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91.
K. Andersen O. P. Anderson T. Miller N. S. Mani T. F. Baumann M. Anderson W. E. Broderick D. M. Eichhorn D. Goldberg W. Jarrell S. J. Lange S. Lee H. Nie M. Sabat J. W. Sibert C. Stern B. M. Hoffman S. Baum L. S. Beall A. S. Cook Q. J. Mccubbin A. Garrido Montalban M. S. Rodriguez-Morgade A. J. P. White D. B. G. Williams D. J. Williams A. G. M. Barrett H. Hope M. M. Olmstead 《Journal of heterocyclic chemistry》1998,35(5):1013-1042
92.
Nguyen HP Ortiz IP Temiyasathit C Kim SB Schug KA 《Rapid communications in mass spectrometry : RCM》2008,22(14):2220-2226
Crude oil fingerprints were obtained from four crude oils by laser desorption/ionization mass spectrometry (LDI-MS) using a silver nitrate cationization reagent. Replicate analyses produced spectral data with a large number of features for each sample (>11,000 m/z values) which were statistically analyzed to extract useful information for their differentiation. Individual characteristic features from the data set were identified by a false discovery rate based feature selection procedure based on the analysis of variance models. The selected features were, in turn, evaluated using classification models. A substantially reduced set of 23 features was obtained through this procedure. One oil sample containing a high ratio of saturated/aromatic hydrocarbon content was easily distinguished from the others using this reduced set. The other three samples were more difficult to distinguish by LDI-MS using a silver cationization reagent; however, a minimal number of significant features were still identified for this purpose. Focus is placed on presenting this multivariate statistical method as a rapid and simple analytical procedure for classifying and distinguishing complex mixtures. 相似文献
93.
The results of an energy decomposition analysis of ortho-, meta-, and para-substituted benzylic cations and para-substituted benzylic anions H2C-C6H4Rq (R = H, F, CN, Me, OH, NH2, NO2, CHO, CO2H; q = +, -) are presented and discussed. The calculated values for the pi bonding between CH2(q) and C6H4R show for substituents which have pi orbitals a linear correlation with the Hammett sigma(p), sigma(+)(p), and sigma(m) constants. 相似文献
94.
Tshuva EY Goldberg I Kol M Goldschmidt Z 《Chemical communications (Cambridge, England)》2001,(20):2120-2121
An amine bis(phenolate) dibenzyl titanium complex having a methoxy donor on a side arm leads, upon activation with tris(pentafluorophenyl)borane, to unique living properties in alpha-olefin polymerization: exceptionally high molecular weight poly(1-hexene) is obtained in a living fashion at room temperature, living polymerization of 1-hexene is obtained above room temperature, and block copolymerization of 1-hexene and 1-octene at room temperature is described as well. 相似文献
95.
Abstract -The rate of formation of the M intermediate ( k M ) in the photocycles of bacteriorhodopsin (bR570 ) and of nitrated bacteriorhodopsin (bR532 n ), is measured over the range between pH 6.5 and 11.5. In the case of bR570 , k M is markedly pH dependent, exhibiting a titration-like curve with pK ∽ 10.3. The pH dependency is completely eliminated by nitration. On the basis of previous work by Lemke and Oesterhelt (1981), the effect is attributed to the specific modification of the Tyr 26 residue. The data are rationalized by a mechanism in which deprotonation of Tyr 26 at the stage of the L intermediate constitutes a prerequisite for deprotonation of the retinal-lysine SchifT base. Both reactions are intimately associated with the photo-induced proton pump mechanism. 相似文献
96.
Wulff WD Korthals KA Martínez-Alvarez R Gómez-Gallego M Fernández I Sierra MA 《The Journal of organic chemistry》2005,70(13):5269-5277
By means of deuterium-labeling experiments, we have carried out a systematic ESI-MS study to determine the mechanism of ESI ionization of alkenyl and alkynyl group 6 Fischer carbene complexes. These compounds can be ionized under ESI conditions only in the presence of additives such as hydroquinone (HQ) or tetrathiafulvalene (TTF). Our results demonstrate that in the ESI source an anion-radical is formed after the initial HQ- or TTF-mediated electron transfer to the metallic carbene complex. For alkenyl carbene complexes, this species evolves by extrusion of a hydrogen radical to form an allenylchromium anion that is detected as the [M - H](-) ion in the mass spectrum. The preference for this mechanistic pathway could be rationalized by DFT calculations. In the case of alkynyl carbene complexes, experiments combining deuterated substrate, additive, and solvent demonstrate that the previously proposed allene-anion carbene complex is not formed. Instead, the H transfer from the ethoxy group in the anion radical, followed by extrusion of a hydrogen radical, leads to an allenyl anion that is detected in the ESI-MS as [M - H - CO](-). 相似文献
97.
