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191.
Bassan A Blomberg MR Siegbahn PE Que L 《Journal of the American Chemical Society》2002,124(37):11056-11063
Density functional theory using the B3LYP hybrid functional has been employed to investigate the reactivity of Fe(TPA) complexes (TPA = tris(2-pyridylmethyl)amine), which are known to catalyze stereospecific hydrocarbon oxidation when H(2)O(2) is used as oxidant. The reaction pathway leading to O-O bond heterolysis in the active catalytic species Fe(III)(TPA)-OOH has been explored, and it is shown that a high-valent iron-oxo intermediate is formed, where an Fe(V) oxidation state is attained, in agreement with previous suggestions based on experiments. In contrast to the analogous intermediate [(Por.)Fe(IV)=O](+1) in P450, the TPA ligand is not oxidized, and the electrons are extracted almost exclusively from the mononuclear iron center. The corresponding homolytic O-O bond cleavage, yielding the two oxidants Fe(IV)=O and the OH. radical, has also been considered, and it is shown that this pathway is inaccessible in the hydrocarbon oxidation reaction with Fe(TPA) and hydrogen peroxide. Investigations have also been performed for the O-O cleavage in the Fe(III)(TPA)-alkylperoxide species. In this case, the barrier for O-O homolysis is found to be slightly lower, leading to loss of stereospecificity and supporting the experimental conclusion that this is the preferred pathway for alkylperoxide oxidants. The difference between hydroperoxide and alkylperoxide as oxidant derives from the higher O-O bond strength for hydrogen peroxide (by 8.0 kcal/mol). 相似文献
192.
The two tetraazamacrocycle-N,N′,N″,N?-tetraacetic acids H4dota and H4teta form with Ni2+, Cu2+, and Zn2+ (M2+) mononuclear complexes MLH2 and M′[ML], M′ being an alkaline earth ion. The structures of Ni(H2dota) and Cu(H2dota) have been solved by X-ray structure analysis. The metal ions are in a distorted octahedral geometry coordinated by four amino N-atoms and two carboxylates. In the case of Cu2+, the distortions are more pronounced than for Ni2+ indicating that the Jahn-Teller effect is operating. Starting from these two structures, the coordination geometry of the other complexes is discussed using VIS and IR spectra. 相似文献
193.
Boström E Jansson B Hammarlund-Udenaes M Simonsson US 《Rapid communications in mass spectrometry : RCM》2004,18(21):2565-2576
Sensitive and reproducible methods for the determination of oxycodone, oxymorphone and noroxycodone in Ringer solution, rat plasma and rat brain tissue by liquid chromatography/mass spectrometry are described. Deuterated analogs of the substances were used as internal standards. Samples in Ringer solution were analyzed by direct injection of 10 microL Ringer solution diluted by an equal volume of water. The limit of quantification was 0.5 ng/mL and the method was linear in the range of 0.5-150 ng/mL for all substances. To analyze oxycodone and oxymorphone in rat plasma, 50 microL of plasma were precipitated with acetonitrile, and the supernatant was directly injected onto the column. To analyze oxycodone, oxymorphone and noroxycodone in rat plasma, 100 microL of rat plasma were subjected to a C18 solid-phase extraction (SPE) procedure, before reconstituting in mobile phase and injection onto the column. For both methods the limit of quantification in rat plasma was 0.5 ng/mL and the methods were linear in the range of 0.5-250 ng/mL for all substances. To analyze the content of oxycodone, oxymorphone and noroxycodone in rat brain tissue, 100 microL of the brain homogenate supernatant were subjected to a C18 SPE procedure. The limit of quantification of oxycodone was 20 ng/g brain, and for oxymorphone and noroxycodone 4 ng/g brain, and the method was linear in the range of 20-1000 ng/g brain for oxycodone and 4-1000 ng/g brain for oxymorphone and noroxycodone. All methods utilized a mobile phase of 5 mM ammonium acetate in 45% acetonitrile, and a SB-CN column was used for separation. The total run time of all methods was 9 min. The intra-day precision and accuracy were <11.3% and <+/-14.9%, respectively, and the inter-day precision and accuracy were <14.9% and <+/-6.5%, respectively, for all the concentrations and matrices described. 相似文献
194.
DFT calculations using the B3LYP functional support the suggestion that the [(terpy)(H(2)O)Mn(IV)(micro-O)(2)Mn(III)(H(2)O)(terpy)](3+) (terpy=2,2':6,2' '-terpyridine) complex functions as a synthetic O(2) catalyst. The calculated barrier for O-O bond formation with water is 23 kcal/mol. In this complex, as well as in models of the oxygen evolving complex in PSII, the active species is a Mn(IV)-oxyl radical. From comparisons with inactive Mn(V)-oxo complexes, it is proposed that radical formation is actually a requirement for O(2) formation activity in Mn-complexes. 相似文献
195.
