全文获取类型
收费全文 | 7850篇 |
免费 | 332篇 |
国内免费 | 36篇 |
专业分类
化学 | 4994篇 |
晶体学 | 31篇 |
力学 | 301篇 |
综合类 | 1篇 |
数学 | 1533篇 |
物理学 | 1358篇 |
出版年
2023年 | 65篇 |
2022年 | 104篇 |
2021年 | 205篇 |
2020年 | 188篇 |
2019年 | 183篇 |
2018年 | 172篇 |
2017年 | 154篇 |
2016年 | 294篇 |
2015年 | 284篇 |
2014年 | 327篇 |
2013年 | 500篇 |
2012年 | 541篇 |
2011年 | 638篇 |
2010年 | 376篇 |
2009年 | 321篇 |
2008年 | 502篇 |
2007年 | 473篇 |
2006年 | 415篇 |
2005年 | 405篇 |
2004年 | 314篇 |
2003年 | 244篇 |
2002年 | 254篇 |
2001年 | 89篇 |
2000年 | 66篇 |
1999年 | 82篇 |
1998年 | 64篇 |
1997年 | 68篇 |
1996年 | 65篇 |
1995年 | 58篇 |
1994年 | 58篇 |
1993年 | 46篇 |
1992年 | 36篇 |
1991年 | 34篇 |
1990年 | 31篇 |
1989年 | 23篇 |
1988年 | 23篇 |
1987年 | 27篇 |
1986年 | 26篇 |
1985年 | 39篇 |
1984年 | 30篇 |
1983年 | 18篇 |
1982年 | 28篇 |
1981年 | 22篇 |
1980年 | 21篇 |
1979年 | 25篇 |
1978年 | 23篇 |
1977年 | 16篇 |
1976年 | 16篇 |
1897年 | 14篇 |
1891年 | 15篇 |
排序方式: 共有8218条查询结果,搜索用时 31 毫秒
71.
Tessore F Roberto D Ugo R Pizzotti M Quici S Cavazzini M Bruni S De Angelis F 《Inorganic chemistry》2005,44(24):8967-8978
Coordination of 4'-(C6H4-p-X)-2,2':6',2'-terpyridines [X = NO2, NBu2, (E)-CH=CH-C6H4-p-NBu2, (E,E)-(CH=CH)2-C6H4-p-NMe2] to Zn(II), Ru(III), and Ir(III) metal centers induces a significant enhancement of the absolute value of the second-order nonlinear optical (NLO) response of the terpyridine, measured by means of both electric field induced second harmonic generation and solvatochromic methods. By varying the nature of the metal center, the enhanced second-order NLO response shifts from positive to negative. Such a shift is controlled by electronic charge-transfer transitions, such as metal-to-ligand or ligand-to-metal transitions, in addition to the intraligand charge transfer. The enhancement generated by coordination is also controlled by the chelation effect and by fine-tuning of the ancillary ligands. 相似文献
72.
Lucarini M Franchi P Pedulli GF Gentilini C Polizzi S Pengo P Scrimin P Pasquato L 《Journal of the American Chemical Society》2005,127(47):16384-16385
ESR spectroscopy has been used to study the interaction of para-pentylbenzyl hydroxyalkyl nitroxide with the monolayer of water-soluble protected gold clusters having a core diameter ranging from 1.6 to 5.3 nm. The solubilization of the nitroxide probe in the more hydrophobic environment of the monolayer strongly depends on the size of the gold core. In particular, the partition equilibrium constant increases as the nanoparticle diameter decreases. These results have been attributed to the different packing of the chains in the monolayer resulting from the different radius of curvature of the investigated nanoparticles. This represents, to the best of our knowledge, the first report demonstrating that the core size of metallic nanoparticles affects the solvating properties of the protective organic monolayer. 相似文献
73.
