全文获取类型
收费全文 | 1843篇 |
免费 | 72篇 |
国内免费 | 7篇 |
专业分类
化学 | 1200篇 |
晶体学 | 16篇 |
力学 | 38篇 |
数学 | 203篇 |
物理学 | 465篇 |
出版年
2023年 | 17篇 |
2022年 | 45篇 |
2021年 | 52篇 |
2020年 | 38篇 |
2019年 | 39篇 |
2018年 | 30篇 |
2017年 | 21篇 |
2016年 | 51篇 |
2015年 | 54篇 |
2014年 | 52篇 |
2013年 | 99篇 |
2012年 | 99篇 |
2011年 | 123篇 |
2010年 | 73篇 |
2009年 | 74篇 |
2008年 | 119篇 |
2007年 | 110篇 |
2006年 | 109篇 |
2005年 | 91篇 |
2004年 | 63篇 |
2003年 | 50篇 |
2002年 | 57篇 |
2001年 | 32篇 |
2000年 | 34篇 |
1999年 | 20篇 |
1998年 | 16篇 |
1997年 | 20篇 |
1996年 | 23篇 |
1995年 | 16篇 |
1994年 | 27篇 |
1993年 | 18篇 |
1992年 | 20篇 |
1991年 | 21篇 |
1990年 | 13篇 |
1989年 | 12篇 |
1988年 | 14篇 |
1987年 | 10篇 |
1986年 | 15篇 |
1985年 | 19篇 |
1984年 | 22篇 |
1983年 | 15篇 |
1982年 | 10篇 |
1981年 | 9篇 |
1980年 | 10篇 |
1979年 | 11篇 |
1978年 | 11篇 |
1977年 | 11篇 |
1976年 | 5篇 |
1975年 | 5篇 |
1974年 | 5篇 |
排序方式: 共有1922条查询结果,搜索用时 15 毫秒
51.
A. Alberola L. F. Antolin A. M. Gonzalez M. A. Laguna F. J. Pulido 《Journal of heterocyclic chemistry》1986,23(4):1035-1038
The base-induced ring cleavage of 4-nitro-(Ia), 4-ethoxycarbonyl- (Ib) and 4-acetyl-5-methylisoxazole (Ic) and the conversion of the resulting β-cyanoenolates and β-enaminonitriles into 5-aminoazoles and 4-cyanoazoles was studied. 相似文献
52.
The synthesis, characteristics, properties and reactions with metallic ions of six aromatic derivatives of 2-thiohydantoin have been studied. The reagents exhibit two pK values in aqueous solution, and with Pd(II), Au(III), Ag(I) and Hg(I) and (II), especially with the first, form stable complexes with molar absorptivities adequate for spectrophotometric measurements. 相似文献
53.
Helical rosette nanotubes are obtained through the self-assembly of low molecular weight synthetic modules in water. Here we demonstrate that despite their dynamic nature, these materials respond very well to directional fluid flow and assume long-range order on flat substrates. Persistence length, order, and packing of the rosette nanotubes were found to depend dramatically on the surface properties of both the substrate and the nanotubes and vary from well-ordered long-range 2D films to bundled nanotubes or amorphous conglomerates. While flow-induced long-range alignment of dynamic nanostructures is unprecedented, the chemical tunability of the rosette nanotubes is anticipated to offer a versatile means for investigating the basis of interfacial forces in self-assembled organo-silicon devices and their effect on the stability and physical properties of organic nanostructures on electroactive surfaces. 相似文献
54.
Quantitative separation of zinc traces from cadmium matrices by solid-phase extraction with polyurethane foam 总被引:1,自引:0,他引:1
Santiago de Jesus D Souza de Carvalho M Spínola Costa AC Costa Ferreira SL 《Talanta》1998,46(6):1525-1530
A system for separation of zinc traces from large amounts of cadmium is proposed in this paper. It is based on the solid-phase extraction of the zinc in the form of thiocyanate complexes by the polyurethane foam. The following parameters were studied: effect of pH and of the thiocyanate concentration on the zinc extraction, shaking time required for quantitative extraction, amount of PU foam necessary for complete extraction, conditions for the separation of zinc from cadmium, influence of other cations and anions on the zinc sorption by PU foam, and required conditions for back extraction of zinc from the PU foam. The results show that zinc traces can be separated from large amounts of cadmium at pH 3.0±0.50, with the range of thiocyanate concentration from 0.15 to 0.20 mol l−1, and the shaking time of 5 min. The back extraction of zinc can be done by shaking it with water for 10 min. Calcium, barium, strontium, magnesium, aluminum, nickel and iron(II) are efficiently separated. Iron(III), copper(II) and cobalt(II) are extracted simultaneously with zinc, but the iron reduction with ascorbic acid and the use of citrate to mask copper(II) and cobalt(II) increase the selectivity of the zinc extraction. The anions nitrate, chloride, sulfate, acetate, thiosulphate, tartarate, oxalate, fluoride, citrate, and carbonate do not affect the zinc extraction. Phosphate and EDTA must be absent. The method proposed was applied to determine zinc in cadmium salts using 4-(2-pyridylazo)-resorcinol (PAR) as a spectrophotometric reagent. The result achieved did not show significant difference in the accuracy and precision (95% confidence level) with those obtained by ICP–AES analysis. 相似文献
55.
