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71.
Karl-Siegfried Boos Anne Rudolphi Stefan Vielhauer Andreas Walfort Dieter Lubda Friedhelm Eisenbeiß 《Fresenius' Journal of Analytical Chemistry》1995,352(7-8):684-690
The direct and repetitive injection of untreated biological fluids (e.g., hemolyzed blood, plasma, serum, cell culture and tissue homogenates) onto an HPLC-system and the subsequent analysis of low-molecular weight compounds (e.g. drugs, xenobiotics, metabolites) is rendered possible by a coupled-column configuration and special precolumn packings. For this purpose a new family of chemically and enzymatically tailored reversed-phase packing materials have been prepared. The LC-integrated sample clean-up with these restricted access (bimodal) phases is based on the complete nonadsorptive size exclusion of macromolecules (e.g. proteins) and on the simultaneous dynamic partitioning of the target molecules. The bonded phase which exclusively covers the internal pore surface of a glyceryl-modified silica is a butyryl-(C-4), capryloyl-(C-8) or stearoyl-(C-18) moiety. These ligands allow a classical reversed-phase or ion-pair chromatography during the sample work-up step. The capacity of the n-alkyl phase is comparable with conventional silica based RP-materials. The broad hydrophobic retentive capability of these packings allows the extraction of a wide variety of compounds of biomedical interest. The electroneutral and hydrophilic particle exterior (glyceryl-residues) was generated using either soluble or immobilized enzymes (lipase, esterase) which cleave the fatty acid esters exclusively at the outer surface. Unwanted macromolecular components of a sample (e.g. proteins) are quantitatively eluted in the void volume due to the restricted access given by the pore size (6 nm) and the nonadsorptive external diol coverage. The lifetime of a precolumn (25 × 4 mm I.D.) packed with these novel bimodal, i.e. RP-SEC phases exceeds more than 200 injections of 500 l plasma. In addition to the synthesis, this paper describes an application of each of these Alkyl-Diol Silica (ADS) precolumn packings in fully automated coupled-column HPLC systems for the analysis of drugs and endogenous compounds in different biological matrices.Dedicated to Professor Dr. Dr. h.c. mult. J.F.K. Huber on the occasion of his 70th birthday 相似文献
72.
We report the details of the first total synthesis of erythromycin B using two different strategies for the end game. The first of these follows a classical approach in which the desosamine and cladinose residues are sequentially appended to a macrocyclic lactone, which was formed by cyclization of a seco acid derivative, to give a bis-glycosylated macrolide intermediate that is converted into erythromycin B. The second strategy features an abiotic approach in which a seco acid bearing a desosamine residue is cyclized to give a monoglycosylated macrocyclic lactone that is then transformed into erythromycin B via a sequence of steps involving refunctionalizations and a glycosylation to introduce the cladinose moiety. Attempts to prepare a bis-glycosylated seco acid by de novo synthesis were unsuccessful. The syntheses of the key seco acid intermediates feature the oxidative transformation of a furan containing C(3)-C(10) to provide a dioxabicyclo[3.3.1]nonenone that served as a template on which to create the stereocenters at C(6) and C(8). A stereoselective aldol reaction was used to establish the C(11)-C(15) segment, and a stereoselective crotylation was implemented to introduce the propionate subunit comprising C(1)-C(2). 相似文献
73.
Kryatov SV Chavez FA Reynolds AM Rybak-Akimova EV Que L Tolman WB 《Inorganic chemistry》2004,43(6):2141-2150
Dioxygen activation by enzymes such as methane monooxygenase, ribonucleotide reductase, and fatty acid desaturases occurs at a nonheme diiron active site supported by two histidines and four carboxylates, typically involving a (peroxo)diiron(III,III) intermediate in an early step of the catalytic cycle. Biomimetic tetracarboxylatodiiron(II,II) complexes with the familiar "paddlewheel" topology comprising sterically bulky o-dixylylbenzoate ligands with pyridine, 1-methylimidazole, or THF at apical sites readily react with O(2) to afford thermally labile peroxo intermediates that can be trapped and characterized spectroscopically at low temperatures (193 K). Cryogenic stopped-flow kinetic analysis of O(2) adduct formation carried out for the three complexes reveals that dioxygen binds to the diiron(II,II) center with concentration dependences and activation parameters indicative of a direct associative pathway. The pyridine and 1-methylimidazole intermediates decay by self-decomposition. However, the THF intermediate decays much faster by oxygen transfer to added PPh(3), the kinetics of which has been studied with double mixing experiments in a cryogenic stopped-flow apparatus. The results show that the decay of the THF intermediate is kinetically controlled by the dissociation of a THF ligand, a conclusion supported by the observation of saturation kinetic behavior with respect to PPh(3), inhibition by added THF, and invariant saturation rate constants for the oxidation of various phosphines. It is proposed that the proximity of the reducing substrate to the peroxide ligand on the diiron coordination sphere facilitates the oxygen-atom transfer. This unique investigation of the reaction of an O(2) adduct of a biomimetic tetracarboxylatodiiron(II,II) complex provides a synthetic precedent for understanding the electrophilic reactivity of like adducts in the active sties of nonheme diiron enzymes. 相似文献
74.
A methodology to synthesize oligonucleotides containing an alkyl interstrand cross-link between the two O6 atoms of deoxyguanosine has been developed. This cross-link is designed to serve as a stable structural mimic of the lesion formed in duplex DNA with the bifunctional alkylating agent hepsulfam. The O6-alkyl coupling is performed via a Mitsunobu reaction between a nucleoside and mono-protected 1,7-heptanediol. Solid-phase oligonucleotide synthesis using a nucleoside bis-phosphoramidite allows for the assembly of the cross-linked duplex. Sufficient quantities of this cross-linked duplex were obtained for various structural and biological investigations. 相似文献
75.