Michelini Mdel C Russo N Alikhani ME Silvi B 《Journal of computational chemistry》2004,25(13):1647-1655
The Density functional theory has been applied to characterize the structural features of Mo(1,2)-NH(3),-C(2)H(4), and -C(2)H(2) compounds. Coordination modes, geometrical structures, and binding energies have been calculated for several spin multiplets. It has been shown that in contrast to the conserved spin cases (Mo(1,2)-NH(3)), the interaction between Mo (or Mo(2)) and C(2)H(4) (or C(2)H(2)) are the low-spin (Mo-C(2)H(4) and -C(2)H(2)) and high-spin (Mo(2)-C(2)H(4) and -C(2)H(2)) complexes. In the ground state of Mo(1,2)-C(2)H(4) and -C(2)H(2), the metal-center always reacts with the C-C center. The spontaneous formation of the global minima is found to be possible due to the crossing between the potential energy surfaces (ground and excited states with respect to the metallic center). The bonding characterization has been performed using the topological analysis of the Electron Localization Function. It has been shown that the most stable electronic structure for a pi-acceptor ligand correlates with a maximum charge transfer from the metal center to the C-C bond of the unsaturated hydrocarbons, resulting in the formation of two new basins located on the carbon atoms (away from hydrogen atoms) and the reduction of the number of attractors of the C-C basin. The interaction between Mo(1,2) and C(2)H(4) (or C(2)H(2)) should be considered as a chemical reaction, which causes the multiplicity change. Contrarily, there is no charge transfer between Mo(1,2) and NH(3), and the partners are bound by an electrostatic interaction. 相似文献
74.
The direct flow injection potentiometric (FIP) analysis of phosphate in hydroponic nutrient solution has been carried out using a cobalt-wire ion-selective electrode (ISE). Synthetic hydroponic nutrient solution, commercial hydroponic nutrient solution and working hydroponic farm nutrient solution were analysed for phosphate using the FIP technique. It is shown that FIP results compare favourably to standard methods of analysis such as spectrophotometry and indirect photometric ion-pair chromatography. Reproducible FIP response curves with a slope of −(47.57±0.03) mV per decade and intercept of −(169.7±0.1) mV were obtained for four separate calibrations in the concentration range 5.0×10−4–1.0×10−2 M H2PO4−. Anion corrections for interferences by Cl−, NO3− and SO42− were applied to all samples using the selectivity coefficients determined independently using a fixed interference method. Nevertheless, it was found that anion corrections were not necessary, as the deviations fell within the bounds of experimental error for the cobalt-wire ISE technique (i.e.±2–5% R.S.D.). The proposed FIP method enables the direct determination of phosphate in hydroponic nutrient solutions. 相似文献
75.
Marco Berettoni Giovanna De Chiara Tommaso Iacoangeli Paola Lo Surdo Rinaldo Marini Bettolo Lorenzo Montagnini Di Mirabello Luca Nicolini Rita Scarpelli 《Helvetica chimica acta》1996,79(7):2035-2041
A novel preparation of methyl (13S)-13-hydroxyisoatisiren-18-oate ( 4 ), a key-intermediate in a synthesis of (+)-methyl trachyloban-18-oate ((+)- 1 ), from (?)-abietic acid, is described. Since (?)- 1 has been previously converted into (?)-methyl 16-oxo-17-norkauran-18-oate ((?)- 16 ), our preparation of 4 constitutes also a formal total synthesis, from (?)-abietic acid, of (+)- 16 . Key steps in this approach were the allene photoaddition to podocarp-8(14)-en-13-one ( 5 ) and the conversion of the endo-toluene-4-sulfonate 11 into the exo-benzoate 12b . 相似文献
76.
The bond dissociation enthalpies (BDE) of several phenols containing electron-withdrawing substituents in the para position have been determined by means of the EPR radical equilibration technique. It has been found that CN, NO(2), CHO, COOR, and COOH induce an increase of the BDE value of the O-H bond, thus producing a worsening of the antioxidant activity of phenols, while Cl, Ph, and CH[double bond]CHPh show an opposite effect. The contributions of these substituents for the calculation of the BDE values in polysubstituted phenols by using the group additivity rule have also been derived. It is shown that this rule provides quite reliable predictions of bond strengths, so that the method can be conveniently used to estimate new data on substituted phenols. 相似文献
77.
