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991.
In order to develop a robust and easy-to-use technique for characterization of bacterial polysaccharides, a pseudo-hydrolysis strategy was investigated. Based on in-source collision-induced dissociation, polysaccharide molecular ions were fragmented within the orifice-skimmer region of an electrospray ionization (ESI) mass spectrometer. The fragment ions thus generated were then analyzed similarly to the conventional ESI mass spectrometry approach. MS/MS scanning was applied to obtain product-ion spectra of the primary fragments for sequencing. To further improve the sensitivity and separation of polysaccharides from other components in the samples, a pressure-assisted capillary electrophoresis/mass spectrometry (CE/MS) system was employed. Using bacterial polysaccharides as model compounds, the mass spectra obtained for polysaccharide repeating units generated through chemical hydrolysis and in-source fragmentation were directly compared, both in positive and negative ion modes. With the additional separation of impurities provided by CE, the success of this technique has been demonstrated for structural analysis of O-chain polysaccharides (O-PS) and capsular polysaccharides (CPS). In-source fragmentation was applied to promote the formation of structurally relevant repeating units of heterogeneous CPS that would remain undetected using conventional ESI conditions. This approach was proven to be particularly useful for probing the subtle structural differences in monosaccharide composition and functionalities arising across bacterial serotypes. 相似文献
992.
Molecular dynamics (MD) simulations and hydrogen bonding energy (HBE) calculations have been performed on the prereactive enzyme-substrate complexes (ES), transition states (TS1), and intermediates (INT1) for acetylcholinesterase (AChE)-catalyzed hydrolysis of acetylcholine (ACh), butyrylcholinesterase (BChE)-catalyzed hydrolysis of ACh, and BChE-catalyzed hydrolysis of (+)/(-)-cocaine to examine the protein environmental effects on the catalytic reactions. The hydrogen bonding of cocaine with the oxyanion hole of BChE is found to be remarkably different from that of ACh with AChE/BChE. Whereas G121/G116, G122/G117, and A204/A199 of AChE/BChE all can form hydrogen bonds with ACh to stabilize the transition state during the ACh hydrolysis, BChE only uses G117 and A199 to form hydrogen bonds with cocaine. The change of the estimated total HBE from ES to TS1 is ca. -5.4/-4.4 kcal/mol for AChE/BChE-catalyzed hydrolysis of ACh and ca. -1.7/-0.8 kcal/mol for BChE-catalyzed hydrolysis of (+)/(-)-cocaine. The remarkable difference of approximately 3 to 5 kcal/mol reveals that the oxyanion hole of AChE/BChE can lower the energy barrier of the ACh hydrolysis significantly more than that of BChE for the cocaine hydrolysis. These results help to understand why the catalytic activity of AChE against ACh is considerably higher than that of BChE against cocaine and provides valuable clues on how to improve the catalytic activity of BChE against cocaine. 相似文献
993.
994.
