全文获取类型
收费全文 | 4936篇 |
免费 | 176篇 |
国内免费 | 13篇 |
专业分类
化学 | 3211篇 |
晶体学 | 61篇 |
力学 | 62篇 |
数学 | 696篇 |
物理学 | 1095篇 |
出版年
2022年 | 45篇 |
2021年 | 64篇 |
2020年 | 87篇 |
2019年 | 85篇 |
2018年 | 68篇 |
2017年 | 55篇 |
2016年 | 108篇 |
2015年 | 116篇 |
2014年 | 118篇 |
2013年 | 204篇 |
2012年 | 225篇 |
2011年 | 242篇 |
2010年 | 161篇 |
2009年 | 127篇 |
2008年 | 210篇 |
2007年 | 187篇 |
2006年 | 199篇 |
2005年 | 178篇 |
2004年 | 165篇 |
2003年 | 128篇 |
2002年 | 126篇 |
2001年 | 75篇 |
2000年 | 99篇 |
1999年 | 81篇 |
1998年 | 68篇 |
1997年 | 75篇 |
1996年 | 72篇 |
1995年 | 56篇 |
1994年 | 79篇 |
1993年 | 78篇 |
1992年 | 67篇 |
1991年 | 58篇 |
1990年 | 59篇 |
1989年 | 42篇 |
1988年 | 40篇 |
1987年 | 50篇 |
1986年 | 60篇 |
1985年 | 65篇 |
1984年 | 52篇 |
1983年 | 55篇 |
1982年 | 48篇 |
1981年 | 46篇 |
1980年 | 66篇 |
1979年 | 47篇 |
1978年 | 56篇 |
1977年 | 59篇 |
1976年 | 52篇 |
1975年 | 57篇 |
1974年 | 44篇 |
1973年 | 51篇 |
排序方式: 共有5125条查询结果,搜索用时 15 毫秒
71.
On the Sodium Tetrahydroxoaluminate Chloride Na2[Al(OH)4]Cl The hitherto unknown compound Na2[Al(OH)4]Cl was prepared by crystallisation from a NaCl containing sodium aluminate solution. According to the X-ray single crystal investigation (tetragonal, space group P4/nmm, a = 7.541 Å, c = 5.059 Å, Z = 2) the compound represents the first example of a crystalline hydroxoaluminate with monomeric [Al(OH)4]? anions. Cl? shows a quadratic anti prismatic coordination to 4 Na+ and over hydrogen bonds to 4 O2? while Na+ is octahedrally coordinated by 4 O2? and 2 Cl? (axial). The results of the crystal structure analysis are confirmed by 27Al and 23Na MAS NMR investigations. Na2[Al(OH)4]Cl decomposes at about 200°C without intermediates under formation of β-NaAlO2 and NaCl. 相似文献
72.
The high frequency acoustic response of liquids is measured in a manner directly analogous to conventional ultrasonic measurements. Two thin metal films act as acoustic transducer and receiver for a liquid layer between them. Pulsed optical excitation generates high bandwidth wave packets in the transducer, and these are detected in the receiver after damping and dispersion by the liquid. This initial measurement probes structural relaxation dynamics of glycerol in the frequency range 2-20 GHz, for temperatures between 235 and 291 K. The analysis presented here demonstrates the presence of excess relaxation, not accounted for by either the alpha or beta relaxation of the mode-coupling theory, and suggests the presence of constant loss in the susceptibility spectrum of supercooled glycerol. 相似文献
73.
F Schneider 《Angewandte Chemie (International ed. in English)》1978,17(8):583-592
The presence of histidine in the active center of an enzyme can be demonstrated by kinetic measurements, chemical modification, NMR spectroscopy or X-ray structure analysis. Histidine is the only naturally occurring amino acid to contain an imidazole residue as a side chain. The role of histidine in enzyme catalysis depends, inter alia, upon the special features of the imidazole residue: it thus tends to form hydrogen bonds, combines donor and acceptor properties and can take part in either nucleophilic or base catalysis. In some of these enzymes the position of each atom is known; however, the theories as to how the catalysis proceeds at a molecular level are controversial. 相似文献
74.
Helga Schneider 《Fresenius' Journal of Analytical Chemistry》1970,249(4):225-228
Zusammenfassung Eine Methode wird beschrieben, die die Analyse sehr geringer Substanzmengen ermöglicht. Eine maximale Einwaage von 2 mg wird in 1 g Borax geschmolzen, und die so erhaltene Boraxscheibe direkt zur röntgenfluorescenzanalytischen Bestimmung der verschiedenen Elemente eingesetzt. Für eine Reihe von Elementen werden die Nachweisgrenzen angegeben und ein Beispiel für die Reproduzierbarkeit der Methode angegeben. Verschiedene Anwendungsbeispiele dieser Methode werden beschrieben und die Ergebnisse mitgeteilt.
