全文获取类型
收费全文 | 3637篇 |
免费 | 137篇 |
国内免费 | 21篇 |
专业分类
化学 | 2769篇 |
晶体学 | 10篇 |
力学 | 65篇 |
数学 | 525篇 |
物理学 | 426篇 |
出版年
2023年 | 17篇 |
2021年 | 37篇 |
2020年 | 40篇 |
2019年 | 51篇 |
2018年 | 31篇 |
2017年 | 21篇 |
2016年 | 80篇 |
2015年 | 102篇 |
2014年 | 77篇 |
2013年 | 139篇 |
2012年 | 183篇 |
2011年 | 222篇 |
2010年 | 159篇 |
2009年 | 111篇 |
2008年 | 216篇 |
2007年 | 200篇 |
2006年 | 199篇 |
2005年 | 196篇 |
2004年 | 134篇 |
2003年 | 109篇 |
2002年 | 116篇 |
2001年 | 59篇 |
2000年 | 69篇 |
1999年 | 64篇 |
1998年 | 57篇 |
1997年 | 66篇 |
1996年 | 59篇 |
1995年 | 55篇 |
1994年 | 51篇 |
1993年 | 64篇 |
1992年 | 56篇 |
1991年 | 25篇 |
1990年 | 32篇 |
1989年 | 26篇 |
1988年 | 32篇 |
1987年 | 39篇 |
1986年 | 36篇 |
1985年 | 58篇 |
1984年 | 24篇 |
1983年 | 30篇 |
1982年 | 44篇 |
1980年 | 37篇 |
1978年 | 27篇 |
1977年 | 38篇 |
1976年 | 17篇 |
1974年 | 18篇 |
1973年 | 19篇 |
1972年 | 19篇 |
1971年 | 19篇 |
1969年 | 20篇 |
排序方式: 共有3795条查询结果,搜索用时 147 毫秒
991.
Barea E Navarro JA Salas JM Quirós M Willermann M Lippert B 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(18):4414-4421
A set of enantiomerically pure cyclic multinuclear complexes with the formula cis-[a(2)PdL](n) (n+) [a(2)=(R,R)-1,2-diaminocyclohexane (R,R-dach), (S,S)-1,2-diaminocyclohexane (S,S-dach); n=4, 6; LH=2-hydroxypyrimidine (2-Hpymo), 4,6-dimethyl-2-hydroxypyrimidine (2-Hdmpymo) and 4-hydroxypyrimidine (4-Hpymo)] were obtained by reaction of cis-[a(2)Pd(H(2)O)(2)](2+) and LH in aqueous media. The polynuclear complexes were studied by (1)H NMR spectroscopy and X-ray crystallography. These studies revealed that the N1,N3-bridging mode exhibited by the pyrimidine moieties is ideally suited for formation of inorganic analogues of calixarenes (metallacalixarenes) in a self-assembly process. The most stable species are the tetranuclear metallacalix[4]arenes, which are obtained in all cases. Hexanuclear species, namely, [a(2)Pd(2-dmpymo)](6) (6+), were also isolated and fully characterised. (1)H NMR experiments show conversion of [a(2)Pd(2-dmpymo)](6) (6+) to [a(2)Pd(2-dmpymo)](4) (4+) on heating. Analogously to organic calixarenes, these systems are also capable of incorporating hard metal ions at the oxo surface. Additionally, investigations on the receptor properties of these metallacalixarenes towards mononucleotides showed that enantioselective recognition processes occur in aqueous media. 相似文献
992.
993.
994.
995.
Christof Niedermayer Christian Bernhard Ulrich Binninger Harald Glückler Jeffery Tallon Eduardo J. Ansaldo Joseph I. Budnick 《Hyperfine Interactions》1994,86(1):585-590
We report transverse field muon spin rotation experiments carried out on Tl2Ba2CuO6+. This system spans the whole overdoped regime, in which Tc is reduced by excess oxygen doping despite an increasing normal state carrier concentration. In the heavily overdoped regime the extrapolated low temperature depolarization rate(0) is found to scale linearly with the superconducting critical temperature Tc similar to the behavior previously observed for other cuprates in the underdoped regime. In the framework of the clean limit London model,(0)
–2 n
s
(0)/m
s
*
, suggesting that the depression of Tc by overdoping is associated with a decrease of the superconducting condensate density. This needs to be understood in view of the observed increase in the normal state carrier density. We discuss some possible explanations for this behavior. 相似文献
996.