Kadish KM Shao J Ou Z Zhan R Burdet F Barbe JM Gros CP Guilard R 《Inorganic chemistry》2005,44(24):9023-9038
Combined electrochemical and UV-visible spectroelectrochemical methods were utilized to elucidate the prevailing mechanisms for electroreduction of previously synthesized porphyrin-corrole dyads of the form (PCY)H2Co and (PCY)MClCoCl where M = Fe(III) or Mn(III), PC = porphyrin-corrole, and Y is a bridging group, either biphenylenyl (B), 9,9-dimethylxanthenyl (X), anthracenyl (A), or dibenzofuranyl (O). These studies were carried out in pyridine, conditions under which the cobalt(IV) corrole in (PCY)MClCoCl is immediately reduced to its Co(III) form, thus enabling direct comparisons with the free-base porphyrin dyad, (PCY)H2Co(III) under the same solution conditions. The compounds are all reduced in multiple one-electron-transfer steps, the first of which involves the M(III)/M(II) process of the porphyrin in the case of (PCY)MClCoCl and the Co(III)/Co(II) process of the corrole in the case of (PCY)H2Co. Each metal-centered redox reaction may be accompanied by the gain or loss of pyridine axial ligands, with the exact stoichiometry of the exchange process depending upon the specific combination of metal ions in the dyad, their oxidation states, and the particular spacer in the complex. Before this study was started, it was expected that the porphyrin-corrole dyads with the largest spacers, namely, O and A, would readily accommodate the formation of cobalt(III) bis-pyridine adducts because of the larger size of the cavity while dyads with the smallest B spacer would seem to have insufficient room to add even a single pyridine within the cavity, as was structurally seen in the case of (PCB)H2Co(py). This is clearly not the case, as shown in the present study. A reversible Co(III)/Co(II) reaction is seen for (PCB)MnClCoCl at -0.62 V, which when combined with spectroscopic data, leads to the assignment of (PCB)Mn(III)(py)2Co(III)(py) as the species in pyridine. The reduction of (PCB)Mn(III)(py)2Co(III)(py) to (PCB)Mn(II)(py)Co(III)(py) is accompanied on the slower spectroelectrochemical time scale by the appearance of a 603 nm band in the UV-vis spectra and is consistent with the addition of a second pyridine ligand to the Co(III)(py) unit of the dyad as one ligand is lost from the electrogenerated manganese(II) porphyrin, thus maintaining one pyridine ligand within the cavity. A different change in the coordination number is observed in the case of (PCB)FeClCoCl. Here the initial Fe(III) complex can be assigned as (PCB)Fe(III)ClCo(III)(py), which has no pyridine molecule within the cavity and the singly reduced form is characterized as (PCB)Fe(II)(py)2Co(III)(py)2, which contains two pyridine ligands inside the cavity. A following one-electron reduction of the Fe(II)/Co(III) complex then gives [(PCB)Fe(II)(py)2Co(II)]-. 相似文献
995.
Jiugui Liu Lizhong Jiang Jiayu Zhan Dezhen Wu Riguang Jin 《Frontiers of Chemistry in China》2007,2(1):13-16
A new route to porous polyimide (PI) films with pore sizes in the nanometer regime was developed. A polyamic acid (PAA)/polyurethane
(PU) blend with PU as the disperse phase was first prepared via in situ polymerization of pyromellitic dianhydride and 4,4-oxydianiline in PU solutions. Porous PI films were obtained from PAA/PU
films by thermolysis of PU at 360°C and imidization of PAA at 300°C, respectively. Fourier transform infrared spectroscopy
and thermal gravimetric analysis were used to detect the imidization and thermolysis processes of PAA/PU blends under thermal
treatment. The microporous structure of the PI films was observed by transmission electron microscopy. It was found that the
size and content of pores increased with an increase in the PU mass fraction in the PAA/PU blend up to 20%. Because of the
existence of nanopores, the dielectric constant of PI films decreased by a wide margin and was less than 2.0 at a PU mass
fraction of 20%. It implies that this is an effective means to reduce the dielectric constant of PI, but it also causes the
decrease of tensile strength and the rise of water absorption.
Translated from Chemistry Journal of Chinese Universities 2006, 27(1): (in Chinese) 相似文献
996.
Xue‐Qin Cao Zhan‐Xiong Li Guo‐Qiang Chen De‐Chun Zhang 《Acta Crystallographica. Section C, Structural Chemistry》2007,63(11):o671-o672
The title molecule, C9H10N4O3, consists of benzene and imidazole rings which are almost perpendicular to each other. A hydroxyimino group is directly linked to the imidazole ring with a double C=N bond, which is the first example in this type of compound. The double bond may be a good location for the initiation of various reactions with a wide range of potential applications. In the crystal structure, there are π–π interactions between molecules related by a centre of symmetry, with the imidazole and benzene rings almost completely overlapped. The molecules are hydrogen bonded in each direction and form a three‐dimensional hydrogen‐bond network. 相似文献
997.
