全文获取类型
收费全文 | 282788篇 |
免费 | 4617篇 |
国内免费 | 2529篇 |
专业分类
化学 | 153153篇 |
晶体学 | 4381篇 |
力学 | 12766篇 |
综合类 | 139篇 |
数学 | 31079篇 |
物理学 | 88416篇 |
出版年
2020年 | 2645篇 |
2019年 | 2938篇 |
2018年 | 3638篇 |
2017年 | 3616篇 |
2016年 | 5190篇 |
2015年 | 3386篇 |
2014年 | 5207篇 |
2013年 | 11886篇 |
2012年 | 9471篇 |
2011年 | 11411篇 |
2010年 | 8101篇 |
2009年 | 7869篇 |
2008年 | 10293篇 |
2007年 | 10470篇 |
2006年 | 9649篇 |
2005年 | 8726篇 |
2004年 | 7983篇 |
2003年 | 7159篇 |
2002年 | 7062篇 |
2001年 | 7660篇 |
2000年 | 5978篇 |
1999年 | 4606篇 |
1998年 | 4067篇 |
1997年 | 4009篇 |
1996年 | 3766篇 |
1995年 | 3478篇 |
1994年 | 3637篇 |
1993年 | 3302篇 |
1992年 | 3773篇 |
1991年 | 3753篇 |
1990年 | 3682篇 |
1989年 | 3531篇 |
1988年 | 3612篇 |
1987年 | 3501篇 |
1986年 | 3325篇 |
1985年 | 4399篇 |
1984年 | 4657篇 |
1983年 | 3888篇 |
1982年 | 4010篇 |
1981年 | 3940篇 |
1980年 | 3914篇 |
1979年 | 3886篇 |
1978年 | 4193篇 |
1977年 | 4113篇 |
1976年 | 4179篇 |
1975年 | 3854篇 |
1974年 | 3895篇 |
1973年 | 4054篇 |
1972年 | 2852篇 |
1971年 | 2380篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
871.
Zusammenfassung Es wurden eine Reihe von Substitutionsreaktionen mit entwässertem K3[Cr(NCS)6] und aromatischen Aminen, ohne Verwendung von Lösungsmitteln, durchgeführt. Es wurde beobachtet, da die sehr schwachen Basen mit pK-Werten 14–16, wie die isomeren Mono- und Dinitro-aniline, die Anthranil- und Sulfanilsäuren, für diese Substitutionsreaktionen ungeeignet sind. Die Alkyl-anilinderivate, wie daso-, m- undp-Toluidin (pK=10–11) bilden dagegen leicht reineckesalzähnliche Verbindungen: Amin· H[Cr(Toluidin)2(NCS)4]. Die Zusammensetzung der neuen Komplexanionen wurde bei einer Reihe von doppelten Umsetzungsreaktionen mit den Chlorhydraten einiger heterocyclischen Aminen festgestellt.Für die Klärung einiger Strukturfragen wurden spektrophotometrische Untersuchungen im UV und IR durchgeführt und die thermische Stabilität bzw. der Mechanismus des Pyrolysenvorganges auf thermogravimetrischem Wege verfolgt.
Mit 8 Abbildungen 相似文献
Substitution reactions of anhydrous K3[Cr(NCS)6] with aromatic amines without applying of solvents were studied.It was observed, that the very weak bases (pK-values 14–16, e.g. isomeric mono- and dinitro-anilines, the anthranilic and sulfanilic acids) are unsuitable for substitution reactions. The alkylaniline derivates e.g.o-, m- andp-toluidine (pK=10–11) form easily reineckesalt analogous compounds: Amin· ·H[Cr(toluidine)2(NCS)4]·The formula of the 3 new complex anions: [Cr(toluidine)2 (NCS)4]- was established by preparative methods, with a series of double decomposition reactions, using chlorohydrates of some heterocyclic amines for this purpose. From spectroscopical investigations (UV and IR) some structural problems are resolved and discussed.The thermal stability and the mechanism of pyrolysis of these salts was studied by thermogravimetric analysis.
Mit 8 Abbildungen 相似文献
872.
N. V. Bondarenko A. L. Shinkarenko G. I. Gerashchenko 《Chemistry of Natural Compounds》1973,6(4):449-451
Conclusions 1. On the basis of literature data on the structure of theVeratrum alkaloids, a definite law for the arrangement of the acyl radicals in these ester alkaloids and an interrelationship between the melting point and the structure of the substituent in the C7 position, and between the melting point and the types of amino alcohols upon which the ester alkaloids are based, has been found.2. The features mentioned may be used in studying the structure of theVeratrum ester alkaloids according to a scheme which we propose.Khimiya Prirodnykh Soedinenii, Vol. 6, No. 4, pp. 440–443, 1970 相似文献
873.
