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121.
Geremia S Di Costanzo L Randaccio L Engel DE Lombardi A Nastri F DeGrado WF 《Journal of the American Chemical Society》2005,127(49):17266-17276
The de novo protein DF1 is a minimal model for diiron and dimanganese metalloproteins, such as soluble methane monooxygenase. DF1 is a homodimeric four-helix bundle whose dinuclear center is formed by two bridging Glu side chains, two chelating Glu side chains, and two monodentate His ligands. Here, we report the di-Mn(II) and di-Co(II) derivatives of variants of this protein. Together with previously solved structures, 23 crystallographically independent four-helix bundle structures of DF1 variants have been determined, which differ in the bound metal ions and size of the active site cavity. For the di-Mn(II) derivatives, as the size of the cavity increases, the number and polarity of exogenous ligands increases. This collection of structures was analyzed to determine the relationship between protein conformation and the geometry of the active site. The primary mode of backbone movement involves a coordinated tilting and sliding of the first helix in the helix-loop-helix motif. Sliding depends on crystal-packing forces, the steric bulk of a critical residue that determines the dimensions of the active site access cavity, and the intermetal distance. Additionally, a torsional motion of the bridging carboxylates modulates the intermetal distance. This analysis provides a critical evaluation of how conformation, flexibility, and active site accessibility affect the geometry and ligand-binding properties of a metal center. The geometric parameters defining the DF structures were compared to natural diiron proteins; DF proteins have a restricted active site cavity, which may have implications for substrate recognition and chemical stability. 相似文献
122.
Grazia Zassinovich Roberto Bettella Giovanni Mestroni Nevenka Bresciani-Pahor Silvano Geremia Lucio Randaccio 《Journal of organometallic chemistry》1989,370(1-3):187-202
Some diastereoisomeric pentacoordinate complexes of the type [Ir(COD)-(NNR)I] (COD = cis,cis-1,5-cyclooctadiene; NNR = 2-pyridinal-1-phenylethylimine (PPEI) (I), 2-acetylpyridine-1-phenylethylimine (APPEI) (II)) have been synthesized. The complexes are active and selective catalysts for asymmetric hydrogen transfer from propan-2-ol to prochiral ketones. Optical yields of up to 84% have been obtained in the reduction of t-butyl phenyl ketone. The structure and absolute configuration of complexes I and II were determined by X-ray diffraction. 相似文献
123.
Eszter?Gács-Baitz Orsolya?EgyedEmail author Aldo?Taticchi Lucio?Minuti Assunta?Marrocchi 《Structural chemistry》2005,16(6):581-585
Our ongoing study on cycloaddition reactions of dienes with different dienophiles afforded a great variety of derivatives
with interesting molecular structures and electronic behavior. A new type of angularly annelated [2.2]paracyclophane (3) has been synthesize by the Diels–Alder reaction of 4-(2-propenyl[2.2]paracyclophane (1) and 1,4-benzoquinone (2) under high pressure conditions. The structure determination of this compound has been achieved by NMR measurements and semiempirical
calculations. 相似文献
124.
Multidimensional nuclear magnetic resonance (NMR) provides one of the foremost analytical tools available to elucidate the structure and dynamics of complex molecules in their native states. Executing this kind of experiment generally requires collecting an n-dimensional time-domain signal S, from which the spectrum arises via an appropriate Fourier analysis of its various time variables. This time-domain signal is actually measured directly only along one of the time axes, while the effects introduced by the remaining time variables are monitored via a parametric incrementation of their values throughout independent experiments. Two-dimensional (2D) NMR experiments thus usually require longer acquisition times than unidimensional experiments, 3D NMR is orders-of-magnitude more time consuming than 2D spectroscopy, etc. Very recently, we proposed and demonstrated an approach whereby data acquisition in 2D NMR can be parallelized, enabling the collection of complete 2D spectral sets within a single transient. The present paper discusses the extension of this 2D NMR methodology to an arbitrary number of dimensions. The principles of the ensuing ultrafast n-dimensional NMR approach are described, and a variety of homo- and heteronuclear 3D and 4D NMR spectra collected within a fraction of a second are presented. 相似文献
125.
