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501.
W. Żmijewska 《Journal of Radioanalytical and Nuclear Chemistry》1972,10(2):187-196
A method has been developed for the substoichiometric separation of Cr(VI) by extraction with tri-n-octylamine solution in
benzene from 0.1M H2SO4 solution. The method has been applied to the determination of chromium in silicon and aluminium by the neutron activation
method. 相似文献
502.
Dietmar Heidrich Dietmar Volkmann Bronisz.xl;law Żurawski 《Chemical physics letters》1981,80(1):60-63
The deprotonation energy of a (H3Si)O-H group as a simplified structural element of silica gel and aluminosilicates is estimated from ab initio calculations on H2O, CH30H and SiH30H to be near 1475 kJ/mol. Results of the effect of Lewis acids on the Brønsted acidity of SiOH are given. 相似文献
503.
Wieslaw Wiczk Leszek Łankiewicz Cezary Czaplewski Stanislaw Oldziej Krystyna Stachowiak Alicja Michniewicz Beata Micewicz Adam Liwo 《Journal of fluorescence》1997,7(4):257-266
Synthesis and photophysical studies of (O-methyl)--tyrosine (-tyrosine; an analogue of tyrosine, in which the amino group is moved from the - to the -carbon, closer to the phenol ring) and its derivatives with a blocked amino and/or carboxyl group were performed to explain the nature of the fluorescence of tyrosine derived analogues. All -tyrosine derivatives, except Ac-Tyr(Me), displayed the monoexponential fluorescence decay. The biexponential fluorescence decay observed for Ac-Tyr(Me) is assumed to be the result of the presence of two low-energy conformations (extended and with an intramolecular hydrogen bond). Higher quenching of the fluorescence of -tyrosine derivatives by the N-acetyl group than by the N-methylamide group moved farther was found, contrary to the data found for the respective derivatives of natural tyrosine. The obtained photophysical data are discussed with theoretical calculations (AMBER, AM1) on the basis of the rotamer model. 相似文献
504.
W. Żabiński I. Wacławska C. Paluszkiewicz 《Journal of Thermal Analysis and Calorimetry》1996,46(5):1437-1447
The thermal decomposition of vesuvianite was studied by means of thermal, FTIR and X-ray methods. It was found that two structural forms of vesuvianite, a high-temperature (disordered) and a low-temperature (ordered) one, differ distinctly in the mechanism of their decomposition (dehydroxylation). Dehydroxylation of low vesuvianites begins at lower temperatures (ca. 900°C), and the strong endothermic peak with maximum at ca. 1020°C is usually followed by an exothermic peak. Dehydroxylation of high vesuvianites begins at ca. 1000°C, and the OTA curve usually displays two endothermic peaks not followed by an exothermic effect. The crystallization products of vesuvianite are grossular, gehlenite and anorthite. Vesuvianite melts in the temperature range 1100–1200°C.The authors thank Mr. A. Gawel for kindly recording the X-ray powder diffractograms. This study was supported by KBN grant No 6 P201 018 04. 相似文献
505.
Justyna Piekielna-Ciesielska Roberto Artali Ammar A. H. Azzam David G. Lambert Alicja Kluczyk Luca Gentilucci Anna Janecka 《Molecules (Basel, Switzerland)》2021,26(1)
In recent years, G protein vs. β-arrestin biased agonism at opioid receptors has been proposed as an opportunity to produce antinociception with reduced adverse effects. However, at present this approach is highly debated, a reason why more information about biased ligands is required. While the practical relevance of bias in the case of µ-opioid receptors (MOP) still needs to be validated, it remains important to understand the basis of this bias of MOP (and other GPCRs). Recently, we reported two cyclopeptides with high affinity for MOP, the G protein biased Dmt-c[d-Lys-Phe-pCF3-Phe-Asp]NH2 (F-81), and the β-arrestin 2 biased Dmt-c[d-Lys-Phe-Asp]NH2 (C-33), as determined by calcium mobilization assay and bioluminescence resonance energy transfer-based assay. The biased character of F-81 and C-33 has been further analyzed in the [35S]GTPγS binding assay in human MOP-expressing cells, and the PathHunter enzyme complementation assay, used to measure β-arrestin 2 recruitment. To investigate the structural features of peptide-MOP complexes, we performed conformational analysis by NMR spectroscopy, molecular docking, and molecular dynamics simulation. These studies predicted that the two ligands form alternative complexes with MOP, engaging specific ligand–receptor contacts. This would induce different displays of the cytosolic side of the seven-helices bundle, in particular by stabilizing different angulations of helix 6, that could favor intracellular coupling to either G protein or β-arrestin. 相似文献
506.
