalls abweichende Titrationskurven, die das Ergebnis verfälschten. Die übrigen untersuchten Salze übten bei den angewendeten Konzentrationen (bis 3 · 10–4 M) keinen signifikanten Einfluß auf die amperometrische Titration der SH-Gruppen aus.
Effect of salts on the amperometric titration of thiol groups with silver nitrate
The following salts, which can be present in animal tissue and meat products were studied: CaCl2, MgCl2, ZnCl2, MnSO4, CuSO4, FeCl3, NaCl, KCl, NaNO2, Na3PO4, ATP or lactate. They were added to reduced glutathion in 0.14 M tris buffer pH 7.4 before titration. CuSO4 caused a strong decrease of the SH content. This effect could be inhibited by the addition of 0,5% of EDTA in the presence of low concentrations of the copper salt. MnSO4 also decreased the SH content. High concentrations of sodium chloride (0.1 M and more) caused a change in the normal titration curve and, therefore, made the determination of the endpoint difficult. Titration in 0.6 M KCl solution also yielded abnormal curves. The remaining salts investigated had no influence on the amperometric titration of SH groups in the concentrations used (up to 3×10–4 M).相似文献
52.
We have extended to molecules a density functional previously parametrized for atomic computations. The Coulomb-hole–Hartree–Fock functional, introduced by Clementi in 1963, estimates the dynamical correlation energy by the computations of a Hartree–Fock-type single-determinant wave function, where the Hartree–Fock potential was augmented with an effective potential term, related to a hard Coulomb hole enclosing each electron. The method was later revisited by S. Chakravorty and E. Clementi [Phys. Rev. A 39 , 2290 (1989)], where a Yukawa-type soft Coulomb hole replaced the previous hard hole; atomic correlation energies, computed for atoms with Z = 2 to Z = 54 as well as for a number of excited states, validated the method. In this article, we parametrized a function, which controls the width of the soft Coulomb hole, by fitting the first and second atomic ionization potentials of the atoms with 1 ? Z ? 18. The parametrization has been preliminarily validated by computing the dissociation energy for a number of molecules. A few-determinant version of the Coulomb-hole–Hartree–Fock method, necessary to account for the nondynamic correlation corrections, is briefly discussed. © 1994 John Wiley & Sons, Inc. 相似文献
53.
Ba3N2 reacts at 950°C under pure N2 with Zr to yield dark red, air-sensitive Ba[ZrN2]. This new compound crystallizes in the tetragonal space group P4/nmm with a = 416.10(2), c = 839.2(1) pm and Z = 2. The crystal structure was solved and refined using X-ray and neutron powder diffraction data. In the nitrido zirconate [ZrN2]2? the Zr atoms exhibit a square-pyramidal coordination by five N atoms at distances of 201(3) and 220.2(2) pm. The pyramids share all the edges in the basal plane to form layers parallel to (001) with their apices alternately pointing up and down. The Ba2+ cations are integrated into these layers at the levels of the pyramidal apices. The structure can be interpreted as a stuffed PbFCl type. Ba2[NbN3] is formed by the reaction of Ba3N2 and NbN or of Ba and Nb at 1 000°C under N2. Isostructural to Ba2[TaN3] it crystallizes in the monoclinic space group C2/c with a = 613.2(3), b = 1 176.8(3), c = 1 322.9(4) pm, β = 91.65(2)°, Z = 8. The nitrido niobate anions form chains of corner sharing NbN4 tetrahedra with distances Nb? N between 188(1) and 199.9(9) pm. 相似文献
54.
