全文获取类型
收费全文 | 77580篇 |
免费 | 1708篇 |
国内免费 | 300篇 |
专业分类
化学 | 40318篇 |
晶体学 | 530篇 |
力学 | 2193篇 |
综合类 | 13篇 |
数学 | 18208篇 |
物理学 | 18326篇 |
出版年
2023年 | 372篇 |
2022年 | 388篇 |
2021年 | 613篇 |
2020年 | 778篇 |
2019年 | 738篇 |
2018年 | 2189篇 |
2017年 | 2347篇 |
2016年 | 2516篇 |
2015年 | 2025篇 |
2014年 | 1915篇 |
2013年 | 3901篇 |
2012年 | 5898篇 |
2011年 | 5177篇 |
2010年 | 3631篇 |
2009年 | 3105篇 |
2008年 | 3121篇 |
2007年 | 2926篇 |
2006年 | 2686篇 |
2005年 | 6405篇 |
2004年 | 5627篇 |
2003年 | 3717篇 |
2002年 | 1798篇 |
2001年 | 1290篇 |
2000年 | 1092篇 |
1999年 | 857篇 |
1998年 | 663篇 |
1997年 | 544篇 |
1996年 | 644篇 |
1995年 | 467篇 |
1994年 | 546篇 |
1993年 | 492篇 |
1992年 | 640篇 |
1991年 | 568篇 |
1990年 | 584篇 |
1989年 | 479篇 |
1988年 | 471篇 |
1987年 | 408篇 |
1986年 | 382篇 |
1985年 | 505篇 |
1984年 | 469篇 |
1983年 | 371篇 |
1982年 | 360篇 |
1981年 | 387篇 |
1980年 | 312篇 |
1979年 | 329篇 |
1978年 | 319篇 |
1977年 | 302篇 |
1976年 | 357篇 |
1974年 | 306篇 |
1973年 | 320篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
921.
Fernández LE Verón MG Varetti EL 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2004,60(1-2):405-411
Force fields and vibrational wavenumbers were calculated for the molecules SO2X2 (X=F, Cl, Br) using DFT techniques. The previously available experimental data and assignments for SO2F2 and SO2Cl2 were compared with the theoretical results and revised, and new low temperature infrared and Raman data were obtained for SO2Cl2. These data were subsequently used in the definition of scaled quantum mechanics force fields for such molecules. Adjusted wavenumbers were also predicted for the still unknown SO2Br2. A comparison is made with results published for the VO2X2- anions. 相似文献
922.
García-Orozco I López-Cortés JG Ortega-Alfaro MC Toscano RA Penieres-Carrillo G Alvarez-Toledano C 《Inorganic chemistry》2004,43(26):8572-8576
The paper describes the synthesis and structural characterization of four novel copper(I) complexes [CuL(PPh(3))(2)] (L = 3-hydroxy-3-(p-R-phenyl)-2-propenedithioate). In addition, a tautomeric equilibrium in solution was found and Hammett correlations with (13)C NMR parameters were studied. The structure of one complex was fully established by X-ray diffraction analysis. 相似文献
923.
Herein, we present results from MD simulations of the Michaelis complex formed between the B. cereus zinc-beta-lactamase enzyme and benzylpenicillin. The structural and dynamical effects induced by substrate-binding, the specific role of the conserved residues, and the near attack conformers of the Michaelis complex are discussed. Quantum chemical methods (HF/6-31G* and B3LYP/6-31G*) are also applied to study the hydrolysis reaction of N-methylazetidinone catalyzed by a monozinc system consisting of the side chains of the histidine residues (His86, His88, and His149) complexed with Zn-OH and the side chains of Asp90 and His210. From this model system, we built molecular-mechanics representations of the prereactive complex and transition state configurations docked into the active site. Linear-scaling semiempirical calculations coupled with a continuum solvent model were then performed on these static models. We propose that the experimental rate data for the B. cereus enzyme is compatible with a one-step mechanism for the hydrolysis of beta-lactam substrates in which His210 acts as a proton donor. 相似文献
924.