The copper(II) polymer Cu(2-qic)Cl (2-qic = quinoline-2-carboxylate) was synthesized and then characterized by X-ray crystal structure determination, spectroscopic and magnetic studies. The crystal structure consists of copper(II) ions with two different chromophores: four-coordinated in a square-planar geometry and five-coordinated in an environment between square-pyramidal and trigonal-bipyramidal. The copper ions are bridged sequentially through the carboxylate groups in a syn–anti conformation, forming an infinite one-dimensional zigzag chain with two alternating non-equivalent copper(II) chromophores. The chloride atom acts as a single chloro-bridge link to adjacent chains, forming a ribbon type structure (1D). The variable-temperature (1.8–300 K) magnetic susceptibility data of the complex were interpreted with the dimer law using the molecular field approximation. The results obtained indicate a very weak ferromagnetic (J2 = 0.37 cm−1) interchain interaction through the syn–anti carboxylate bridge. A relatively strong antiferromagnetic interaction, transmitted through the chloro-bridge with an exchange coupling of J1 = −57.0 cm−1, dominates the magnetic properties of this complex. The magnitude and the nature of the exchange coupling are explained on the basis of the structural results. 相似文献
507.
Heats of solution of acetonitrile, solvated or hydrated perchlorates, Mn(ClO4)2·6AN, Co(ClO4)2·6AN, Ni(ClO4)2·6AN, Cu(ClO4)2·4AN, Cu(ClO4)2·6H2O, Zn(ClO4)2·6AN, and Zn(ClO4)2·6H2O have been determined in NMF, DMF, DMA, and water. Complete or almost complete exchange of AN and water molecules in amides is inferred from the visible spectra determinations. The heats of transfer of perchlorate anion from water to DMA and NMF have been obtained from separately determined heats of solution of NaBPh4, AsPH4Cl·H2O, NaClO4, NaCl, and SiPh4 in the respective solvents. The heats of transfer of cations from water to amides have been determined from the above data and the heats of solvation of cations using literature data for the heats of hydration. 相似文献
508.
Metalloproteases utilize their active site divalent metal ions to generate a nucleophilic water/hydroxide. For methionine aminopeptidases (MetAPs), the exact location of this nucleophile, as well as of the substrate, with respect to the active site metal ion is unknown. In order to address this issue, we have examined the catalytically competent Fe(II)-loaded form of PfMetAP-II ([Fe(PfMetAP-II)]) in the absence and presence of both nitric oxide (NO) and the substrate-analogue inhibitor butaneboronic acid (BuBA) by kinetic and spectroscopic (EPR, UV-vis) methods. NO binds to [Fe(PfMetAP-II)] with a Kd of 200 microM forming an {FeNO}7 complex. UV-vis spectra of the resulting [Fe(PfMetAP-II)]-NO complex indicate that the Fe(II) ion is six coordinate. These data suggest that NO binding occurs without displacing the bound aquo/hydroxo moiety in [Fe(PfMetAP-II)]. On the basis of EPR spectra, the resulting Fe-NO complex is best described as NO- (S = 1) antiferromagnetically coupled to a high-spin Fe(III) ion (S = 5/2). The addition of BuBA to [Fe(PfMetAP-II)]-NO displaces the coordinated water molecule forming a six-coordinate adduct. EPR data also indicate that an interaction between the bound NO- and BuBA occurs forming a complex that mimics an intermediate step between the Michaelis complex and the tetrahedral transition-state. 相似文献
509.