Three cationic tetrakis(nucleobase) complexes of Pt(II) have been synthesized: [Pt(Hmhyp-N7)4](NO3)2.H2O 1, [Pt(Hegua-N7)4](NO3)2.2KNO(3).5H2O and trans-[Pt(Hmcyt-N3)2(Hegua-N7)2](NO3)2 3 (Hmhyp = 9-methylhypoxanthine, Hegua = 9-ethylguanine, Hmcyt = 1-methylcytosine). The X-ray crystal structure of has been determined. All three cationic compounds rapidly react with Hg(II), but gel formation prevented an adequate characterization of the products formed. However, a Cu(II) adduct of was isolated in crystalline form and characterized crystallographically. [{(H2O)Cu(Hmhyp)4Pt}2Cu(ClO4)4)](ClO4)2(NO3)4.6H2O crystallizes in a centrosymmetric Cu-Pt-Cu-Pt-Cu chain structure with Cu-Pt separations of 2.791(1)A(outside) and 3.8980(9)A(inside). Two of the three Cu(II) ions are bound via exocyclic O(6) sites of the Hmhyp nucleobases. At neutral and moderate alkaline pH both and form virtually insoluble precipitates, which redissolve at strongly alkaline pH to give eventually anionic [Pt(L)4]2- species (L = mhyp, egua). Finally, interacts with complementary Hmcyt to give Watson-Crick associates, as demonstrated by 1H NMR spectroscopy in DMSO-d(6). 相似文献
997.
Zusammenfassung Sehr kleine Wolframkonzentrationen von etwa 0,1–20 g/ml werden am besten in Grundlösungen mit 9–10 m PerchlorsÄure + 0,6–0,7 m WeinsÄure polarographisch bestimmt. Für höhere Konzentrationen von etwa 20–100 g/ml sind Lösungen mit 9–10 m SalzsÄure + 0,3 m WeinsÄure (oder 0,1 m CitronensÄure) + 0,01% Gelatine vorzuziehen.
Herrn Prof. Dr., Dr. h.c. Dr. h.c. Dr. h.c. Wilhelm Klemm zum 70. Geburtstag gewidmet. 相似文献
Summary For the determination of very small concentrations of tungsten (0.1 to 20 g/ml) solutions with 9–10 M perchloric + 0.6–0.7 M tartaric acid are especially suitable. The determination of higher concentrations (20–100 g/ml) should be preferably performed in solutions with 9–10 M hydrochloric + 0.3 M tartaric acid (or 0.1 M citric acid) + 0.01% gelatin.
Herrn Prof. Dr., Dr. h.c. Dr. h.c. Dr. h.c. Wilhelm Klemm zum 70. Geburtstag gewidmet. 相似文献
998.
G. Bernhard 《Journal of Radioanalytical and Nuclear Chemistry》1994,177(2):321-325
The distribution of different radionuclides depends on their speciation in the real fission product solution and the electric charge of the alumina. The pH-value is the main parameter that influences the disyribution coefficient. By combination of an acidic adsorption and an alkaline desorption process, the interesting fission molybdenum can be separated. The distribution ratios of the relevant radionuclides are described. 相似文献
999.
Marko D. Mihovilovic Markus Spina Bernhard Müller Peter Stanetty 《Monatshefte für Chemie / Chemical Monthly》2004,135(7):899-909
Summary. The synthesis of carbo- and heterocyclic aldehydes bearing an ipso-methoxy group is investigated. The synthetic sequence is based on an initial Grignard addition of an olefin to a cyclic ketone followed by methylation of the resulting tertiary alcohol. The terminal olefin serves as precursor for the aldehyde functionality. Oxidation by ozonolysis turned out to depend significantly on the distance of the donor methoxy group. The observed side reactions could be circumvented by applying a one-pot OsO4 mediated diol formation followed by Malaprade oxidation using KIO4. A series of carbo- and heterocyclic precursors were successfully converted to the title products. 相似文献
1000.
the ethylenediamine/pyridine-2-carbaldehyde/copper(I) system is used in a new spectrophotometric method for the determination of ethylenediamine. The batch procedure involves the formation of an orange chelate between the Schiff's base and copper(I) ions at pH 8.5 (borate buffer) and measurement of the absorbances at 475 nm against water after 10/2-15 min; Beer's law is obeyed over the range 0.5/2-11.2 μg ml?1 and the molar absorptivity is 6.21 × 103 l mol?1 cm?1. Tolerance limits for different amines [36] and other organic compounds [12] are reported. In the optimized flow-injection system, ethylenediamine (1.4/2-84.6 μg ml?1 is determined at a sample throughput of 55 h?1. The method is sensitive and selective and is satisfactory for the determination of the diamine in aminophylline and pharmaceutical preparations (ethylenediamine contents from 0.031 to 3.23%) with relative errors ranging from ?7.4 to +11.1% and relative standard deviations of about 0.65% for both procedures. 相似文献