Optical sensors based on hybrid DNA/conjugated polymer complexes 总被引:2,自引:0,他引:2
Ho HA Béra-Abérem M Leclerc M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(6):1718-1724
Single-stranded DNA (ss-DNA) can specifically bind to various targets, including a complementary ss-DNA, ions, proteins, drugs, and so forth. When binding takes place, the oligonucleotide probe often undergoes a conformational transition. This conformational change of the negatively charged ss-DNA can be detected by using a water-soluble, cationic polythiophene derivative, which transduces the complex formation into an optical (colorimetric or fluorometric) signal without any labeling of the probe or the target. This simple and rapid methodology has enabled the specific and sensitive detection of nucleic acids and human thrombin. This new biophotonic tool can easily be applied to the detection of various other biomolecules and is also useful in the high-throughput screening of new drugs. 相似文献
78.
The kinetics of the process AuCl3(nu) + Cl−→ AuCl
4
−
+ nu (nu = one of a number of five-membered N-donor heterocycles covering a wide range of basicity, namely: oxazole, 2,4,5-trimethyloxazole,
thiazole, 5-methylthiazole, 4-methylthiazole, 4,5-dimethylthiazole, 2,4-dimethylthiazole, 2,4,5-trimethylthiazole, imidazole
and 2-methylimidazole) have been studied in methanol at 25 °C. The reactions follow the usual two-term rate law, rate = (k
1 + k
2[Cl−])[complex], observed in a square-planar substitution associative-mechanism. The second-order rate constants, k
2, indicate that the discriminating ability of Au(III) in these complexes is good and markedly influenced by the nature of the leaving group. A linear-free-energy relationship,
logk
2 = 0.53pK
a
+ constant, is observed between the rate constant and the basicity of the leaving group for its replacement by chloride.
The results are compared with data from the literature regarding a series of complexes of the type AuCl3(py) (py = one of a number of pyridines) reacting with the Cl− anion under the same experimental conditions. The reactivity depends not only upon ligand basicity but also upon the nature
of the ligand in the order: pyridines> five-membered heterocycles. Steric factor due to the presence of a single methyl group
ortho to the sp2 nitrogen atom in the ring has no influence on the rate of substitution while, surprisingly, when there are two ortho methyl groups a remarkable steric retardation effect is observable. The results are discussed in terms of reaction-profile
in the associative-substitution reaction and bonding interactions in the ground and transition states. 相似文献
79.
Scrimin P Tecilla P Tonellato U Veronese A Crisma M Formaggio F Toniolo C 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(12):2753-2763
Three copies of peptide sequences from the peptaibol family, known to affect the permeability of the lipid bilayer of membranes, were connected to tris(2-aminoethyl)amine (TREN), a tripodal metal ion ligand, to prepare functional peptides capable of modifying the permeability of liposomal membranes. Some of the resulting tripodal polypeptide derivatives are very effective in promoting carboxyfluorescein (CF) leakage from CF-loaded unilamellar vesicles composed of a 70:30 phosphatidylcholine/cholesterol blend. The activity of these novel compounds was shown to be tunable upon metal ion coordination of the TREN subunit; the tripodal apopeptide was far more effective than its ZnII complex. Leakage experiments showed that a minimum number of five amino acids per peptide chain is required to form active systems. A mechanism is proposed in which the ZnII ion changes the conformation of the template from extended to globular and thus acts as an allosteric regulator of the activity of the systems. Molecular modeling studies indicate that when the three peptide chains are connected to the template in the extended conformation, the resulting tripodal polypeptide is able to span across the membrane, thus allowing the formation of permeable channels made of a cluster of molecules. The same change of conformation induces, to some extent, fusion of the membranes of different liposomes. 相似文献
80.
The systematic preparation of partially fluorinated pentafluorosulfur alkanes containing no additional halogens is reported. Thus, the indirect addition of “HF” (via KF/formamide) to SF5CH=CF2, SF5CFCF2, and SF5C(CF3)CF2 produces SF5CH(in2)CF3, SF5CHFCF3, and SF5C(CF3)2H respectively. The monohydryl-pentafluorosulfur-F- alkanes react readily with S2O6F2 to form the corresponding fluorosulfates by oxidative displacement of hydrogen, while the dihydryl derivative undergoes cleavage to produce F-acetyl fluoride. Efforts to convert some of the new materials to the important but unknown pentafluorosulfur “ketone,” SF5C(O)CF3, were unsuccessful. 相似文献