全文获取类型
收费全文 | 3251篇 |
免费 | 36篇 |
国内免费 | 18篇 |
专业分类
化学 | 1981篇 |
晶体学 | 21篇 |
力学 | 65篇 |
数学 | 674篇 |
物理学 | 564篇 |
出版年
2018年 | 26篇 |
2017年 | 18篇 |
2016年 | 32篇 |
2015年 | 20篇 |
2014年 | 42篇 |
2013年 | 95篇 |
2012年 | 72篇 |
2011年 | 74篇 |
2010年 | 60篇 |
2009年 | 48篇 |
2008年 | 73篇 |
2007年 | 93篇 |
2006年 | 111篇 |
2005年 | 81篇 |
2004年 | 93篇 |
2003年 | 73篇 |
2002年 | 74篇 |
2001年 | 47篇 |
2000年 | 54篇 |
1999年 | 49篇 |
1998年 | 36篇 |
1997年 | 50篇 |
1996年 | 61篇 |
1995年 | 55篇 |
1994年 | 48篇 |
1993年 | 61篇 |
1992年 | 55篇 |
1991年 | 52篇 |
1990年 | 41篇 |
1989年 | 45篇 |
1988年 | 30篇 |
1987年 | 42篇 |
1986年 | 43篇 |
1985年 | 72篇 |
1984年 | 42篇 |
1983年 | 42篇 |
1982年 | 52篇 |
1981年 | 42篇 |
1980年 | 60篇 |
1979年 | 56篇 |
1978年 | 60篇 |
1977年 | 59篇 |
1976年 | 56篇 |
1975年 | 36篇 |
1974年 | 45篇 |
1973年 | 45篇 |
1972年 | 31篇 |
1971年 | 27篇 |
1970年 | 28篇 |
1930年 | 18篇 |
排序方式: 共有3305条查询结果,搜索用时 31 毫秒
61.
Ntaoleng M. Makunya Reinout Meijboom Alfred Muller 《Journal of organometallic chemistry》2005,690(18):4159-4167
Cyclopentadienyldicarbonylmethyliron, [CpFe(CO)2Me] (1), undergoes migratory carbonyl insertion under the influence of isosteric phosphine ligands P(4-FC6H4)3 and P(4-MeC6H4)3. The products of the reaction, [CpFe(CO)(COMe)P(4-FC6H4)3] (2a) and [CpFe(CO)(COMe)P(4-MeC6H4)3] (2b), were characterised by X-ray crystallography. In both structures, the iron atom adopts a pseudo octahedral coordination geometry. Fe-P bond distances are the same at 2.1932(8) Å in 2a and 2b, respectively. Thus, contrary to what was expected, X-ray data could not be used to quantitatively differentiate between the two phosphine ligands in 2a and 2b. Therefore, additional spectroscopic techniques such as IR and NMR were employed. Similarly, the Fe-C bond lengths of the carbonyl (Fe-CO) and acetyl (Fe-COMe) are 1.748(3) and 1.955(3) in 2a, and 1.744(3) and 1.951(3) Å in 2b, respectively.The migratory carbonyl insertion was studied by NMR, IR, and UV-vis spectroscopies to determine the mechanism and the rate law. Results from NMR spectroscopy show that the formation of the product is accompanied by oxidation of the corresponding phosphine ligand. An increase in the reactivity of migratory carbonyl insertion for P(4-MeC6H4)3 was observed when the solvent was changed from CH2Cl2 to MeCN. The kinetic data showed that P(4-MeC6H4)3 reacts faster than P(4-FC6H4)3. 相似文献
62.
Synthese of sulfonated derivatives of 2-amino-p-xylene Sulfonation of 2-amino-p-xylene (2) gave 2-amino-p-xylene-5-sulfonic acid (1) . The 2-amino-p-xylene-6-sulfonic acid (3) was prepared via three routes: (1) sulfonation of 2-amino-5-chloro-p-xylene (19) to 5-amino-2-chloro-p-xylene-3-sulfonic acid (20) followed by hydrogenolysis; (2) sulfur dioxide treatment of the diazonium salt derived from 2-amino-6-nitro-p-xylene (21) to 2-nitro-p-xylene-6-sulfonyl chloride (11) followed by hydrolysis to 2-nitro-p-xylene-6-sulfonic acid (4) and Béchamp reduction; (3) Béchamp reduction of 2-chloro-3-nitro-p-xylene-5-sulfonic acid (13) to 3-amino-2-chloro-p-xylene-5-sulfonic acid (16) and subsequent hydrogenolysis. Catalytic reduction of 13 in aqueous sodium carbonate solution gave mixtures of 3 and 16 . 2-Amino-p-xylene-3-sulfonic acid (27) was synthesized via two routes: (1) reaction of 19 with sulfamic acid to 2-amino-5-chloro-p-xylene-3-sulfonic acid (26) followed by hydrogenolysis; (2) sulfur dioxide treatment of the diazonium salt derived from 2-amino-3-nitro-p-xylene (28) to 2-nitro-p-xylene-3-sulfonyl chloride (12) , hydrolysis to 2-nitro-p-xylene-3-sulfonic acid (7) and Béchamp reduction. 相似文献
63.
C. Bellmann A. Opfermann H.-J. Jacobasch H.-J. Adler 《Analytical and bioanalytical chemistry》1997,358(1-2):255-258
In the field of corrosion prevention, the characterisation of metal surfaces is useful to obtain information about the necessary
preparation of the surface and about the quality of thin coatings of coupling and protective agents. Electrokinetic measurements
can be used to determine the properties of modified polymer surfaces, metal sheets, and different surface preparations. However,
it was necessary first to create special conditions for measuring the streaming potential, such as electrical isolation of
the sample sheets together with the use of inert systems, as shown by the example of aluminium. According to the usual procedure
the metal was first coated with a thin layer of a coupling agent. The effect of this adhesive agent depends on the degree
of its adsorption by the metal surface. The quality of this layer can be characterised by streaming potential measurements,
which can show the degree of coverage, the adhesive strength of the coated layer, and the effective chemical properties of
the treated surface. These results were confirmed by wetting measurements. The production of surfaces which allow us to couple
a coating varnish to the adhesive agent in good quality was demonstrated.
