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91.
The oxidation of water catalyzed by [Ru(tpy)(bpy)(OH(2))](ClO(4))(2) (1; tpy = 2,2';6',2'-terpyridine; bpy = 2,2'-bipyridine) is evaluated in different acidic media at variable oxidant concentrations. The observed rate of dioxygen evolution catalyzed by 1 is found to be highly dependent on pH and the identity of the acid; e.g., d[O(2)]/dt is progressively faster in H(2)SO(4), CF(3)SO(3)H (HOTf), HClO(4), and HNO(3), respectively. This trend does not track with thermodynamic driving force of the electron-transfer reactions between the terminal oxidant, (NH(4))(2)[Ce(NO(3))(6)] (CAN), and Ru catalyst in each of the acids. The particularly high reactivity in HNO(3) is attributed to the NO(3)(-) anion: (i) enabling relatively fast electron-transfer steps; (ii) participating in a base-assisted concerted atom-proton transfer process that circumvents the formation of high energy intermediates during the O-O bond formation process; and (iii) accelerating the liberation of dioxygen from the catalyst. Consequently, the position of the rate-determining step within the catalytic cycle can be affected by the acid medium. These factors collectively contribute to the position of the rate-determining step within the catalytic cycle being affected by the acid medium. This offering also outlines how other experimental issues (e.g., spontaneous decay of the Ce(IV) species in acidic media; CAN/catalyst molar ratio; types of catalytic probes) can affect the Ce(IV)-driven oxidation of water catalyzed by homogeneous molecular complexes. 相似文献
92.
To overcome the unfavorable steric pressures associated with 2,6-disubstitution in encumbering pyridine ligands, the coordination chemistry of a 2,5-disubstituted variant, namely, 2,5-dimesitylpyridine (2,5-Mes(2)py), is reported. This diaryl pyridine shows good binding ability to a range of transition-metal fragments with varying formal oxidation states and coligands. Treatment of 2.0 equiv of 2,5-Mes(2)py with monovalent Cu and Ag triflate sources generates complexes of the type [M(2,5-Mes(2)py)(2)]OTf (M = Cu, Ag; OTf = OSO(2)CF(3)), which feature long M-OTf distances and a substrate-accessible primary coordination sphere. Combination of 2,5-Mes(2)py with Cu(OTf)(2) and Pd(OAc)(2) produces four-coordinate complexes featuring cis- and trans-2,5-Mes(2)py orientations, respectively. The four-coordinate palladium complex Pd(OAc)(2)(2,5-Mes(2)py)(2) is found to resist py-ligand dissociation at room temperature in solution, but functions as a precatalyst for the aerobic C-H bond olefination of benzene at elevated temperatures. This C-H bond activation chemistry is compared with a similar Pd-based system featuring 2,6-disubstituted pyridines. 2,5-Mes(2)py also readily supports mono- and dinuclear divalent Co complexes, and the solution-phase equilibria between such species are detailed. The coordination studies presented highlight the potential of 2,5-Mes(2)py to function as an encumbering ancillary for the stabilization of low-coordinate complexes and as a supporting ligand for metal-mediated transformations. 相似文献
93.
Zaleski CM Tricard S Depperman EC Wernsdorfer W Mallah T Kirk ML Pecoraro VL 《Inorganic chemistry》2011,50(22):11348-11352
The magnetic behavior of the pentanuclear complex of formula Mn(II)(O(2)CCH(3))(2)[12-MC(Mn(III)(N)shi)-4](DMF)(6), 1, was investigated using magnetization and magnetic susceptibility measurements both in the solid state and in solution. Complex 1 has a nearly planar structure, made of a central Mn(II) ion surrounded by four peripheral Mn(III) ions. Solid state variable-field dc magnetic susceptibility experiments demonstrate that 1 possesses a low value for the total spin in the ground state; fitting appropriate expressions to the data results in antiferromangetic coupling both between the peripheral Mn(III) ions (J = -6.3 cm(-1)) and between the central Mn(II) ion and the Mn(III) ones (J' = -4.2 cm(-1)). In order to obtain a reasonable fit, a relatively large single ion magnetic anisotropy (D) value of 1 cm(-1) was necessary for the central Mn(II) ion. The single crystal magnetization measurements using a microsquid array display a very slight opening of the hysteresis loop but only at a very low temperature (0.04 K), which is in line with the ac susceptibility data where a slow relaxation of the magnetization occurs just around 2 K. In frozen solution, complex 1 displays a frequency dependent ac magnetic susceptibility signal with an energy barrier to magnetization reorientation (E) and relaxation time at an infinite temperature (τ(o)) of 14.7 cm(-1) and 1.4 × 10(-7) s, respectively, demonstrating the single molecule magnetic behavior in solution. 相似文献
94.