F. R. Gonzalez J. L. Alessandrini A. M. Nardillo 《Journal of chromatography. A》1998,810(1-2):105-117
The contributions to the free energy change involved in the chromatographic retention of n-alkanes, which are functions of the solute chain length, are discussed with the aid of recent advances in chain statistics. In the context of a simplified theoretical analysis, a discussion is promoted on the molecular nature of the observed slight deviations from linearity of ln tR′ vs. n. A non-linear function ln tR′(n) is derived from the application of the generalized Van der Waals fluid partition function to the liquid stationary phase. The excluded volume, particularly its dependence on n, is analyzed through the theory of Flory for dilute solutions of chain molecules. 相似文献
56.
Felix H. Cano Concha Foces-Foces Manuel Bernabe Jesus Jimenez-Barbero Manuel Martin-Lomas Soledad Penades-Ullate 《Tetrahedron》1985,41(18):3875-3886
X-Ray and 1H N.M.R. studies on pyranoid rings 1,2--fused to dioxolane rings in acetylated -gluco- and --galactopyranose derivatives demonstrate that the configuration of the dioxolane ring influences the conformation of the pyranoid ring in the -gluco but not in the -galactopyranose series. The crystal structure of 3,4,6-tri--acetyl-1,2--(R)--(l-cyano-ethylidene)-α--glucopyranose () and 3,4,6-tri--acetyl-1,2--()-(1-cyano-ethylidene)-α--galactopyranose ()have been determined by X-ray analysis. Lattice parameters for are a=20.6021 (11), b=8.0438 (2), c=5.5541 (1) Å and β= 95.588 (3)° for a cell with P21 symmetry. These parameters for are a=20.3361 (7), b=10.0907 (2), c=18.9115 (5) Å, β =112.399 (2)°, C2, with two crystallographycally independent molecules. The conformation of the pyranoid ring in both compounds can be described as flattened 4C1 and that of the dioxolane ring as distorted E1. The importance of the torsion angles for describing problems of configuration is remarked and the use of relative configurational angles is stressed. The 1H N.M.R. spectra of and and 3,4,6-tri--acetyl-1,2-O-(S)- and (R)-ethylidene-α--glucopyranose ( and ), 3,4,6-tri-O-acetyl--1,2-O-(S)- and ()-ethylidene-α--galactopyranose ( and ), and 3,4,6-tri--acetyl-1,2--()-and ()-benzylidene-α--glucopyranose ( and ) have been analyzed by using iterative computer methods and N.O.E. measurements. The results indicate that the major solution conformation of the pyranoid ring of the derivatives in the -gluco series , and may be described as flattened 4C1 and that of and as 25. The major solution conformation of the pyranoid ring in all compounds in the -galacto series (,,,) may be described as flattened 41. 相似文献
57.
We prove some facts concerning surfaces of minimal area bounding regions of prescribed volume in n. The main result we prove is that the mean curvature of such a surface is constant, if possibly a discontinuous function of the enclosed volume. The boundary behaviour of the solutions is also discussed. 相似文献
58.
L.M. León M.C. Gonzalez J.R. Quintana F. Zamora Armando Martinez G.M. Guzman 《European Polymer Journal》1982,18(3):229-231
The Ptitsyn-Eizner λ flexibility parameter for poly(N-vinylcarbozole) solutions under theta conditions has been found to be 6.31, demonstrating the rigidity imparted to the polymer chain by the carbazole group. A study has been made of the variation of this parameter with solvent and temperature. A study has been made of the variation of λ in a conformational transition of poly(methyl methacrylate). 相似文献
59.
Dabkowska I Gonzalez HV Jurecka P Hobza P 《The journal of physical chemistry. A》2005,109(6):1131-1136
Stabilization energies of the H-bonded and stacked structures of a DNA base pair were studied in the crystal structures of adenine-thymine, cytosine-guanine, and adenine-cytosine steps as well as in the 5'-d(GCGAAGC)-3' hairpin (utilizing the NMR geometry). Stabilization energies were determined as the sum of the complete basis set (CBS) limit of MP2 stabilization energies and the Delta E(CCSD(T)) - Delta E(MP2) correction term evaluated with the 6-31G*(0.25) basis set. The CBS limit was determined by a two-point extrapolation using the aug-cc-pVXZ basis sets for X = D and T. While the H-bonding energies are comparable to those of base pairs in a crystal and a vacuum, the stacking energies are considerably smaller in a crystal. Despite this, the stacking is still important and accounts for a significant part of the overall stabilization. It contributes equally to the stability of DNA as does H-bonding for AT-rich DNAs, while in the case of GC-rich DNAs it forms about one-third of the total stabilization. Interstrand stacking reaches surprisingly large values, well comparable to the intrastrand ones, and thus contributes significantly to the overall stabilization. The hairpin structure is characterized by significant stacking, and both guanine...cytosine pairs possess stacking energies larger than 11.5 kcal/mol. A high portion of stabilization in the studied hairpin comes from stacking (similar to that found for AT-rich DNAs) despite the fact that it contains two GC Watson-Crick pairs having very large H-bonding stabilization. The DFT/B3LYP/6-31G** method yields satisfactory values of interaction energies for H-bonded structures, while it fails completely for stacking. 相似文献
60.