Yan XingYunling Liu Zhan ShiPing Zhang Yunlong FuChang Cheng Wenqin Pang 《Journal of solid state chemistry》2002,163(2):364-368
A novel two-dimensional zinc phosphate Zn4P3O11(OH)·3CN2H4 in which the structure-directing organic amine acts as a ligand has been synthesized hydrothermally. The structure was solved by single-crystal X-ray diffraction analysis. Crystal data: triclinic, space group P1? (No. 2) with lattice parameters a=9.5663(15) Å, b=9.8530(16) Å, c=12.3658(19) Å, α= 77.495(4)°, β=77.893(4)°, γ=68.175(3)°, V=1045.6(3) Å3, Z=2, R1[I>2σ(I)]=0.0309, and wR2[I>2σ(I)]=0.0809. Interestingly the structure involves a network of ZnO4, PO4, PO3(OH), and the unusual ZnO3N and ZnO2N2 tetrahedra with shared vertices. There are 10-membered rings in the layers, in which the structure-directing imidazole molecules reside. The other amine molecules protrude from the Zn centers and occupy spaces between the layers. 相似文献
998.
Peng Cheng Daizheng Liao Shiping Yan Zonghui Jiang Genglin Wang 《Transition Metal Chemistry》1996,21(6):515-518
Summary Four binuclear CuII complexes, [Cu2(L)(-X)], have been prepared and characterized, where H3L is a 2:1 Schiff base derived from 2-hydroxy-1-naphthaldehyde and 1,3-diamino-2-propanol and X = 1,3-N3
(1), C3H3N2 (pyrazolate) (2), AcO (3) and PhCO2
(4). The CuIIions are bridged by endogenous alkoxide and bidentate exogenous bridges. The variable-temperature magnetic analyses show that complexes (2)–(4) exhibit antiferro-magnetic coupling with the exchange integrals, 2J, from- 188 to -158 cm-1. The -1,3-azide-bridged binuclear CuII complex (1) shows the essential features of diamagnetism through measurements of its variable-temperature susceptibility and e.p.r. spectra. The results suggest that (1) can act as a diamagnetic model for metazidohemocyanin. 相似文献
999.
A new poly(acrylamidrazone-hydrazide) chelating fiber has been synthesized using polyacrylonitrile fiber as a starting material. An ICP-OES method for applying the fiber to preconcentrate and separate trace Au(III) and Pd(IV) ions in solution has been established. The experiments show that 8 ng/ml Au(III) and 6 ng/ml Pd(IV) in 1000 ml of solution can be enriched quantitatively by the fiber column at a flow rate of 12 ml/min at pH 2. These ions can be desorbed quantitatively with 10 ml of 2.5% CS(NH2)2 + 6% H2SO4 containing 0.2% Fe(II) from the column at an elution rate of 6 ml/min. A fiber treated with 12M HCl or 15M HNO3 can be re-used 10 times with above 95% recoveries of Au(III) and Pd(IV), and 120–800-fold excesses of Cu(II), Mn(II), Fe(III), Al(III), Ni(II), Mg(II) and Ca(II) ions cause little interference. The RSDs are 2.0% for 8 ng/ml Au and 3.5% for 6 ng/ml Pd. The recovery of added standard in a solution sample from a metal smelter is 96.2% for Au and 100% for Pd, and the content of each ion in the sample determined by the method is in agreement with the analysed value from the smelter laboratory. 相似文献
1000.
The one-color resonant two -photon ionization technique is employed to study jetcooled van der Waals(vdW) complex o-xylene?N2 through the S0-S1 transition around the band. The spectra obtained exhibit rich information about the complex intermolecular vdW vibrational modes. We have tentatively assigned all the observed spectral features. The structure of the complex has been obtained by calculation of the minimum energy structure. 相似文献