Roman Eminger Silvio Fallab Margareta Zehnder Max Dobler 《Helvetica chimica acta》1992,75(4):1245-1250
The Crystal and Molecular Structures of Two trans -Bis(N,N-dimethylethylenediamine)cobalt(III) Complexes. Estimate of Steric Ligand-Ligand Interactions by a Molecular-Mechanics Model Two trans-bis (N,N-dimethylethylenediamine)cobalt(III)m complexes with different axial ligands (SNC?, NO) have been synthesized starting from the corresponding CoII-amine species. An X-ray crystal-structure analysis revealed widely differing Co? N bond distances. While for the primary amino groups, as in complexes of unsubstituted bidentate amines, bond distances are close to 196 pm, those for the tertiary amino groups are stretched by ca. 10 pm. The good agreement between the molecular geometry as calculated by a molecular-mechanics model and experimental data suggests that the differences in bond distances must be primarily due to simple steric effects. 相似文献
196.
197.
To circumvent the detrimental effects of large‐volume injection with fixed‐loop injector in modern supercritical fluid chromatography, the feasibility of performing multiple injection was investigated. By accumulating analytes from a certain number of continual small‐volume injections, compounds can be concentrated on the column head, and this leads to signal enhancement compared with a single injection. The signal to noise enhancement of different compounds appeared to be associated with their retention on different stationary phases and with type of sample diluent. The diethylamine column gave the best signal to noise enhancement when acetonitrile was used as sample diluent and the 2‐picolylamine column showed the best overall performance with water as the sample diluent. The advantage of multiple injection over one‐time large‐volume injection was proven with sulfanilamide, with both acetonitrile and water as sample diluents. The multiple injection approach exhibited comparable within‐ and between‐day precision of retention time and peak area with those of single injections. The potential of the multiple injection approach was demonstrated in the analysis of sulfanilamide‐spiked honey extract and diclofenac‐spiked ground water sample. The limitations of this approach were also discussed. 相似文献
198.
Radical addition to 2-cyclohexyl-5-methylidene-6-methyl-1,3-dioxan-4-one ( 2 ) affords stereoselectively 5,6-trans-products trans- 3 . The size of the radical has no influence on the selectivity. These trans-acetals are converted into threo-3-hydroxy-butanoates 4 . Methyl 2-methylidene-3-[(tert-butyl)diphenylsilyloxy]butanoate ( 5 ), treated under the same conditions, leads mainly to the erythro-isomer of 4 after deprotection. An influence of the steric bulkyness of the radical is observed. The stereochemical course of the reactions is discussed. 相似文献
199.
Krook Margareta Lindbladh Christer Eriksen Jon Amund Mosbach Klaus 《Molecular diversity》1997,3(3):149-159
A cyclic nonapeptide library displayed on filamentous bacteriophages was selected 6 times against α-chymotrypsin (EC 3.4.21.1)
at three different pH conditions (6.5, 7.0, and 7.5). Phage peptide clones from the sixth selection, at all three pH conditions,
interacted more strongly with α-chymotrypsin than the original library and a wild-type phage did. DNA sequencing of the selected
phage peptide clones showed that different cyclic nonapeptide sequences had been selected at the different pH conditions.
The oxidized form of the synthetic peptide, Cys-Cys-Phe-Ser-Trp-Arg-Cys-Arg-Cys, selected at pH 7.5, could completely inhibit
the enzymatic activity of α-chymotrypsin. The structurally related enzymes trypsin (bovine) and elastase (porcine) were only
marginally inhibited by the same peptide under the same conditions. The inhibition constant for α-chymotrypsin was estimated
to be 10-6 M. Phage clones expressing this peptide had a lower affinity for phenylmethylsulfonylfluoride-modified α-chymotrypsin than
for natural α-chymotrypsin as determined by an enzyme immunosorbent assay. This peptide phage clone was also competitively
prevented from binding to α-chymotrypsin by the corresponding synthetic oxidized peptide. Collectively, the results suggest
that the oxidized form of the selected peptide Cys-Cys-Phe-Ser-Trp-Arg-Cys-Arg-Cys interacts with the active site of α-chymotrypsin
and acts as a specific inhibitor to the enzyme. To our knowledge, the selected sequence Cys-Cys-Phe-Ser-Trp-Arg-Cys-Arg-Cys
has not been found in nature.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献