全文获取类型
收费全文 | 13901篇 |
免费 | 407篇 |
国内免费 | 47篇 |
专业分类
化学 | 11124篇 |
晶体学 | 79篇 |
力学 | 213篇 |
数学 | 1705篇 |
物理学 | 1234篇 |
出版年
2023年 | 122篇 |
2022年 | 480篇 |
2021年 | 550篇 |
2020年 | 308篇 |
2019年 | 346篇 |
2018年 | 268篇 |
2017年 | 225篇 |
2016年 | 462篇 |
2015年 | 467篇 |
2014年 | 437篇 |
2013年 | 851篇 |
2012年 | 889篇 |
2011年 | 999篇 |
2010年 | 600篇 |
2009年 | 584篇 |
2008年 | 840篇 |
2007年 | 823篇 |
2006年 | 695篇 |
2005年 | 645篇 |
2004年 | 542篇 |
2003年 | 458篇 |
2002年 | 443篇 |
2001年 | 174篇 |
2000年 | 136篇 |
1999年 | 115篇 |
1998年 | 114篇 |
1997年 | 100篇 |
1996年 | 131篇 |
1995年 | 86篇 |
1994年 | 84篇 |
1993年 | 78篇 |
1992年 | 62篇 |
1991年 | 77篇 |
1990年 | 56篇 |
1989年 | 78篇 |
1988年 | 64篇 |
1987年 | 55篇 |
1986年 | 57篇 |
1985年 | 65篇 |
1984年 | 85篇 |
1983年 | 48篇 |
1982年 | 74篇 |
1981年 | 60篇 |
1980年 | 60篇 |
1979年 | 51篇 |
1978年 | 57篇 |
1977年 | 41篇 |
1976年 | 33篇 |
1975年 | 25篇 |
1963年 | 20篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Ewa Katzenellenbogen Nina A. Kocharova Georgy V. Zatonsky Maria Bogulska Danuta Witkowska Aleksander S. Shashkov 《Journal of carbohydrate chemistry》2013,32(5):545-558
ABSTRACT Lipopolysaccharide (LPS) of Hafnia alvei 23 has an acid-labile O-specific polysaccharide (OPS) with a pentasaccharide-phosphate repeating unit containing D-Glc1P, D-GlcNAc, L-Fuc, 6-deoxy-D-talose (D-6dTal), 4-acetamido-4,6-dideoxy-D-glucose (D-Qui4NAc), and an O-acetyl group. A partially degraded OPS was obtained by hydrolysis of LPS with 0.25 M sodium acetate in aqueous 0.5% acetic acid. Fractionation of LPS on Sephadex G-200 in DOC buffer allowed isolation of long-chain LPS species which, together with OPS, were studied by methylation analysis, chemical degradations (O-deacetylation, dephosphorylation with 48% hydrofluoric acid, Smith degradation), and 1H and 13C NMR spectroscopy, including 2D COSY, TOCSY, NOESY, and H-detected 1H,13C heteronuclear single-quantum coherence (HSQC) experiments. The following structure of the repeating unit of OPS was established: 相似文献
62.
Teresa Olszewska Maria J. Milewska Maria Gdaniec Tadeusz Połoński 《Tetrahedron: Asymmetry》2012,23(3-4):278-283
The crystal structures of several bicyclic N-nitrosamines indicate that they crystallize in the chiral (Sohncke) space group P212121 as conglomerates. This allows the resolution of these compounds by manual picking of the enantiomorphous crystals. The optical activity of the single crystals was confirmed by their CD spectra taken in KBr disks. The absolute configurations of the title nitrosamines were assigned by crystallographic measurements and by a comparison of their CD spectra with those of a reference compound resolved by classical methods. The observed Cotton effect signs, corresponding to the n–π1 transition, were correlated with the helicity of the inherently chiral nitrosamine chromophore. 相似文献
63.
64.
65.
We present a simplified correlation for calculating the dissolved gas moles in a pendant drop during the diffusion time, for several drop shapes. After this correlation is determined, the Yang and Gu (Ind Eng Chem Res 44:4474–4483, 2005) dynamic pendant drop volume analysis (DPDVA) method for calculation of mass diffusivity from the pendant drop volume variation against time can be used. We solved the differential equation in cylindrical coordinates for the mass transfer model of the gas diffusion into the liquid inside the pendant drop, using a different characteristic length (LC), instead of the outer radius of the syringe needle (rn) used in Yang and Gu (Ind Eng Chem Res 44:4474–4483, 2005) for defining the dimensionless variables. LC is the relationship between the pendant drop volume and its mass transfer surface area at the initial conditions. The generalized correlation saves time, simplifies the method application and the deviations in the diffusion coefficient calculation respect to the complete Yang and Gu model are below 6%. 相似文献
66.