Jeffery B. Press Corris M. Hofmann Gretchen E. Wiegand Sidney R. Safir 《Journal of heterocyclic chemistry》1982,19(2):391-394
Pyrido[3,2-b]thieno[3,4-e][1,4]diazepines ( 1a-d ) were synthesized to investigate their potential CNS activity. Synthesis of the desired ring system was effected by condensation of 2,3-diaminopyridine ( 3 ) with methyl tetrahydro-4-oxo-3-thiophenecarboxylate ( 4 ). Structural assignment of the major condensation product 5 was determined by comparison of 1H nmr absorptions of 5 with those of related methyl lactam derivatives 11 and 14. A discussion of the possible mechanism leading to 5 in preference to isomeric lactam 6 is presented. Biological evaluation of 1a-d revealed no interesting properties. 相似文献
55.
Hofmann A Pelletier M Michot L Stradner A Schurtenberger P Kretzschmar R 《Journal of colloid and interface science》2004,271(1):163-173
Hydrous ferric oxides (HFO) are efficient sorbents for inorganic and organic pollutants and therefore have great potentials in environmental science and engineering applications. Freezing and thawing of HFO suspensions leads to the formation of dense HFO aggregates. It facilitates the handling and increases the drying rate of HFO. In this study, we used a combination of pycnometry, gas adsorption (N(2) gas, water vapor), and small-angle neutron scattering (SANS) to characterize the porosity and pore size distribution of dense HFO aggregates formed by freezing dialyzed HFO suspensions at -25 degrees C and thawing them at room temperature. The crystallinity of the HFO, which was a 2-line ferrihydrite, was not affected by this treatment. Wet sieving and laser diffraction analysis showed that the dense HFO aggregates had a unimodal size distribution with an average diameter of 235+/-35 microm. Increasing the freezing rate by cooling with liquid N(2) (-196 degrees C) resulted in much smaller aggregates with an average diameter of 20 microm. Adding NaNO(3) electrolyte to the HFO suspensions prior to freezing also resulted in the formation of smaller aggregates. The dense HFO aggregates formed at -25 degrees C had a porosity of 0.73+/-0.02 ll(-1). SANS revealed a unimodal size distribution of pores, with an average pore diameter of 2.0 nm. The diameter of the HFO crystallites was estimated by transmission electron microscopy to be 1.9+/-0.5 nm. Geometrical considerations taking into account the unit particle and average pore size suggest that the crystallites retain 1-2 layers of hydration water during the coagulation induced by freezing. Analysis by N(2) gas adsorption showed that drying the dense HFO aggregates induced a reduction in porosity by about 25% and shifted the pore size distribution to smaller diameters. Rewetting during water vapor adsorption did not induce significant changes of the aggregate structure. The specific surface area of the dry HFO aggregates was between 320 and 380 m(2)g(-1). 相似文献
56.
Ford S Hofmann M Morley CP Roberts JL Di Vaira M 《Organic & biomolecular chemistry》2005,3(21):3990-3995
Bis(1,2,3,4-tetramethylcyclopentadienyl)selane (1) has been prepared by the reaction of tetramethylcyclopentadienyllithium (Cp(t)Li) with selenium bis(diethyldithiocarbamate). Treatment of Cp(t)Li with elemental selenium, followed by air oxidation, led to loss of the allylic hydrogen atom, and formation of the novel tricyclic compound 1,4,5,6,7,10,11,12-octamethyltricyclo[7.3.0.0]-2,8-diselenadodeca-3,5,9,11-tetraene (2). The sulfur analogue of 2 has been obtained by a similar procedure. The X-ray crystal structures of compounds 1 and 2 have been determined, and the molecular geometry observed for has been probed using DFT calculations. 相似文献
57.