The thermal properties of cyanatocopper complexes with pyridine, bipyridine and phenanthroline are described in this paper. It was found that the thermal stabilities of the complexes were found to increase in the order pybipy2(NCS)2 (L=pyridine and its methyl derivates) which are composed of the liberation of ligandsL and redox reactions of thiocyanate ligands with the central Cu(II) atom [1, 2]. The decomposition temperature of thiocyanate ligands depends on the properties of the ligandsL present in the coordination sphere. An analogous course of thermal decomposition was also observed for [Cu(bipy)2(NCX)](NCX) (X=S or Se) [3]. For the phenanthroline complexes [Cu(phen)2(NCX)2] (X=S or Se) the thermal stability was found to increase significantly (by about 140C) and the decomposition stoichiometry was also changed [3].The present paper contains the results of thermoanalytical studies on bipyridine and phenanthroline cyanatocopper complexes, and a comparison of the thermal properties of pyridine cyanato and isothiocyanatocopper complexes. 相似文献
925.
The ability of enzymes to work in non-aqueous media offers new and almost unexploited possibilities for the development of new optical biosensors. The advantages of performing biocatalytic reactions in non-aqueous media are discussed in relation to their possible application in optical biosensor design. Attention is focused on the factors that influence enzymatic catalysis in organic solvents, including the role of enzyme-associated water, criteria for solvent selection and the alteration of enzyme specificity. Recent examples of relevant applications and future prospects of organic-phase optical biosensing are discussed. 相似文献
926.
Iliyana V. Berlinova Aissa Amzil Svetlana Tsvetkova Ivan M. Panayotov 《Journal of polymer science. Part A, Polymer chemistry》1994,32(8):1523-1530
Amphiphilic graft copolymers were synthesized via activated ester substitution of derivatives of fumaric acid with amino-functionalized methoxypoly (oxyethylene)s (MPEO-NH2) of different molecular weights. The monomeric activated esters, isopropyl pentachlorophenyl fumarate (PCPFA) and isopropyl succinimido fumarate (SIFA), were copolymerized with styrene (St) or N-vinyl pyrrolidone (VP) at equimolar ratio. The polymeric-activated esters proved to be good precursors for grafting of definite amounts of MPEO-NH2. The aminolysis of the succinimide esters and VP-containing copolymers proceeded with gel formation due to extensive hydrogen bonding. The hydrodynamic behavior, the emulsifying ability, the thermal properties, and crystallinity of the graft copolymers were studied as a function of their molecular characteristics. The length of the PEO grafts and the degree of grafting are the factors which affect the melting parameters and the crystallinity of the side chains. © 1994 John Wiley & Sons, Inc. 相似文献
927.
José M. Sánchez-Montero JOSé V. Slnisterra Antonio Ballesteros 《Applied biochemistry and biotechnology》1989,22(2):205-214
The endonuclease from S. aureus has been immobilized on ground maize cob, previously activated with tosyl chloride. Pretreatment of the support on acid before tosylation yielded the best insoluble enzyme derivatives. The catalytic activity has been evaluated as percent of total hydrolysis attained in a batch reactor using DNA as a model substrate. The derivatives prepared are very resistant to high temperatures under conditions of catalysis (24 h at 45 degrees C). For these long reaction times, the extent of hydrolysis in the presence of small amounts of organic solvent (dimethyl sulfoxide at 2 percent) is larger than in plain buffer (Tris). This type of derivative could be very useful for the removal of nucleic acids from single-cell protein concentrates. 相似文献
928.
When excesses of ammonium thiocyanate and tetrabutylammonium chloride are added to a cobalt(II) solution, a water-insoluble ion-pair is formed; this compound is soluble in chloroform, isoamyl alcohol, and other solvents. The variables which affect the extractions by these two solvents are studied in order to obtain the optimal conditions and two alternative extraction procedures are proposed. Thus, the separation of cobalt as a previous step for its spectrophotometric determination is studied. 相似文献
929.
José I. Concepción Cosme G. Francisco Rosendo Hernández José A. Salazar Ernesto Suárez 《Tetrahedron letters》1984,25(18):1953-1956
Photolysis of several hydroxy compounds in presence of iodosobenzene diacetate and iodine leads to alkoxy radical derivatives which undergo intramolecular hydrogen abstraction to produce cyclic ethers in good yield. 相似文献
930.
The influence of coulomb centers (coulomb traps and photogeneration centers) on the intensity of a thermally stimulated current (TSC) transport peak is examined. We apply these concepts to account for the field dependent transport charge associated to the area under the TSC traces resulting from poly(N-vinylcarbazole).Dedicated to Professor Dr. F. H. Müller. 相似文献