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51.
The difference in steric strain between the oxidized and the reduced forms of tetraaminecopper complexes is correlated with the corresponding reduction potentials. The experimentally determined data considered range from ?0.54 to ?0.04 V (vs. NHE) in aqueous solution and from ?0.35 to ?0.08 V (vs. NHE) in MeCN. The observed and/or computed geometries of the tetraaminecopper(II) complexes are distorted octahedral or square-pyramidal (4 + 2 or 4+1) with (distorted) square-planar CuN4 chromophores (CuII? N = 1.99–2.06 Å; Cu? O ≈ 2.5 Å; Cu? O ≈ 2.3 Å), those of the tetraaminecopper(I) complexes are (distorted) tetrahedral (four-coordinate; CuI? N = 2.12–2.26 Å; tetrahedral twist angle ?? = 30–90°). The reduction potentials of CuII/I couples with primary-amine ligands and those with macrocyclic secondary-amine ligands were correlated separately with the corresponding strain energies, leading to slopes of 70 and 61 kJ mol?1 V?1, with correlation coefficients of 0.89 and 0.91, respectively. The approximations of the model (entropy, solvation, electronic factors) and the limits of applicability are discussed in detail and in relation to other approaches to compute reduction potentials of transition-metal compounds. 相似文献
52.
[reaction: see text] Fmoc-protected amino acid fluorides were found to be excellent reagents for the acylation of sulfonamide safety-catch linkers (SCL) suitable for the subsequent preparation of peptide C-terminal thioesters. High loadings were obtained on different types of resins with low levels of epimerization. 相似文献
53.
Susanne Bredendiek-Kämper Holger Jenett 《Fresenius' Journal of Analytical Chemistry》1993,346(1-3):315-317
Summary The system of plasma sprayed Al2O3 on Ni substrates is investigated by means of AES/depth profiling. The influence of two process parameters — preoxidation procedure and spraying temperature — is examined. Rupture between substrate and ceramic layer occurs between a residual — or, in the case of excessive preoxidation, a superfluous — NiO layer on Ni, the thickness of the former depending on preoxidation conditions and the Al2O3 layer, the back side of which being partially covered with NiO. The thickness of this NiO layer increases up to about 1 m with the thickness of the initial NiO layer on the substrate, until this layer is about 1.3 m thick, and remains constant thereafter. The same dependence is observed for the width (0.1–1 m range) of the mixed oxide interface between the sprayed Al2O3 layer and the NiO layer below. These results represent the chemical contribution to adherence. Contrary to excessive preoxidation, an increase of the spraying temperature from 300°C to 500°C effects broader interfaces.This poster was awarded the First Prize in Poster Section C by the Deutscher Arbeitskreis für Spektroskopie (DASp) 相似文献
54.
55.
The stability of gold phosphine complexes of the form [Au(PH(3))(n)()](+) (n = 1-4) and [AuCl(PH(3))(n)()] (n = 1-3) is analyzed in detail by applying quantum theoretical methods and compared to the coordination behavior of the lighter group 11 elements copper and silver. It is shown that, once [M(PH(3))(2)](+) or [MClPH(3)] (M = Cu, Ag, and Au) is formed, further coordination by PH(3) ligands is relatively weak; i.e., the energy gain to form [M(PH(3))(3)](+) from [M(PH(3))(2)](+) is less than 60 kJ mol(-)(1), and less than 100 kJ mol(-)(1) to form [MCl(PH(3))(2)] from [MClPH(3)]. Relativistic effects in gold significantly influence these factors and reduce the tendency for phosphine coordination beyond two-coordination. This implies that the most favored coordination number for gold is two with either a linear P-Au-P or P-Au-X arrangement (X = a strongly coordinating ligand like Cl(-)). Instead, X-Au-PH(3) units prefer to interact via close Au-Au contacts (aurophilic interactions) keeping the linear structure approximately intact, while the corresponding copper and silver compounds prefer PH(3) coordination to strongly bound M(2)Cl(2) units (M = Cu or Ag) where two chlorine atoms bridge the two metal atoms thus having the formal coordination number of three for copper or silver. 相似文献
56.
