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21.
22.
The synthesis and binding properties of a new tricationic guanidiniocarbonyl pyrrole receptor 7 are described. Receptor 7 binds citrate 9 and other tricarboxylates such as trimesic acid tricarboxylate 8 with unprecedented high association constants of K(assoc) > 10(5) M(-1) in water as determined by UV and fluorescence tritration studies. According to NOESY experiments and molecular modeling calculations, the tricarboxylates are bound within the inner cavity of receptor 7 by ion pairing between the carboxylate groups and the guanidiniocarbonyl pyrrole moieties, favored by the nonpolar microenvironment of the cavity. Hence, receptor 7 can be regarded as a molecular flytrap. In the case of the aromatic tricarboxylate 8, additional aromatic interactions further strengthen the complex. The complexes with the tricarboxylates are so strong that even the presence of a large excess of competing anions or buffer salts does not significantly affect the association constant. For example, the association constant for citrate changes only from K(assoc) = 1.6 x 10(5) M(-1) in pure water to K(assoc) = 8.6 x 10(4) M(-1) in the presence of a 170-fold excess of bis-tris buffer and a 1000-fold excess of chloride. This makes 7 one of the most efficient receptors for the binding of citrate in aqueous solvents reported thus far. 相似文献
23.
A dimethylsiloxane diluted polysiloxane side chain co-polymer with non-chiral banana-shaped mesogenic units shows an optically isotropic ferroelectric switching polar smectic C phase (SmCPF) consisting of a conglomerate of homogeneously chiral domains with opposite handedness. 相似文献
24.
Ottmar Steinhauser Paul von Fragstein und Claus Harzdorf 《Fresenius' Journal of Analytical Chemistry》1967,232(4):241-251
Zusammenfassung Die Analyse hochprozentigen Flußspats wird in ihrer derzeitigen Ausführungsform beschrieben. Nach Beseitigung carbonatischer Begleiter durch eine Behandlung mit verd. Essigsäure wird die Kieselsäure entweder mit Flußsäure verflüchtigt und aus dem Gewichtsverlust bestimmt oder in einer gesonderten, mit Soda aufgeschlossenen Probe als Tetrapyridiniumdodecamolybdatosilicat gefällt und als SiO2 + 12 MoO3 ausgewogen. Der Bariumsulfatgehalt wird nach Verflüchtigung der Kieselsäure durch Abrauchen des verbleibenden Rückstandes mit Schwefelsäure und anschließendes Kochen mit verd. Salzsäure/Schwefelsäure in der üblichen Art ermittelt. Im Filtrat der Bariumsulfatfällung wird Calcium nach Abtrennung oder Maskierung der dreiwertigen Begleiter als Oxalat gefällt und zum Oxid verglüht oder direkt durch eine photometrische Titration mit ÄDTA bestimmt. Der kleine Sulfidschwefelgehalt in Flußspat wird durch Säure als Schwefelwasserstoff ausgetrieben, als Cadmiumsulfid gebunden und anschließend jodometrisch bestimmt.Für Kieselsäure, Bariumsulfat und Calciumfluorid ergibt die Fehlerrechnung Standardabweichungen von
s
SiO2=0.04%; sBaSO4=±0,04%; sCaF2=±0,09%.
Summary The present form of the analysis of high-percentage fluorspar is described. After removal of accompanying carbonates by treatment with dilute acetic acid, the silica is either volatilized with hydrofluoric acid and determined from the loss of weight or precipitated in a separate sample, decomposed with sodium carbonate, as tetrapyridinium dodecamolybdatosilicate and weighed as SiO2 + 12 MoO3. After volatilization of silica the remaining residue is fumed with sulphuric acid, boiled with a dilute hydrochloric/sulphuric acid mixture and the barium sulphate determined gravimetrically. In the filtrate of the barium sulphate precipitation the trivalent elements are either separated or masked. Then calcium is precipitated as oxalate and weighed after conversion to the oxide or directly determined by photometric titration with EDTA. After conversion to hydrogensulphide, the small sulphide-sulphur content in fluorspar is bound as cadmium sulphide and determined iodometrically.The error calculation results in the following standard deviations for silica, barium sulphate, and calcium fluoride: s SiO2=0.04%; sBaSO4=±0,04%; sCaF2=±0,09%.相似文献
25.