Analysis of very small quantities of substances by X-ray fluorescence spectrometry
A maximum sample weight of 2 mg is molten in 1 g borax und the disk obtained is directly analysed by X-ray fluorescence spectrometry. For a number of elements the limits of detection are given and by one example the reproducibility of the method is shown. Different examples of application are described and the results are presented.相似文献
75.
Yang Shen Ioanna Kalograiaki Alessio Prunotto Matthew Dunne Samy Boulos Nicholas M. I. Taylor Eric T. Sumrall Marcel R. Eugster Rebecca Martin Alicia Julian-Rodero Benjamin Gerber Petr G. Leiman Margarita Menndez Matteo Dal Peraro Francisco Javier Caada Martin J. Loessner 《Chemical science》2021,12(2):576
Endolysins are bacteriophage-encoded peptidoglycan hydrolases targeting the cell wall of host bacteria via their cell wall-binding domains (CBDs). The molecular basis for selective recognition of surface carbohydrate ligands by CBDs remains elusive. Here, we describe, in atomic detail, the interaction between the Listeria phage endolysin domain CBD500 and its cell wall teichoic acid (WTA) ligands. We show that 3′O-acetylated GlcNAc residues integrated into the WTA polymer chain are the key epitope recognized by a CBD binding cavity located at the interface of tandem copies of beta-barrel, pseudo-symmetric SH3b-like repeats. This cavity consists of multiple aromatic residues making extensive interactions with two GlcNAc acetyl groups via hydrogen bonds and van der Waals contacts, while permitting the docking of the diastereomorphic ligands. Our multidisciplinary approach tackled an extremely challenging protein–glycopolymer complex and delineated a previously unknown recognition mechanism by which a phage endolysin specifically recognizes and targets WTA, suggesting an adaptable model for regulation of endolysin specificity.Combining genetic, biochemical and computational approaches, we elucidated the molecular mechanisms underlying the recognition of Listeria wall teichoic acid by bacteriophage-encoded SH3b repeats. 相似文献
76.
The nitridorhenium(V) complexes ReNCl(2)(PCy3)(2) (1), ReNBr(2)(PCy3)(2) (2), ReNCl(2)(PPh3)(2) (3), and ReNBr(2)(PPh3)(2) (4) produce structured emission spectra upon excitation at low temperature. The origin, E(00), occurs at 15 775, 16 375, 15 875, and 16 300 cm(-1), respectively. The vibronic peaks are regularly spaced with an average energy separation corresponding to the Re triple bond N stretching frequency. The nitridorhenium stretching frequency ranges from 1095 to 1101 cm(-1), as determined by Raman and IR spectroscopy. The excited-state distortions are calculated by fitting the emission spectra. The excited state arises primarily from a d(xy) (ReN nonbonding) to d(yz) (ReN pi antibonding) transition. The rhenium-nitrogen bond length in the excited state is 0.08 A longer than in the ground electronic state, which is consistent with the difference in bond lengths of ReN bonds of bond order 3 and bond order 2.5 as determined from molecular structures. 相似文献
77.
A total synthesis of the optically active tetrahydroesterastin β -lactam analogue 2 using Miller's hydroxamate approach is described (Scheme 2). Significant modification of published procedures has resulted in a short and facile stereospecific preparation of the N-[(benzyloxycarbonyl)methyl]-β -lactam 17 starting from the readily available D -serine. This material served as intermediate for the preparation of a variety of N-[(benzyloxycarbonyl)methyl]tetrahydroesterastin β-lactam analogues (Scheme 5). 相似文献
78.
79.
Grotjahn DB Van S Combs D Lev DA Schneider C Incarvito CD Lam KC Rossi G Rheingold AL Rideout M Meyer C Hernandez G Mejorado L 《Inorganic chemistry》2003,42(10):3347-3355
Inter- and intramolecular hydrogen bonding of an N-H group in pyrazole complexes was studied using ligands with two different groups at pyrazole C-3 and C-5. At C-5, groups such as methyl, i-propyl, phenyl, or tert-butyl were present. At C-3, side chains L-CH(2)- and L-CH(2)CH(2)- (L = thioether or phosphine) ensured formation of chelates to a cis-dichloropalladium(II) fragment through side-chain atom L and the pyrazole nitrogen closest to the side chain. The significance of the ligands is that by placing a ligating side chain on a ring carbon (C-3), rather than on a ring nitrogen, the ring nitrogen not bound to the metal and its attached proton are available for hydrogen bonding. As desired, seven chelate complexes examined by X-ray diffraction all showed intramolecular hydrogen bonding between the pyrazole N-H and a chloride ligand in the cis position. In addition, however, intermolecular hydrogen bonding could be controlled by the substituent at C-5: complexes with either a methyl at C-5 or no substituent there showed significant intermolecular hydrogen bonding interactions, which were completely avoided by placing a tert-butyl group at C-5. The acidity of two complexes in acetonitrile solutions was estimated to be closer to that of pyridinium ion than those of imidazolium or triethylammonium ions. 相似文献
80.