Frédérique Malbec René Milcent Patrick Vicart Anne Marie Bure 《Journal of heterocyclic chemistry》1984,21(6):1769-1774
Thirty new 2-substituted-4-amino-5-alkyl or aryl-2,4-dihydro-1,2,4-triazol-3-ones and ten 2-substituted-5-alkyl or aryl-4-(5-nitro-2-furfurylidene)amino-2,4-dihydro-1,2,4-triazol-3-ones were synthesized and characterised by their sharp melting points, elemental analysis, ir and 1H nmr spectra. These new derivatives of 5-nitro-2-furaldehyde were screened for their antibacterial activities. Most of the compounds showed good activity against one test organism, Staphylococcus aureus. For a few compounds, C.M.I. ranged from 4 to 8 μg/ml (higher results than nitrofurantoin). 相似文献
76.
Solid state (19)F NMR revealed the conformation and alignment of the fusogenic peptide sequence B18 from the sea urchin fertilization protein bindin embedded in flat phospholipid bilayers. Single (19)F labels were introduced into nine distinct positions along the wild-type sequence by substituting each hydrophobic amino acid, one by one, with L-4-fluorophenylglycine. Their anisotropic chemical shifts were measured in uniaxially oriented membrane samples and used as orientational constraints to model the peptide structure in the membrane-bound state. Previous (1)H NMR studies of B18 in 30% TFE and in detergent micelles had shown that the peptide structure consists of two alpha-helical segments that are connected by a flexible hinge. This helix-break-helix motif was confirmed here by the solid-state (19)F NMR data, while no other secondary structure (beta-sheet, 3(10)-helix) was compatible with the set of orientational constraints. For both alpha-helical segments we found that the helical conformation extends all the way to the respective N- and C-termini of the peptide. Analysis of the corresponding tilt and azimuthal rotation angles showed that the N-terminal helix of B18 is immersed obliquely into the bilayer (at a tilt angle tau approximately 54 degrees), whereas the C-terminus is peripherally aligned (tau approximately 91 degrees). The azimuthal orientation of the two segments is consistent with the amphiphilic distribution of side-chains. The observed 'boomerang'-like mode of insertion into the membrane may thus explain how peptide binding leads to lipid dehydration and acyl chain perturbation as a prerequisite for bilayer fusion to occur. 相似文献
77.
The preparation and identification of a series of trifluoromethylated and bis(trifluoromethylated) precursors and monomers for bridge substituted trifluoromethylated poly(p-phenylenevinylene) (PPV) is reported. These monomers were prepared in several steps from terephthaldicarboxaldehyde and they contain symmetrical and unsymmetrical combinations of leaving groups and/or polarizers, including chlorides, sulfoxides, sulphones, tosylates and S-methyl (and also O-ethyl) xanthates. The electron withdrawing effect of the trifluoromethyl group dominated the regioselectivity of substitution, thus allowing the convenient and selective formation of unsymmetrically substituted products in high yield. 相似文献
78.
Baudouin Jambe Jacqueline Marchand-Brynaert Jacques Devaux 《Journal of polymer science. Part A, Polymer chemistry》1995,33(8):1283-1292
Thermal degradation of poly(methyl-n-hexylsilane) in the solid state in absence of oxygen reveals formation of a cyclic pentamer between 150 and 250°C. Polymer is gradually degraded to an intermediate molecular weight distribution. The weight average of this new distribution is not only temperature-dependent, but is also a function of viscosity of the polymer and nature of chain ends. As no insolubles or Si? H groups are formed, the degradation mechanism is most likely a back-biting mechanism induced by active chain ends such as silyl anions or Si? Cl rather than a homolytic cleavage of the main chain. A concurrent intramolecular rearrangement reaction is also proposed. Moreover, this study proposes an explanation to the trimodal molecular weight distribution obtained by the Wurtz coupling of dichlorosilanes with molten sodium in refluxing toluene. © 1995 John Wiley & Sons, Inc. 相似文献
79.
Reduced quantum efficiencies for the photocatalytic degradation of cyclohexanol, cyclododecanol, 2-hexanol, and benzyl alcohol
are observed on TiO2 particles included within small pore zeolitic supports suspended in acetonitrile. A modest reduction in photocatalytic activity
observed with TiO2 included within large pore zeolites and pillared clays is attributed to reflective and refractive losses of incident light.
No appreciable substrate size selectivity could be observed in this series of supported TiO2 composites. TS-1, a titanium-containing zeolite with Ti in tetrahedral lattice sites, was photoactive, although ETS-10, a
titanium-containing zeolite with Ti in octahedral lattice sites, was less so. The low quantum yield residual photoactivity
observed with these alcoholic substrates on unmodified zeolites may derive from surface charge transfer complexation, as suggested
by diffuse reflectance absorption measurements. 相似文献
80.
Lahboub Bouyazza Jean-Charles Lancelot Sylvain Rault Max Robba Marie Anne Quermonne 《Journal of heterocyclic chemistry》1991,28(2):373-377
Cyclization of 2-(1-pyrrolyl)piperonylcarboxamide derivatives gave iminium perchlorates which afforded 9-(N-substituted-imino) and 9-(N-substituted amino)-6,7-methylenedioxypyrrolo[1,2-a]indoles. 相似文献