Received: 24 June 1996 / Revised: 30 January 1997 / Accepted: 30 January 1997 相似文献
64.
65.
66.
67.
Paulette A. Greenidge Alfred Merz Gerd Folkers 《Journal of computer-aided molecular design》1995,9(6):473-478
Summary A representative range of pyrimidine nucleoside analogues that are known to inhibit herpes simplex virus (HSV) replication have been used to construct receptor binding site models for the varicella-zoster virus (VZV), thymidine kinase (TK) and human TK1. Given a set of interacting ligands, superimposed in such a manner as to define a pharmacophore, the pseudoreceptor modelling technique Yak provides a means of building binding site models of macromolecules for which no three-dimensional experimental structures are available. Once the models have been evaluated by their ability to reproduce experimental binding data [Vedani et al., J. Am. Chem. Soc., 117 (1995) 4987], they can be used for predictive purposes. Calculated and experimental values of relative binding affinity are compared. Our models suggest that the substitution of one residue may be sufficient to determine ligand subtype affinity. 相似文献
68.
Alfred Baszczyk Matthias Fischer Carsten vonHnisch Marcel Mayor 《Helvetica chimica acta》2006,89(9):1986-2005
The synthesis, characterization, and optical properties of a series of new 2,6‐disubstituted naphthalene‐bisimide dyes as molecular rods comprising terminal AcS groups is reported. The first series of dyes ( 1 – 3 ), comprising phenylhetero (Ph‐X) core substituents, cover a broad range of the VIS spectrum, ranging from yellow ( 2 ) over red ( 3 ) to blue ( 1 ). The second series of dyes contains benzylhetero (Bn‐X) core substituents ( 4 – 7 ). For the same heteroatom connecting the substituent to the naphthalene core, both series were found to display comparable colors. For the second series, the colors were blue ( 4 ), red ( 5 ), and violet ( 6, 7 ). The Ph‐X‐substituted dyes 1 – 3 are nonfluorescent, in contrast to the Bn‐X‐substituted compounds 4 – 7 . This rich variety of optical features that can be adjusted by rather small alterations of the core substituents makes these structurally very comparable molecular rods ideal candidates for optically triggered molecular‐transport investigations. Also, thanks to the terminal AcS groups, these compounds can be placed between nobel‐metal electrodes for optically triggered transport experiments. 相似文献
69.
Holzwarth AR Müller MG Niklas J Lubitz W 《The journal of physical chemistry. B》2005,109(12):5903-5911
The fluorescence kinetics of photosystem I core particles from Chlamydomonas reinhardtii have been measured with picosecond resolution in order to test a previous hypothesis suggesting a charge recombination mechanism for the early electron-transfer steps and the fluorescence kinetics (Müller et al. Biophys. J. 2003, 85, 3899-3922). Performing global target analyses for various kinetic models on the original fluorescence data confirms the "charge recombination" model as the only acceptable one of the models tested while all of the other models can be excluded. The analysis allowed a precise determination of (i) the effective charge separation rate constant from the equilibrated reaction center excited state (438 ns(-1)) confirming our previous assignment based on transient absorption data (Müller et al. Biophys. J. 2003, 85, 3899-3922), (ii) the effective charge recombination rate constant back to the excited state (52 ns(-1)), and (iii) the intrinsic secondary electron-transfer rate constant (80 ns(-1)). The average energy equilibration lifetime core antenna/RC is about 1 ps in the "charge recombination" model, in agreement with previous transient absorption data, vs the 18-20 ps energy transfer lifetime from antenna to RC within "transfer-to-the-trap-limited" models. The apparent charge separation lifetime in the recombination model is about three times faster than in the "transfer-to-the-trap-limited" model. We conclude that the charge separation kinetics is trap-limited in PS I cores devoid of red antenna states such as in C. reinhardtii. 相似文献
70.
Werner Korsatko Christian Knopp Alfred Fuchsgruber Gustav Zigeuner 《Monatshefte für Chemie / Chemical Monthly》1976,35(2):745-755
The structures of tetrahydro-4-phenylspiro([1]benzopyran-2,4(1H)-pyrimidin)-2(3H)-ones and-thiones4 a, b resp., are proved by synthesis. 3-(2-methoxy-3,5-dimethylphenyl)-3-phenylpropionic acid11 b is prepared from 3,4-dihydro-6,8-dimethyl-4-phenylcoumarin10. The lithium salt of11 b reacts with isobutenyl-lithium to 1-(2-methoxy-3,5-dimethylphenyl)-5-methyl-1-phenyl-4-hexen-3-on12 a. 12 a is transferred with urea in acid medium and NH4CNS resp. in a mixture of dihydro-6-[2-(2-methoxy-3,5-dimethylphenyl)-2-phenyläthyl]-4,4-dimethyl-2(1H)-pyrimidinone and-thione13 a, b and tetrahydro-6-[2-(2-methoxy-3,5-dimethylphenyl)-2-phenyläthyliden]-4,4-dimethyl-2(1H)-pyrimidinone and-thione14 a, b resp.14 b leads to13 a, b with H2O2. Heating of13 a, 14 a and14 b resp. with pyridin-HCl leads to the spiro compounds4 a, b. 相似文献