The application of an inductively coupled high-frequency plasma source to the determination of iodine, mercury, arsenic and selenium by atomic emission spectrometry at wavelengths less than 200 nm is described. Optimal conditions have been established, and the spectral interference effects at different atomic lines for each element have been investigated. With the type of instrumentation employed, the determination of iodine at 183.04 nm, mercury at 184.96 nm, arsenic at 189.0 nm and selenium at 196.09 nm is recommended to minimize spectral interferences. No chemical or physical interferences resulting from the influence of foreign ions on the solute vaporization process have been noted. 相似文献
874.
1,2-Dihydroquinolines were obtained by the reduction of 3-substituted 2-methyl-4-phenylquinolines with sodium borohydride in aliphatic carboxylic acids; N-alkyl derivatives are also formed. The corresponding 1,4-dihydroquinoline was obtained in the reaction of 2-methyl-3-nitroquinolinium perchlorate with sodium borohydride.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1680–1686, December, 1991. 相似文献
875.
A simple method for the determination of molybdenum and tungsten in sea and surface water is presented. Molybdenum and tungsten are concentrated on activated charcoal by adsorption as the ammonium pyrrolidine dithiocarbamate complex; the optimal pH for adsorption is 1.3. Mo and W are then determined by thermal neutron activation, forming 99Mo (T = 66.7 h) and 137W (T = 23.8 h), respectively. The 99mTc daughter of 99Mo is measured as soon as the equilibrium between 99mTc(T= 6 h) and 99Mo is established. The detection limits are 0.05 μg Mo l-1 and 0.05 μg W l-1 (or 0.001 μg W l-1 after a simple chemical separation). 相似文献
876.
Hydrodynamic voltammetry employing empirically determined mass transport coefficients is used to determine heterogeneous rate constants and transport coefficients for the ferricyanide/ferrocyanide system in 0.1 M phosphate buffer and other supporting electrolytes with turbulent tubular and rotated disk electrodes of platinum, gold, glassy carbon, carbon film, and boron carbide. Different kinetic parameters are obtained at the various electrode materials. For the platinum, gold, and boron carbide electrodes, the magnitudes of the rate parameters depend on scan direction. The nature of this hysteresis varies with the electrode material and is explained in terms of adsorbed oxide and ionic layers or other phenomena not described by simple double layer theory. 相似文献
877.
In order to elucidate certain controversies in interpreting the π-electronic structure of some simple quinone and indigo dyes the Pariser–Parr–Pople SC β,γ-wave-functions have been subjected to configuration analyses. Whereas 2,5-diamino-quinone (1) can be excellently represented by coupling of two trimethine-merocyanine chains, the analogous consideration is less appropriate with bispyrrolindigo (2). In this case the results of the configuration analyses indicate clearly the limited applicability of the Longuet/Higgins–Murrell–method. 相似文献
878.
The fractional dissociation,32Γ, of 32SF6 by the output of a pulsed TEA CO2 laser has been found to depend upon Φ, the total integrated laser intensity, as 32Γ ∞ Φm for both the P(20) and P(16) laser lines, where m ≈ 3 in the range 2 < Φ < 7.2 J/cm2 and increases from 3 to ≈ 4 as Φ is reduced from 2 to 0.9 J/cm2. 相似文献
879.
Castiñeiras A Sicilia-Zafra AG González-Pérez JM Choquesillo-Lazarte D Niclós-Gutiérrez J 《Inorganic chemistry》2002,41(26):6956-6958
By reaction of Cu(2)CO(3)(OH)(2), 2-benzylmalonic acid (H(2)Bzmal), and 1,10-phenanthroline (phen), [Cu(Bzmal)(phen)(H(2)O)] x 3H(2)O (compound 1) has been obtained and characterized by thermal, spectral, magnetic, and X-ray diffraction methods. The molecular structure of 1 is remarkably similar to that of [Cu(Bzmal)(bipy)(H(2)O)] x 2H(2)O (compound 2, bipy = 2,2'-bipyridine). In both complexes, the aryl(Bzmal) ring produces an unexpected pi,pi-stacking interaction with the Cu(II)-(aromatic alpha,alpha'-diimine) chelate ring, at an average distance d(pi)(-)(pi) of 3.40 A, involving roughly parallel and smoothly slipped rings. This insight is discussed as new structural evidence for metalloaromaticity of Cu(II)-(aromatic alpha,alpha'-diimine) chelate rings. Interestingly, 1 recognizes itself by a weak intermolecular pi,pi-stacking interaction between aryl(Bzmal) ligands to give pairs of complex molecules. In contrast, there is an intermolecular pyridyl-pyridyl pi,pi-stacking interaction also forming pairs of complex molecules in 2. 相似文献
880.
Zusammenfassung Kürzlich wurde in einer Publikation vonA. K. Bose undS. Garratt über eine neue Synthese von substituierten Barbitursäuren aus Malonsäuren und substituierten Malonsäuren mit Carbodiimiden berichtet. Hierdurch sehen wir uns veranlaßt, über unsere bisherigen Ergebnisse im Rahmen ähnlicher Untersuchungen kurz zu berichten. 相似文献