Single crystal structure determinations prove the two pyridine substituents in di(2-pyridyl)ketone (H4C4NC)2C=O to be twisted out of the carbonyl skeleton plane by torsion angles (OCCN) of 41° and –163°, in contrast to their planar arrangement in azo-di(2-pyridine) H4C4NC)-N=N-(CNC4H4). In order to rationalize the surprising difference between the two isoelectronic molecules, approximate PM3 enthalpy of formation hypersurfaces have been calculated for each of the two ring torsions, which are assumed to be the dominant ones among the 3N – 6 = 60 degrees of freedom. For both the ketone and the azo derivative, global minima are calculated, the torsion angles of which deviate from the crystal structure results, and, therefore, support the assumption that both the experimentally determined twisting of di(2-pyridyl)ketone as well as the flattening of azo-di(2-pyridine) might be affected by the crystal packing. 相似文献
57.
Lindén AA Hermanns N Ott S Krüger L Bäckvall JE 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,11(1):112-119
Eight different flavin derivatives have been synthesized and the electronic effects of substituents in various positions on the flavin redox chemistry were investigated. The redox potentials of the flavins, determined by cyclic voltammetry, correlated with their efficiency as catalysts in the H2O2 oxidation of methyl p-tolyl sulfide. Introduction of electron-withdrawing groups increased the stability of the reduced catalyst precursor. 相似文献
58.
59.
Joachim Janssen Holger Rumpf Hartwig Modrow Ralf Rablbauer Georg Frommeyer Josef Hormes 《无机化学与普通化学杂志》2003,629(10):1701-1708
The surface oxidation of FeCr alloys with 18, 28, and 43 mass‐% Cr was investigated in situ using grazing‐incidence X‐ray absorption spectroscopy (GIXAS) at the chromium and iron K‐edges. Oxidation in air was monitored in situ in the temperature range from 290 K to 680 K. The standard GIXAS data analysis is extended for the treatment of a single layer model in order to estimate the chromium concentrations of the oxide layer and of the near‐interface substrate as well as the oxide layer thickness. XANES analysis shows transitions from b.c.c. Fe to corundum type Fe2O3 and from b.c.c. Cr to corundum type Cr2O3. The initial oxide layers are 1.1‐1.4 nm thick and contain 60‐90 mass‐% chromium, while the near‐interface substrate is depleted in Cr. During heating, iron oxide growth dominates up to 560‐600 K. Then the chromium oxide layer loses its passivation effect and Cr oxidation sets in. 相似文献
60.
Matthias Westerhausen Mathias Krofta Stefan Schneiderbauer Holger Piotrowski 《无机化学与普通化学杂志》2005,631(8):1391-1396
Transmetallation of Tin(II) in [Sn(μ3‐PSitBu3)]4 by Barium – from Sn4P4 Heterocubane Structures to Heterobinuclear Cage Compounds with a Central BanSn4?nP4 Heterocubane Polyhedron (n = 1, 2 and 3) For the preparation of compounds of the type [BanSn4?n(PSitBu3)4] (n = 1 ( 2 ), 2 ( 3 ) and 3 ( 4 )) two synthetic routes are applicable: in the transmetallation reaction homometallic [Sn4(PSitBu3)4] ( 1 ) reacts with barium metal and in a deprotonation reaction (metallation) tri(tert‐butyl)silylphosphane reacts simultaneously with (thf)2Ba[N(SiMe3)2]2 and Sn[N(SiMe3)2]2. During the transmetallation reaction mixtures of the heterobimetallic cage compounds 2 to 4 are obtained, however, analytically pure compounds 2 and 3 are accessible by the metallation reaction. Compound 4 is formed as a minor product together with 3 . Due to the larger Ba‐P bond lengths compared to the Sn‐P values the substitution of tin by barium leads to strong distortions of the heterocubane moiety. With NMR‐spectroscopic experiments one could show that all the above mentioned compounds form BanSn4?nP4 heterocubane cage structures. 相似文献