Coating Platinum Nanoparticles with Methyl Radicals: Effects on Properties and Catalytic Implications 下载免费PDF全文
Dr. Ronen Bar‐Ziv Dr. Israel Zilbermann Dr. Michael Shandalov Prof. Dr. Vladimir Shevchenko Prof. Dr. Dan Meyerstein 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(52):19000-19009
It was recently reported that the reaction of methyl radicals with Pt0 nanoparticles (NPs), prepared by the reduction of Pt(SO4)2 with NaBH4, is fast and yields as the major product stable (Pt0‐NPs)?(CH3)n and as side products, in low yields, C2H6, C2H4, and some oligomers. We decided to study the effect of this coating on the properties of the Pt0‐NPs. The results show that the coating can cover up to 75 % of the surface Pt0 atoms. The rate constant of the reaction, k( . CH3+Pt0‐NPs), decreases with the increase in the surface coverage, leading to competing reaction paths in the solution, which gradually become dominant, affecting the composition of the products. The methyl coating also affects the zeta potential, the UV spectra, and the electrocatalytic reduction of water in the presence of the NPs. Thus, the results suggest that binding alkyl radicals to Pt0 surfaces might poison the NPs catalytic activity. When the Pt0‐NPs are prepared by the reduction of a different precursor salt, PtCl62?, nearly no C2H4 and oligomers are formed and the methyl coating covers a larger percentage of the surface Pt0 atoms. The difference is attributed to the morphology of the Pt0‐NPs: those prepared from Pt(SO4)2 are twinned nanocrystals, whereas those prepared from PtCl62? consist mostly of single crystals. Thus, the results indicate that the side products, or most of them at least, are formed on the twinned Pt0 nanocrystal edges created between (111) facets. In addition, the results show that Pt0‐NPs react very differently compared with other noble metals, for example, Au0 and Ag0; this difference is attributed in part to the difference in the bond strength, (M0‐NP)?CH3, and should be considered in heterogeneous catalytic processes involving alkyl radicals as intermediates. 相似文献
98.
Díaz MC Illescas BM Martín N Viruela R Viruela PM Ortí E Brede O Zilbermann I Guldi DM 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(8):2067-2077
A new class of pi-extended TTF-type electron donors (11 a-c) has been synthesized by Wittig-Horner olefination of bianthrone (9) with 1,3-dithiole phosphonate esters (10 a-c). In cyclic voltammetry experiments, donors 11 a-c reveal a single, electrochemically irreversible oxidation-yielding the corresponding dicationic products-at relatively low oxidation potentials (approximately 0.7-0.8 V). Theoretical calculations, performed at the DFT level (B3 P86/6-31 G*), predict a highly-folded C(2h) structure for 11 a. In the ground state, the molecule adopts a double saddle-like conformation to compensate the steric hindrance. The calculations suggest that the intramolecular charge transfer associated with the HOMO-->LUMO transition is responsible for an absorption band observed above 400 nm. While the radical cation 11 a*+ retains the folded C(2h) structure predicted for the neutral molecule as the most stable conformation, the dication 11 a(2+) has a fully aromatic D(2) structure, formed by an orthogonal 9,9'-bianthryl central unit to which two singly-charged dithiole rings are attached. The drastic conformational changes that compounds 11 undergo upon oxidation account for their electrochemical properties. By means of pulse radiolysis measurements, radical-induced one-electron oxidation of 11 a-c was shown to lead to the radical cation species (11 a-c*+), which were found to disproportionate with generation of the respective dication species (11 a-c(2+)) and the neutral molecules (11 a-c). 相似文献
99.
Michaela Shmilovits Mikki Vinodu Israel Goldberg 《Journal of inclusion phenomena and macrocyclic chemistry》2004,50(3-4):165-171
The X-ray crystal structure of IX, perchlorate salt of R-(–-2-ethyl-N-benzyl-4,7,19,13-tetraoxa-8,9-benzo-1-azacyclopentadec-8-ene has been determined. In the molecule, the protonated nitrogen atom participates in two N-HO hydrogen bonds. The unusually high proton affinity of aza crown ether leads to the formation of diastreomer instead of complex formation with chiral R-(+)-1-phenyl ethyl ammonium perchlorate and S-(–)-1-phenyl ethyl ammonium perchlorate. The complex ability of host ethers was evaluated in terms of structural modification. 相似文献
100.
Batjargal S Wang YJ Goldberg JM Wissner RF Petersson EJ 《Journal of the American Chemical Society》2012,134(22):9172-9182
Thioamide modifications of the peptide backbone are used to perturb secondary structure, to inhibit proteolysis, as photoswitches, and as spectroscopic labels. Thus far, their incorporation has been confined to single peptides synthesized on solid phase. We have generated thioamides in C-terminal thioesters or N-terminal Cys fragments and examined their compatibility with native chemical ligation conditions. Most sequence variants can be coupled in good yields with either TCEP or DTT as the reductant, though some byproducts are observed with prolonged TCEP incubations. Furthermore, we find that thioamides are compatible with thiazolidine protection of an N-terminal Cys, so that multiple ligations can be used to construct larger proteins. Since the acid-lability of the thioamide prohibits on-resin thioester synthesis using Boc chemistry, we devised a method for the synthesis of thioamide peptides with a masked C-terminal thioester that is revealed in situ. Finally, we have shown that thioamidous peptides can be coupled to expressed protein fragments to generate large proteins with backbone thioamide labels by synthesizing labeled versions of the amyloid protein α-synuclein for protein folding studies. In a proof-of-principle experiment, we demonstrated that quenching of fluorescence by thioamides can be used to track conformational changes during aggregation of labeled α-synuclein. 相似文献