全文获取类型
收费全文 | 4786篇 |
免费 | 217篇 |
国内免费 | 26篇 |
专业分类
化学 | 3274篇 |
晶体学 | 72篇 |
力学 | 83篇 |
数学 | 549篇 |
物理学 | 1051篇 |
出版年
2023年 | 19篇 |
2022年 | 24篇 |
2021年 | 80篇 |
2020年 | 72篇 |
2019年 | 89篇 |
2018年 | 87篇 |
2017年 | 70篇 |
2016年 | 177篇 |
2015年 | 138篇 |
2014年 | 151篇 |
2013年 | 324篇 |
2012年 | 323篇 |
2011年 | 390篇 |
2010年 | 238篇 |
2009年 | 241篇 |
2008年 | 307篇 |
2007年 | 295篇 |
2006年 | 263篇 |
2005年 | 224篇 |
2004年 | 196篇 |
2003年 | 185篇 |
2002年 | 172篇 |
2001年 | 90篇 |
2000年 | 75篇 |
1999年 | 55篇 |
1998年 | 24篇 |
1997年 | 43篇 |
1996年 | 47篇 |
1995年 | 30篇 |
1994年 | 26篇 |
1993年 | 29篇 |
1992年 | 24篇 |
1991年 | 17篇 |
1990年 | 30篇 |
1989年 | 34篇 |
1988年 | 17篇 |
1987年 | 16篇 |
1986年 | 23篇 |
1985年 | 28篇 |
1984年 | 29篇 |
1983年 | 18篇 |
1982年 | 21篇 |
1981年 | 28篇 |
1980年 | 23篇 |
1979年 | 20篇 |
1978年 | 28篇 |
1977年 | 23篇 |
1976年 | 24篇 |
1975年 | 21篇 |
1973年 | 16篇 |
排序方式: 共有5029条查询结果,搜索用时 15 毫秒
131.
Kang KR Kim JS Chung SI Park MH Kim YW Lim D Lee SY 《Experimental & molecular medicine》2002,34(6):489-495
Deoxyhypusine synthase catalyzes the first step in the posttranslational synthesis of an unusual amino acid, hypusine, in the eukaryotic translation initiation factor 5A (eIF-5A) precursor protein. We earlier observed that yeast recombinant deoxyhypusine synthase was phosphorylated by protein kinase C (PKC) in vitro (Kang and Chung, 1999) and the phosphorylation rate was synergistically increased to a 3.5-fold following treatment with phosphatidylserine (P.Ser)/diacylglycerol (DAG)/ Ca(2+), suggesting a possible involvement of PKC. We have extended study on the phosphorylation of deoxyhypusine synthase in vivo in different cell lines in order to define its role on the regulation of eIF5A in the cell. Deoxyhypusine synthase was found to be phosphorylated by endogenous kinases in CHO, NIH3T3, and chicken embryonic cells. The highest degree of phosphorylation was found in CHO cells. Moreover, phosphorylation of deoxyhypusine synthase in intact CHO cells was revealed and the expression of phosphorylated deoxyhypusine synthase was significantly diminished by diacyl ethylene glycol (DAEG), a PKC inhibitor, and enhanced by phorbol 12-myristate 13-acetate (PMA) or Ca(2+)/DAG. Endogenous PKC in CHO cell and cell lysate was able to phosphorylate deoxyhypusine synthase and this modification is enhanced by PMA or Ca(2+) plus DAG. Close association of PKC with deoxyhypusine synthase in the CHO cells was evident in the immune coprecipitation and was PMA-, and Ca(2+)/phospholipid dependent. These results suggest that phosphorylation of deoxyhypusine synthase was PKC-dependent cellular event and open a path for possible regulation in the interaction with eIF5A precursor for hypusine synthesis. 相似文献
132.
Seung‐Chul Choi Jaejung Ko Sang Ook Kang Won‐Sik Han Ki‐Young Choi 《Acta Crystallographica. Section C, Structural Chemistry》2007,63(3):m85-m87
The copper(II) ion in the syn–anti carboxylate‐bridged one‐dimensional zigzag chain title complex, {[Cu(C16H18N3O2)]ClO4}n, exhibits a distorted trigonal–bipyramidal environment. Two N atoms and one carboxylate O atom of the ligand form the basal plane, while the axial positions are filled by an N atom of the ligand and one O atom belonging to the carboxylate group of an adjacent molecule. The crystal packing is enhanced by C—H⋯O(perchlorate) hydrogen bonds. 相似文献
133.