95.
96.
The high temperatures and large strain limits associated with superplastic materials amplify the possibility of the tensile test outcomes being sensitive to the shape and size of the specimen geometry. In spite of that, the disparities in the specimen geometries used throughout the numerous efforts on characterising this unique class of materials are rather astonishing. There is an urge to evaluate the dependency of a superplastic tensile test on specimen geometry, before a much-needed universally-adopted standard specimen can be designed; which is the main objective of this comprehensive experimental investigation. More than 20 geometries, covering multiple variations in gauge length, gauge width, grip length and grip width values, are tested at identical conditions, and the corresponding material behaviour is compared in terms of deformation uniformity, material flow and the extracted stress/strain curves. The results reveal the influences of each geometrical parameter, as well as their combined effects, and guide the selection of an optimum specimen geometry for accurate and unified tensile testing of superplastic metallic materials. 相似文献
97.
98.
S.?MaterazziEmail author S.?Vecchio L.?W.?Wo S.?De?Angelis Curtis 《Journal of Thermal Analysis and Calorimetry》2011,103(1):59-64
Four manganese(II) coordination compounds with bis(1-methylimidazol-2-yl)ketone (BIK) of general formula Mn(BIK)2X2 (X = Cl, Br, NO3, ClO4) were synthesized and characterized by elemental analysis, by UV–vis, and FTIR spectroscopies to be compared with the literature
data. Following our previous thermoanalytical studies on imidazole-substituted coordination compounds, the thermal behavior
of the synthesized Mn(II) complexes was investigated using TG and DTG techniques: the thermal profile is characterized by
three substantial consecutive releasing steps for all the three complexes and the releasing supposed behavior is confirmed
by EGA analysis performed by coupling the TG analyzer to an MS spectrometer. In particular, the first step is ascribed to
the release of the two anions, followed by the loss of four methyl groups (side chains of the ligand) and two bridge-carbonyl
groups. The residual tetra-imidazole manganese compound decomposes in a final step to give MnO as the final residue. Both
the initial decomposition temperatures and the kinetic rate constants associated to the first decomposition step indicated
a higher stability of the Mn(BIK)2Cl2 complex, the bromide complex being very close to the chloride one (first-step thermal stability: ClO4
− <NO3
− ≤Br− <Cl−). Finally, the three-dimensional diffusion reaction model (D3) was selected to describe the first decomposition step for
all the four complexes examined. 相似文献
99.
Robyn Curtis 《Comptes Rendus Mathematique》2002,334(1):31-35
We define the Hecke von Neumann algebra associated with a group G, a subgroup H and a unitary representation σ of H. We show that when σ is finite dimensional, can be seen as a corner algebra of the tensor product of the group von Neumann algebra of a locally compact group and a matrix algebra. To cite this article: R. Curtis, C. R. Acad. Sci. Paris, Ser. I 334 (2002) 31–35 相似文献
100.
B. Curtis Eaves 《Mathematical Programming》1990,48(1-3):359-367
Subdivisions are continuously deformed subject to three local conditions and shown to retain the global property of being a subdivision. Cell maps, which are introduced, enable elementary arguments.This research is supported in part by the National Science Foundation Grant DMS-8902662. 相似文献