Zusammenfassung Die Anwendbarkeit des von uns vorgeschlagenen Verfahrens1 für die Bestimmung von Sauerstoff im Mikromaßstab setzt geringfügige Veränderungen der Methode voraus (es wurde von dem Absaugen der überschüssigen Flüssigkeit über dem Niederschlag abgesehen und bei einigen Lösungen andere Konzentrationen verwendet), die aber das Prinzip der Methode nicht verändern.Bei visueller Indikation ist die Bestimmung von 0,5 mg O2/l als untere Grenze mit einer Genauigkeit von 5% rel. möglich. Die Titration niedrigerer Sauerstoffkonzentrationen ist zwar möglich, aber mit grundsätzlich größeren Fehlern.Bei biamperometrischer oder potentiometrischer Indikation ist die Bestimmung von 10g O2/l möglich. Für die Bestimmung geringerer Mengen sind diese elektronischen Methoden nicht mehr geeignet. Bei der Potentiometrie ist im Äquivalenzbereich eine kaum meßbare Potentialänderung wahrzunehmen, auch bei biamperometrischen Titrationen war es nicht möglich, eine brauchbare Kurve für die Bestimmung des Äquivalenzpunktes zu erzielen.Bei der Bestimmung des in Mikromengen gelösten Sauerstoffes sind die Vorteile der beschriebenen Semimikromethode gegenüber der jodometrischen Methode nachWinkler gewährleistet. Die Technik der Probenahme von Wasser mit sehr kleinen Sauerstoffkonzentrationen, die einen maßgebenden Einfluß auf die Genauigkeit der Bestimmung hat, bleibt die gleiche, wie bei anderen Methoden.
Summary The applicability of the process1 proposed by us for the determination of oxygen on the micro scale presupposes minor modifications of the method (the drawing off of the excess fluid over the precipitate was omitted and solutions of other concentrations employed for some solutions) but these did not alter the basic principle of the procedure.When visual indication is employed, it is possible to determine 0.5 mg O2/l as the lower limit with an accuracy of 5% (relative). The titration of lower oxygen concentrations is actually feasible but with fundamentally greater errors.When biamperometric or potentiometric indication is used, it is possible to determine as little as 10g O2/l. These electronic methods are no longer suitable for the determination of smaller amounts. In the case of potentiometry, a scarcely measurable potential difference was to be seen in the equivalence region, and likewise in the biamperometric titrations it was not possible to obtain a usable curve for the determination of the equivalence point.The advantages of the semimicro method described as compared with the iodometric method ofWinkler are shown definitely in the determination of the oxygen dissolved in micro quantities. The technique of taking samples of water that has a very small concentration of oxygen, which may have a decisive influence on the preciseness of the determination, is the same as in the other procedures.
Résumé L'applicabilité du procédé1 qui est proposé ici pour le dosage de l'oxygène à l'échelle micro suppose de légères modifications de la méthode (on n'a pas cherché à aspirer l'excès de liquide sur le précipité et on a utilisé d'autres concentrations pour quelques solutions); le principe en reste toutefois inchangé.Avec indication visuelle, le dosage de 0,5 mg O2/l comme limite inférieure avec une précision de 5% (rel.) est possible. Le titrage de concentrations plus faibles d'oxygène peut encore s'effectuer, mais avec de plus grandes erreurs de base.Avec indication biampérométrique ou potentiométrique, il est possible de doser 10g O2/l. Pour le dosage de quantités plus faibles, ces méthodes électroniques ne conviennent plus. Par potentiométrie, on peut déceler dans le domaine équivalent une modification à peine mesurable du potentiel, mais par titrages biampérométriques, il devient impossible d'atteindre une courbe utilisable pour la détermination du point équivalent.Dans le cas du dosage de l'oxygène dissous en microquantités, les avantages des semimicrométhodes qui sont décrites, par rapport à la méthode iodométrique deWinkler, sont certains. La technique du prélèvement de l'eau pour de très petites concentrations en oxygène, qui a une influence déterminante sur l'exactitude du dosage, reste la même que dans les autres méthodes.相似文献
58.