J. Żurakowska-Orszagh K. Mirowski H. Wilczura 《Journal of Thermal Analysis and Calorimetry》1981,21(1):27-34
The thermal properties of complexes of N-tert-butylacrylamide and N-tert-amylacrylamide with ZnCl2, ZnBr2, CdCl2 and CdBr2 have been studied in air and in argon by TG and DTA. The decomposition proceeds in two steps with evolution of simple, low molecular weight gaseous products. The use of various heating rates revealed an additional exothermic DTA peak for complexes with zinc and cadmium chlorides corresponding to a solid state polymerization reaction which was confirmed by IR spectral analysis. This effect was not observed for corresponding complexes of zinc and cadmium bromides, which show much lower thermal stability.
Zusammenfassung Die thermischen Eigenschaften der Komplexe von N-Tertbutylacrylamid und N-Tertamylacrylamid mit ZnCl2, ZnBr2, CdCl2 und CdBr2 wurden in Luft und in Argon durch TG und DTA studiert. Die Zersetzung verläuft in zwei Stufen unter Entwicklung einfacher gasförmiger Produkte niedriges Molekulargewichts. Der Einsatz verschiedener Aufheizgeschwindigkeiten zeigte für Komplexe mit Zink- und Cadmiumchloriden einen zusätzlichen DTA Peak, der Festphasen-Polymerisationsreaktion entsprechend, welche durch IR-Spektralanalyse bestätigt wurde. Dieser Effekt wurde bei den entsprechenden Komplexen der Zink- und Cadmiumbromide, welche eine viel geringere Thermostabilität besitzen, nicht beobachtet.
Résumé On a étudié, par TG et ATD, dans l'air et l'argon, les propriétés thermiques des complexes du N-tertbutyl-acrylamide et du N-tétramylacrylamide avec ZnCl2, ZnBr2, CdCl2 et CdBr2. La décomposition s'effectue en deux étapes, avec dégagement de produits gazeux simples de faible poids moléculaire. L'application de différentes vitesses de chauffage a révélé, pour les complexes des chlorures de zinc et de cadmium, un pic exothermique additionnel en ATD qui correspond à la réaction de polymérisation dans l'état solide et qui a été confirmé par spectrométrie infrarouge. On n'a pas observé cet effet dans le cas des complexes correspondants des bromures de zinc et cadmium dont la stabilité thermique est bien plus faible.
N- - N- ZnCl2, ZnBr2, CdCl2 CdBr2. , . . , . , .相似文献
510.
Adam Liwo Krzysztof Sokoŀowski Alicja Wawrzynów Lech Chmurzyński 《Journal of solution chemistry》1990,19(11):1113-1124
Using a UV-spectrophotometric method, an attempt has been made to estimate quantitatively the influence of traces of water in aprotic solvents on the acidic-basic equilibria involving heterocyclic N-oxides. The N-oxides under study were pyridine N-oxide (PyO), 4-methoxy-pyridine N-oxide (4-MeOPyO), and 2-, 3-, and 4-picoline N-oxide (2-, 3-, and 4-PicO). For particular N-oxide the UV-spectra of acetonitrile solutions containing the free base and/or its simple or semiperchlorate have been recorded. To carry out the calculations various equilibrium models which include the protolytic equilibrium with water and basic species present in the solvent have been tested using the program STOICHIO which is based on non-linear regression analysis. It turned out that apart from the acidic-basic dissociation of a protonated N-oxide and cationic homoconjugation (the equilibria which are usually considered in such systems) it is absolutely necessary to take into account the protolytic equilibrium between the cationic acid and water present as impurity. Implications concerning investigations of other equilibrium systems in aprotic solvents and, in particular, the quality of the acidity constants for the calibration agents used in potentiometry are discussed. 相似文献