Francesco Nicotra Luigi Panzal Fiamma Ronchetti Giovanni Russo Lucio Toma 《Tetrahedron letters》1985,26(6):807-808
Methyl 3,4,6-tri-O-benzyl-α-D-glucopyranoside and methyl 3,4,6-tri-O-benzyl-β-D-mannopyranoside were directly obtained by m-chloroperbenzoic acid treatment of (Z)-(2R,3R,4R)-6-methoxy-1,3,4-tribenzyloxy-5-hexen-2-ol. 相似文献
126.
A new composite material constituted by mu-{5,10,15,20-tetra(4-pyridyl)porphyrinato cobalt(iii)}-tetrakis-{chloro-bis-(2,2'-bipyridine)ruthenium(ii)} complex (or CoTRP) and cobalt oxide, exhibiting high stability and sensitivity for the quantification of hydrogen peroxide, was obtained by the electrochemical polymerization of the tetraruthenated cobalt porphyrin in alkaline medium. The optimized experimental conditions for the preparation of the modified glassy carbon electrodes and for analysis of H2O2 were carefully determined. Fast sequential analysis (120 determinations h(-1)) in a wide linear dynamic range (5.0 x 10(-7) mol L(-1) to 2.0 x 10(-3) mol L(-1)), with high sensitivity and low detection limit (2.0 x 10(-7) mol L(-1)), was achieved by using these electrodes and the batch injection analysis (BIA) technique. Such characteristics allied to a good stability were explored for the specific determination of hydrogen peroxide in six commercial cosmetics and pharmaceutical product samples, giving results in excellent agreement with those obtained by the spectrophotometric method. 相似文献
127.
Lucio Doretti Daniela Ferrara Gilberto Barison Silvano Lora 《Applied biochemistry and biotechnology》1994,49(3):191-202
Amperometric glucose biosensors based on glucose oxidase immobilized onto poly(2-hydroxyethylmethacrylate) membranes obtained by γ radiation-induced polymerization were constructed. In a threeelectrode configuration, smooth or platinized platinum electrodes with different shapes were used, in order to detect the amount of hydrogen peroxide produced in the glucose oxidation. A saturated calomel electrode and a platinum foil were used as a reference and counterelectrode, respectively. The biocompatible obtained sensors were characterized as regards the temperature effect, the response, and lifetime. The determination of glucose in standard solutions was carried out, and linear calibration curves were obtained. Depending on the electrode configuration, the sensor had a response time of 1–4 min, and the measuring range extended from 5 × 10?5 to 4 × 10?3M. 相似文献
128.
Lucio Forni Elena Selli Ernesto Fumagalli Daniela Mainini 《Reaction Kinetics and Catalysis Letters》1997,61(2):267-273
The poisoning effect of CO2 on a HY catalyst for the dehydration of 2-(2-hydroxyethyl)-pyridine (HEP) to 2-vinylpyridine (VP) has been investigated by
FT-IR analysis in the presence and absence of pyridine. CO2 was found to adsorb on sites not occupied by pyridine,i.e. on weak basic sites accompanying the strongly acidic sites, characteristic of Y zeolites in the protonated form. When the
basic sites are occupied by preadsorbed CO2, HEP dehydration cannot take place any more through the minor mechanism, involving a couple of acid-base sites, and the reaction
proceeds only through the major mechanism, involving a carbocation intermediate, on Br?nsted acid sites only. 相似文献
129.
Diego Colombo Fiamma Ronchetti Antonio Scala Lucio Toma 《Journal of carbohydrate chemistry》2013,32(1):89-94
Abstract Methyl 6-O-butyryl-α-D- and L-glucopyranosides and methyl 6-deoxy-α-D- and L-glucopyranosides have been submitted to lipase catalyzed butyrylation, using porcine pancreatic, Candida cylindracea, and Pseudomonas cepacia lipases in organic solvents. The influence of the orientation of the secondary hydroxyl groups on the regioselectivity of the butyrylation is discussed. 相似文献
130.
Mono‐ and diamino‐derivatives of naphthazarin, 2‐methoxynaphthazarin, juglone and their methylethers were prepared by reaction with ammonia. The structure of the products was established by NMR studies. Some compounds were tested for cytotoxic activity. 相似文献