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81.
A chiral stationary phase (CSP) based on (-)-(18-crown-6)-2,3,11,12-tetracarboxylic acid was evaluated for the direct resolution of the enantiomers of dipeptides and tripeptides. The type and concentration of the acid and the methanol content were optimized with regard to retention time and resolution using Ala-Phe as model peptide. A mobile phase consisting of 10 mM sulfuric acid in 70% aqueous methanol was applied to the separation of a set of 16 structurally diverse dipeptides and tripeptides. Generally, the configuration of the amino acid at the N-terminus determined the enantiomer elution order. With a few exceptions the LL- and LD-enantiomers interacted stronger with the CSP compared to the corresponding DD- or DL-enantiomers. The experimental conditions also allowed the simultaneous separation of all four stereoisomers of Ala-Phe. Addition of ammonium sulfate generally reduced retention times and enantiomer resolution. Addition of triethylamine as modifier led to an overall increase of the retention times while the resolution did not show a general trend, increasing in the case of Ala-Ala but decreasing in the case of Ala-Phe. 相似文献
82.
Summary The base catalyzed oxidative dimerization of emodin anthrone exclusively yields hypericin. However, on oxidative dimerization of trimethylemodinanthrone a mixture of hexamethylhypericin and hexamethylisohypericin was obtained. Chromatographic separation of the hexabenzoyl derivatives was achieved, and by saponification about equal amounts of hypericin and isohypericrin were produced. Isohypericin could be characterized for the first time by its spectroscopic data and its protonation and deprotonationpK
a
andpK
a
*
values.
Zur Synthese von Hypericin durch oxidative Dimerisierung von Trimethylemodinanthron und Emodinanthron: Isohypericin
Zusammenfassung Die basenkatalysierte oxidative Dimerisierung von Emodinanthron liefert ausschließlich Hypericin. Oxidative Dimerisierung von Trimethylemodinanthron ergibt jedoch ein Gemisch von Hexamethylhypericin und Hexamethylisohypericin. Die Hexabenzoylderivate wurden chromatographisch getrennt, und Hypericin und Isohypericin konnten aus diesen Derivaten durch Verseifung freigesetzt werden. Isohypercin konnte erstmals durch seine spektroskopischen Daten und seine Protonierungs- und Deprotonierungs-pK aundpK a * -Werte charakterisiert werden.相似文献
83.
Gerhard Fieck 《Theoretical chemistry accounts》1988,73(4):247-277
The reducible representations of the point groups are generally studied because of their relevance to molecular orbital and vibration theory. Triple correlations within the polyhedra are described by group-theoretical invariants that are related to the permutation representations and termed polyhedral isoscalar factors. These invariants are applied in theorems on matrix elements referring to the symmetry-adapted bases at different centres. Further invariants or geometrical weight factors inter-relate different types of reduced matrix elements of irreducible tensors (generalization of the Wigner-Eckart theorem to the polycentric case). As a demonstration a complete tabulation is given for the point group C
4. 相似文献
84.
Planer-Friedrich B Matschullat J Merkel BJ Roewer G Volke P 《Analytical and bioanalytical chemistry》2002,374(7-8):1191-1198
The formation of volatile organic and inorganic metals and metalloids in aquatic environments is a known, but not very intensively investigated, process. Several techniques have been developed over the past 10 years to determine these trace components. These techniques are of limited use in wetland environments, where samples have to be taken from the soil-water interface, and require an immediate sample analysis due to thermodynamic instabilities of the volatile metal(loid)s. This paper presents an innovative sampling technique for total concentrations of volatile metal(loid)s in wetlands, based on an in situ gas-water separation via a porous PTFE membrane and stabilising the volatile metal(loid)s in a liquid sorbent (NaOCl solution). Samples may thus be collected even at remote sites, where longer storage times have to be accounted for. The sampling system was tested by means of a laboratory facility simulating the generation of arsine and dimethyl arsine under abiotic conditions as well as under field conditions. Results for sampling efficiency, reproducibility, and long-term storage are presented. Application of the sampling system in the field is shown. 相似文献
85.
E. Asmus und J. Peters 《Fresenius' Journal of Analytical Chemistry》1970,249(2):106-109
Zusammenfassung Tetrahydroxyphenazin (THP) bildet mit Titan im sauren Medium Komplexe mit sehr hohen Werten des Extinktionskoeffizienten. Es wird ein Bestimmungsverfahren für Titan in molybdänhaltigen Stählen beschrieben, bei dem die störenden Elemente mit einem Gemisch aus Tri-n-butylphosphat und Chloroform extrahiert werden. Das in der wäßrigen Phase verbleibende Titan wird mit THP photometrisch bestimmt. Die hochempfindliche Methode erlaubt Titanbestimmungen im Nanomol-Bereich. Bei Stahlproben mit 0,2–0,7% Ti (in Gegenwart von 0–2% Mo) ergab sich ein Variationskoeffizient von 0,4 bis 2,4%.
Für die Durchführung dieser Arbeit wurden dankenswerterweise Forschungsmittel aus dem ERP-Sondervermögen bereitgestellt.Dem Verbande der Chemischen Industrie und der Deutschen Forschungsgemeinschaft, die diese Arbeit unterstützt haben, danken wir bestens. 相似文献
Photometric determination of titanium in molybdenum steels with tetrahydroxyphenazine
Complexes with very high molar extinction coefficients are produced in the reaction of tetrahydroxyphenazine (THP) with titanium in acid solution. In the method described titanium is determined photometrically by complexing with THP in the aqueous phase after extracting the interfering elements with a mixture of tri-n-butyl-phosphate and chloroform. This highly sensitive method is applicable for determination of titanium in the nanomolar region. With steel samples containing 0.2–0.7% of Ti (in presence of 0–2% of Mo) variation coefficients of 0.4 to 2.4% have been obtained.