W. Kołos 《Chemical physics letters》1976,39(3):416-417
The interaction energy between two hydrogen atoms in the D and D′ 1Πu states of the hydrogen molecule has been calculated for large internuclear distances (12 ? R ? 25 bohr). The variational method and a very flexible trial wave-function were used. The results indicate that for the states under consideration the Rayleigh-Schrödinger perturbation theory with the multipole expansion of the interaction hamiltonian gives reliable results only for R > 25 bohr i.e. in the region where the interaction energies are practically negligible. 相似文献
134.
Renata Wietecha Paweł Kościelniak Teresa Lech Tomasz Kielar 《Mikrochimica acta》2005,149(1-2):137-144
This study describes a simple, rapid and reliable method for simultaneous determination of selenium and arsenic in human hair by means of atomic fluorescence spectrometry combined with a hydride generation technique (HG-AFS). The procedure developed encompasses microwave digestion of a sample in the nitric acid environment only. The interferences caused by nitrous oxides are eliminated by removing a gas from above the digested solution with a stream of argon. The sample is then chemically treated in a flow-through hydride generation system and exposed to measurements in a double-channel atomic fluorescence spectrometer. The method permits determining both analytes in the linear range of 0.5–100µgL–1 with a detection limit equal to 0.2µgL–1, as well as with very good repeatability not exceeding 1% for Se and 2% for As. No mutual interferences from either of the analytes in the concentrations ranges matching the hair composition were found. The method was verified in terms of accuracy with the use of a reference material and then applied to the analysis of the natural samples of human hair. 相似文献
135.
The reaction of trimethylbenzene (TMB) over alumina-pillared montmorillonite (Al-PM) had been investigated in a fixed-bed flow reactor at 300 °C and atmospheric pressure. Al-PM calcined in the range 300-500 °C exhibited decreased layer distance, surface area and acid amount. The strong acid sites of Al-PM calcined at 500 °C were greatly diminished according to measurements of ammonia temperature-programmed desorption. Under our conditions the reaction proceeded mainly via isomerization and disproportionation with little dealkylation. The conversion of reactant decreased in the order 1,2,3-TMB ≥ 1,2,4-TMB > 1,3,5-TMB and the selectivity of isomerization in the order 1,2,3-TMB > 1,3,5-TMB > 1,2,4-TMB, corresponding to the order of diminishing thermal instability. Both the catalytic activity and the selectivity ratio of disproportionation to isomerization decreased with increased calcined temperature and time-on-stream. o-Xylene content in the total xylene produced from 1,2,4-TMB or 1,2,3-TMB greatly exceeded the composition at thermodynamic equilibrium. These results arc consistent with the decrease of both the strong acid sites and the pore size of Al-PM which exhibits the restricted transition state selectivity. 相似文献
136.
Comparative investigation on non-IPR C68 and IPR C78 fullerenes encaging Sc3N molecules 总被引:2,自引:0,他引:2
A computational study on the experimentally detected Sc(3)N@C(68) cluster is reported, involving quantum chemical analysis at the B3LYP/6-31G level. Extensive computations were carried out on the pure C(68) cage which does not conform with the isolated pentagon rule (IPR). The two maximally stable C(68) isomers were selected as initial Sc(3)N@C(68) cage structures. Full geometry optimization leads to a confirmation of an earlier assessment of the Sc(3)N@C(68) equilibrium geometry (Nature 2000, 408, 427), namely an eclipsed arrangement of Sc(3)N in the C(68) 6140 frame, where each Sc atom interacts with one pentagon pair. From a variety of theoretical procedures, a D(3h) structure is proposed for the free Sc(3)N molecule. Encapsulated into the C(68) enclosure, this unit is strongly stabilized with respect to rotation within the cage. The complexation energy of Sc(3)N@C(68) cage is found to be in the order of that determined for Sc(3)N@C(80) and exceeding the complexation energy of Sc(3)N@C(78). The cage-core interaction is investigated in terms of electron transfer from the encapsulated trimetallic cluster to the fullerene as well as hybridization between these two subsystems. The stabilization mechanism of Sc(3)N@C(68) is seen to be analogous to that operative in Sc(3)N@C(78). For both cages, C(68) and C(78), inclusion of Sc(3)N induces aromaticity of the cluster as a whole. 相似文献
137.