67.
Maria Esmeralda C. de Sousa Valria Wanderley-Teixeira lvaro A.C. Teixeira Herbert A.A. de Siqueira Fbio A.B. Santos Luiz C. Alves 《Micron (Oxford, England : 1993)》2009,40(7):743-749
The insect midgut has ultimately been the focus of researches tempting to control insect pests because alterations in the insect gut may affect not only its development, but also physiological events such as nutrient absorption and transformation. The objective of the present work was to describe morphologically, histochemically, and ultrastructurally the larva midgut of Alabama argillacea (Hübner) (Lepidoptera: Noctuidae), a cotton key pest in Brazil. Light and electronic transmission microscopy was used to obtain images from midgut sections of late fourth-instar larvae of A. argillacea. In general, the morphology, histochemistry, and ultrastructure characteristics of A. argillacea midgut follow that described in the literature for other lepidopteran species. However, the results showed a mitochondrial polymorphism and branched microvilli, which suggest an ultrastrucutural and physiological modification possibly associated with a high absorption and secretion activity by the columnar cells of this species. This intense activity may favor a faster response related to the action of ingested microbial agents and/or toxins, and can explain the high susceptibility of A. argillacea to the agents of control such as the toxin of Bacillus thuringiensis. 相似文献
68.
Elena O. Alonso Maria E. R. Marcondes Vicente G. Toscano 《Phosphorus, sulfur, and silicon and the related elements》2013,188(3-4)
Abstract As part of our continuing interest in the mechanistic photochemistry of phosphonium salts (Toscano et al., Chez. Commun., 567 (1973); J. Phys. Chem., 83(9), 1213 (1979)) we have investigated the photodecomposition (313 nm irradiation) of (1-naphthyl-methyl) triphenylphosphonium (I) chloride and tetrafluoroborate in deaerated isopropanol: 相似文献
69.
Giovanna Delogu Ottorino De Lucchi Maria Paola Fois Giovanni Valle 《Phosphorus, sulfur, and silicon and the related elements》2013,188(3-4):417-425
Abstract Ketimines la-d derived from ortho-aminosubstituted phenylthioethers were prepared in order to determine the degree of chirality transfer from the chiral auxiliary to the sulfur atom in the formation of the sulfoxide or to the α-carbon atom in the reaction of the anion with alkyl halides or benzaldehyde. Oxidation to the sulfoxide occurred with little or no asymmetric induction. The crystalline benzyl sulfone 4c was deprotonated by alkyllithium or Grignard reagents and reacted with alkyl halides and benzaldehyde, in all cases with little to fair transfer of chirality. The major diastereoisomer from methylation of the anion of 4c with methyl iodide, was isolated, and afforded the enantiomerically pure amine 5 after removal of the chiral auxiliary. An X-ray structure determination of 4d allowed the assignment of the absolute configuration of the asymmetric carbon and revealed that the conformation of the ketimine in the crystal state is not homogeneous. 相似文献
70.
Mauro Formica Luca Giorgi Eleonora Macedi Giovanni Piersanti Maria Antonietta Varrese 《Supramolecular chemistry》2013,25(6):365-379
The synthesis and anion binding studies of the new neutral receptor 1,1′-(2,2′-(4,10-dimethyl-1,4,7,10-tetraazacyclododecane-1,7-diyl)bis(2-oxoethane-2,1-diyl))bis(3-(3,5-bis(trifluoromethyl)phenyl)urea) (L1) are reported. L1 is a macrocyclic ligand containing the 3,5-trifluoromethylphenylureido-binding fragment attached as a side arm on the tetraazacyclododecane. L1 is soluble in numerous organic solvents; the binding properties of L1 towards several simple anions (G) were investigated by NMR, UV–vis and fluorescence techniques in DMSO and CH3CN solutions. L1 is able to bind F? , Cl? and AcO? in both solvents; in addition, it binds Br? in CH3CN. Fluoride shows the highest constant values in the halide series (F? > Cl? > Br? ) and AcO? is the most strongly bound among all the anions investigated. L1 is able to signal the presence of the anions in solution by fluorescence change; in the case of acetate, this occurs in the visible range. 相似文献