Für die Durchführung dieser Arbeit wurden dankenswerterweise Forschungsmittel aus dem ERP-Sondervermögen bereitgestellt.Dem Verbande der Chemischen Industrie und der Deutschen Forschungsgemeinschaft, die diese Arbeit unterstützt haben, danken wir bestens. 相似文献
86.
The present study was conducted in order to evaluate the cyclodextrin (CD)-mediated chiral separation of peptide enantiomers as uncharged analytes at pH 5.3 using randomly sulfated beta-cyclodextrin, heptakis-6-sulfato-beta-CD and heptakis-(2,3-diacetyl-6-sulfato)-beta-CD as chiral selectors. Although less effective compared to stronger acidic conditions, the CDs proved to be suitable chiral selectors for the present set of peptides at pH 5.3. The carrier ability of the negatively charged CDs upon reversal of the applied voltage may also be exploited leading to a reversal of the migration order. In addition, reversal of the enantiomer migration order upon increasing the buffer pH from 2.5 to 5.3 was also observed for Ala-Tyr in the presence of randomly sulfated beta-CD, for Ala-Phe, Ala-Tyr, Phe-Phe, Asp-PheNH(2) and Gly-Ala-Phe in the presence of heptakis-6-sulfato-beta-CD, and for Phe-Phe and Ala-Leu in the presence of heptakis-(2,3-diacetyl-6-sulfato)-beta-CD. The migration behavior could be explained on the basis of the complexation constants and the mobilities of the peptide-CD complexes. While a change in the affinity pattern of the CDs upon increasing the pH was observed for some peptides, complex mobility was the primary factor for other peptide-CD combinations affecting the enantiomer migration order at the two pH values studied. 相似文献
87.
Gyula Záray Tibor Kántor Gerhard Wolff Zdravka Zadgorska Hubertus Nickel 《Mikrochimica acta》1992,107(3-6):345-358
The analytical capability of high-temperature halogenation with carbon tetrachloride vapour in a graphite furnace was investigated for silicon carbide powder with known chemical composition and particle size. Intensity vs heating time curves were determined for analytical lines of Al, B, Ba, Ca, Cr, Cu, Fe, Mg, Mn, Ti, V and Si constituents, volatilized with and without the presence of CCl4 vapour in the furnace atmosphere. Igniting 10 mg SiC at 2100 °C for 60 s in chlorinating atmosphere, the evaporated fraction of most of the constituents was higher than 90% (for Al about 50%). The line intensity vs sample mass (4–26 mg) relationships were linear for all impurities studied, while the intensity of silicon line showed a relatively small change with the sample mass. BEC (background equivalent concentration) values for this solid sampling technique (10 mg loaded sample) were 2–20 fold lower than those calculated for the conventional solution sample introduction method. 相似文献
88.
Maas G 《Chemical Society reviews》2004,33(3):183-190
The transition-metal catalysed cyclopropanation of olefinic bonds using diazo compounds as a carbene source is among the best developed and most useful transformations available to the synthetic organic chemist. Nevertheless, the quest for new catalyst/ligand systems continues in order to further extend the scope of this method and to identify more economical catalytic systems. In this tutorial review, several different ruthenium complexes are presented which have recently emerged as suitable catalysts for carbenoid cyclopropanation. For the model reaction--cyclopropanation of styrene(s) with diazoacetates--and also for some intramolecular cyclopropanation reactions highly remarkable results in terms of catalyst efficiency, product yields, dia- and enantioselectivity have been reported. 相似文献
89.
Dr. Meaghan M. Deegan Dr. Kareem I. Hannoun Prof. Dr. Jonas C. Peters 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(50):22820-22826
The prototypical reactivity profiles of transition metal dihydrogen complexes (M-H2) are well-characterized with respect to oxidative addition (to afford dihydrides, M(H)2) and as acids, heterolytically delivering H+ to a base and H− to the metal. In the course of this study we explored plausible alternative pathways for H2 activation, namely direct activation through H-atom or hydride transfer from the σ-H2 adducts. To this end, we describe herein the reactivity of an isostructural pair of a neutral S= and an anionic S=0 Co-H2 adduct, both supported by a trisphosphine borane ligand (P3B). The thermally stable metalloradical, (P3B)Co(H2), serves as a competent precursor for hydrogen atom transfer to tBu3ArO⋅. What is more, its anionic derivative, the dihydrogen complex [(P3B)Co(H2)]1−, is a competent precursor for hydride transfer to BEt3, establishing its remarkable hydricity. The latter finding is essentially without precedent among the vast number of M-H2 complexes known. 相似文献
90.
Different synthetic routes for the preparation of diiodo(methylthio)gallane are given. Some reactions of this compound with Lewis bases, such as O(CH3)2, S(CH3)2, S2(CH3)2, N(CH3)3, P(CH3)3, and other compounds, such as CH3I, CH3OH, C2H5OH, C2H5SH, i-C3H7SH, and C2H5SeH are investigated. Spectra and some physical and chemical properties of the new compounds are reported. The structure of diiodo(methylthio)gallane is discussed in view of some interesting differences of this molecule in solution and in crystal form. 相似文献