Marijan Kočevar Branko Stanovnik Miha Tišler 《Monatshefte für Chemie / Chemical Monthly》1988,119(5):597-603
Pyrido[2,3:3,4]pyrazolo[5,1-c][1,2,4]benzotriazin-4(1H)ones were transformed via their oximes in aSemmler-Wolff aromatization process in the tetracyclic heteroaromatic amines4 or bySchmidt reaction into a mixture of the same amine4 and a ring enlarged lactam3. Syntheses of some halo pyrazolo[3,4-b]pyridines and a photochemical transformation of 3-azidopyrazolo[3,4-b]pyridine are also described.
Über dieSemmler-Wolff- undSchmidt-Reaktion einiger Pyrido[2,3:3,4]pyrazolo[5,1-c][1,2,4]benzotriazine
Zusammenfassung Pyrido[23:3,4]pyrazolo[5,1-c][1,2,4]benzotriazin-4(1H)one werden über Oxime in einerSemmler-Wolff-Reaktion in die tetracyklischen aromatischen Amine4 umgewandelt. In einerSchmidt-Reaktion wurden dieselben Ketone in ein Gemisch aus Amin4 und Lactam3 übergeführt. Synthesen von halogensubstituierten Pyrazolo[3,4-b]pyridinen und photochemische Umwandlung von 3-Azidopyrazolo[3,4-b]pyridin werden beschrieben.相似文献
138.
Won Young Jung Seung Hee Baek Seong Soo Park Gun-Dae Lee Euh Duck Jeong Hyun Gyu Kim Seong-Soo Hong 《Reaction Kinetics and Catalysis Letters》2007,91(2):233-240
The nanosized titania and TiO2/SiO2 particles were prepared by the microwave-hydrothermal method. The effect of physical properties TTIP/TEOS ratio and calcination
temperature has been investigated. The major phase of the pure TiO2 particle is of the anatase structure, and a rutile peak was observed above 800°C. In TiO2/SiO2 particles, however, no significant rutile phase was observed, although the calcination temperature was 900°C. No peaks for
the silica crystal phase were observed at either silica/titania ratio. The crystallite size of TiO2/SiO2 particles decreases as compared to pure TiO2 at high calcination temperatures. The TiO2/SiO2 particles show higher activity on the photocatalytic decomposition of orange II as compared to pure TiO2 particles. 相似文献
139.
New (diamine)platinum(II) complexes A(2)PtX(2) (A(2) = trans-(+/-)-1,2-diaminocyclohexane (DACH), tetrahydro-4H-pyran-4,4-diylbis(methylamine)(THPDMA); X(2) = 9-fluorenylidenemalonate(FM), benzhydrylidenemalonate(BHM)) have been synthesized and characterized by means of multinuclear NMR spectroscopy and X-ray analysis. (DACH)Pt(FM) crystallizes in space group P2(1)/c with eight formula units in a cell of dimensions a = 20.071(7) ?, b = 12.717(3) ?, c = 24.512(6) ?, beta = 103.25(2) degrees, and V = 6090(3) ?.(3) (DACH)Pt(BHM) crystallizes in space group P&onemacr; with four molecular units in a cell of dimensions a = 11.048(3) ?, b = 13.639(3) ?, c = 14.043(6) ?, alpha = 90.17(3) degrees, beta = 91.31(4) degrees, gamma = 89.98(3) degrees, and V = 2116(1) ?(3). The platinum atom in both complexes adopts a typical square planar arrangement with two nitrogen atoms in cis position. The 9-fluorenylidene and benzhydrylidene groups of the amine ligands chelated to platinum are strikingly bent up by 88.8(3) and 80.8(2) degrees, respectively, from the platinum square plane in the solid state. Variable temperature (1)H NMR spectra of the title complexes in dimethyl sulfoxide solution reveals that the amine proton resonances are sensitive to the fluxional motion of the remote arylidene groups, and suggests that interconversion occurs between two "bent-up" and "bent-down" forms. The prominent difference between the FM and BHM complexes is observed in solution, due to the presence or absence of the angle constraint of the anionic coligands. 相似文献
140.
Aromatic molecular "clips" bearing two symmetrically bound platinum moieties have been prepared. The molecular "clip" 4 readily self-assembled with linear linkers such as 4,4'-bipyridyl, 1,4-bis[2-(4-isocyano-3,5-diisopropylphenyl)ethynyl]benzene, and nicotinic acid to form molecular rectangles. The overall dimensions of the rectangle 7 were 7.3 Angstroms x 15.3 Angstroms. The molecular "clip" also self-assembled with tritopic pyridyl and isocyanide ligands to form trigonal prismatic frameworks. The characterization of the supramolecules by multinuclear NMR, electrospray mass spectrometry, and X-ray crystal structures is also reported. 相似文献