A New Oxogermanate: Li8GeO6 ? Li8O[GeO4] Transparent colourless single crystals of Li8GeO6(P63cm, a = 550.09(8), c = 1072.2(3) pm, Z = 2; 4-circle-diffractometer Siemens AED 2, MoKα; 326 Io(hkl), R = 2.4%, Rw = 2.0%), have been prepared. As by-product we always got colourless isometric single crystals of Li4GeO4. For the first time we could grow single crystals of Li8SiO6 of suitable size and quality. Our structure refinement confirms the assumed structure model [2]: Li8GeO6 and Li8SiO6 are isotypic with Li8CoO6[3] (Li8SiO6: a = 542.43(8), c = 1062.6(2) pm, Z = 2; 4-circle-diffractometer Siemens AED 2, MoKα; 306 Io(hkl), R = 3.6%, Rw= 3.0%). The known crystal structure of Li4GeO4 [4] is confirmed and refined (Cmcm, a = 776.6(2), b = 735.7(3), c = 604.9(2) pm, Z = 4; 4-circle-diffractometer Siemens AED 2, MoKα, 298 Io(hkl), R = 1.9%, Rw = 1.4%). The Madelung Part of Lattice Energy, MAPLE, and Effective coordination-Numbers, ECoN, these via Mean Fictive Ionic Radii, MEFIR, are calculated. 相似文献
59.
Reactions of Ferrocenol and 1,1′-Ferrocendiol with Cyclotriphosphazenes, P3N3F6 and P3N3Cl6 The hexahalogeno-cyclotriphosphazenes, P3N3X6 (X ? F ( 1 a ), Cl ( 1 b )), react with ferrocenol (FcOH) in a molar ratio 1 : 1 to give the ferrocenoxy derivatives, FcO[P3N3X5] (X ? F ( 3 a ), Cl ( 3 b )); in an analogous manner the tetrameric ring P4N4Cl8 ( 2 b ) is converted to FcO[P4N4Cl7] ( 4 b ). 1 Abkürzungen: Fc = Ferrocenyl, (C5H5)Fe(C5H4?); fc = 1,1′-ferrocendiyl, Fe(C5H4?)2; rc = 1,1′-ruthenocendiyl, Ru(C5H4?)2. Fluorphosphazene werden mit a , Chlorphosphazene mit b gekennzeichnet. With 1,1′-ferrocenediol, (fc(OH) 2 ), the cyclo triphosphazenes react in a molar ratio 1 : 1 to produce fcO 2 [P 3 N 3 X 4 ] (X ? F ( 5 a ), Cl ( 5 b )). According to the x-ray structure analysis, the 1,1′-ferrocenediolato group in 5 a , b is bound to two different phosphorus atoms. On the contrary, the 1,1′-ferrocenedithiolato- and 1,1′-ferrocenediselenolato units in fcS 2 [P 3 N 3 X 5 ] (X ? F ( 6 a ), Cl ( 6 b )) and fcSe 2 [P 3 N 3 X 5 ] (X ? F ( 7 a ), Cl ( 7 b )) are attached to only one phosphorus atom, and spirocyclic 1,3-dichalcogena-2-phospha-[3]ferrocenophanes are formed. All new products have been characterized on the basis of their 1 H, 13 C and 31 P NMR as well as EI mass spectra. The molecular structures of 5 a , b and 6 a have been determined by x-ray structure analyses. 相似文献
60.
Gaetano Riggio Wolfgang Herbert Hopff Alfred Andr Hofmann Peter Gaudenz Waser 《Helvetica chimica acta》1983,66(4):1039-1045
Hydroxyphenyl-1-methylpyridinium-iodide as Potential Reactivators of Acetylcholinesterase Poisoned with Organophosphorus Compounds . It was our aim to reactivate acetylcholinesterase poisoned with sarin. We synthesized 2-(o-hydroxyphenyl)-1-methylpyridinium-iodide ( 9 ), 2-(p-hydroxyphenyl)-1-methylpyridinium-iodide ( 19 ) and 4-(o-hydroxyphenyl)-1-methylpyridinium-iodide ( 14 ) as potential reactivators. All substances showed moderate toxicity against mice; their reactivity potency in vitro and in vivo was negligible. 相似文献