The aim of this work was the preparation of enantiomerically pure bis(pinene-bipyridine) ligands containing the ferrocenyl moiety. Several such ligands (1-3) were synthesized and completely characterized. These molecules can be diastereoselectively deprotonated at the acidic methylene group of the pinene moiety using a strong and sterically hindered base such as LDA. Subsequent reaction of the formed anion with alkyl halides yield the family of C(2)-symmetric enantiopure compounds (1a-c). Copper(I), silver(I), or zinc(II) complexes with several ligands (C1-C8) were prepared and structurally characterized in the solid state and in solution. Self-assembled helical species are formed in several cases. It became evident that the chiral groups present in the ligand do not completely determined the helical configuration of the assemblages. Diastereoselectivity is thus not complete with this type of ligands, contrary to other, similar ligands studied before. 相似文献
26.
The spinels of the system LixMn1?xV2O4 (0 ? x ? 1) have been prepared at 700–750°C from LiV2O4 and MnV2O4. The lattice constants decrease linearly with increasing x. In the region x>0.75, the d-electrons of V should be delocalized as the VV distances are lower than the critical VV separation of 2.94 Å. Experimentally, the samples with x>0.6 show no IR absorption bands and the Seebeck coefficient is near zero. The Seebeck coefficient can be described with a model of intermediate polarons and can be expressed by the equation . 相似文献
27.
The structure and configuration of harpagoside, a bitter principle from the roots of Harpagophytum procumbens DC, have been elucidated by degradation reactions and NMR. spectra. The hitherto uncertain position of a secondary hydroxyl group could be assigned to C-6 of the aglycone moiety. As earlier postulated, harpagoside ( 1 ) as well as its saponification product, harpagide ( 12 ), belong to a series of natural enol ethers known as iridoids. 相似文献
28.
Ulrich Vgeli Wolfgang von Philipsborn Kuppuswamy Nagarajan Mohan D. Nair 《Helvetica chimica acta》1978,61(2):607-617
Heterocyclic compounds obtained by addition of acetylenedicarboxylic acid esters to thioureas, cyclic amidines and o-difunctionalized aromatic systems have been studied by 13C-NMR. In particular, C, H-spin-coupling constants over two and three bonds were used to differentiate between the various constitutional isomers and to establish the configuration of trisubstituted exocyclic C, C-double bonds. The configurational significance and diagnostic value of vicinal cis and trans C,H-spin coupling is again demonstrated in the present series. 相似文献
29.
A reactive ionized cluster beam technique (RICB) which was composed of a conventional ICB source and a radical beam source has been used to deposit stable and metastable polycrystalline AlNx (0x1) films. Using in-situ high energy electron diffraction (HEED) at grazing incidence geometry, crystallographic properties such as structure, preferred orientation and interplanar dspacing values were determined and the relation to deposition parameters investigated. It could be shown that the simultaneous use of the ICB technique and a radical beam source to separately control the kinetic energy of the Al ions and the dissociation rate of molecular nitrogen, allows AlN films to be deposited with variable composition and crystal structures. In-situ HEED used in the transmission mode is an effective tool to investigate the crystallography of growing compound films such as AlNx. 相似文献
30.
Rate coefficients for the collisional quenching of O2*(1Δg) by NO and CO2 at 2–8 torr and 300 K have been determined. kNO = (2.48 ± 0.23) × 10?17 cm3 molecule?1 s?1 and = (2.56 ± 0.12) × 10?18 cm3 molecule?1 s?1. 相似文献