全文获取类型
收费全文 | 711篇 |
免费 | 48篇 |
国内免费 | 2篇 |
专业分类
化学 | 511篇 |
晶体学 | 6篇 |
力学 | 31篇 |
数学 | 53篇 |
物理学 | 160篇 |
出版年
2024年 | 3篇 |
2023年 | 23篇 |
2022年 | 15篇 |
2021年 | 32篇 |
2020年 | 29篇 |
2019年 | 40篇 |
2018年 | 35篇 |
2017年 | 33篇 |
2016年 | 50篇 |
2015年 | 30篇 |
2014年 | 43篇 |
2013年 | 65篇 |
2012年 | 66篇 |
2011年 | 57篇 |
2010年 | 23篇 |
2009年 | 22篇 |
2008年 | 39篇 |
2007年 | 37篇 |
2006年 | 16篇 |
2005年 | 17篇 |
2004年 | 7篇 |
2003年 | 6篇 |
2002年 | 9篇 |
2001年 | 5篇 |
2000年 | 4篇 |
1999年 | 3篇 |
1997年 | 2篇 |
1996年 | 3篇 |
1995年 | 4篇 |
1994年 | 2篇 |
1992年 | 1篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1989年 | 2篇 |
1988年 | 1篇 |
1987年 | 3篇 |
1986年 | 1篇 |
1985年 | 3篇 |
1984年 | 6篇 |
1983年 | 3篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 4篇 |
1973年 | 1篇 |
排序方式: 共有761条查询结果,搜索用时 250 毫秒
101.
102.
We have investigated the magnetic properties of four recently synthesized stable TMM-type nitroxide diradicals. Four new diradicals are proposed by tailoring one of the species in such a way that both conjugation and planarity increase. As a remarkable consequence, the intramolecular ferromagnetic exchange interaction was found to be quite high in the proposed radicals. The calculated coupling constants were in the range of +102 to +140 cm-1. The MO and spin density analysis are provided to interpret the exchange interactions. We observed the existence of intramolecular pi-pi-interactions for the species 2. This slightly increased the J value by shortening the length of the spacer between the two spin sources. 相似文献
103.
Iqbal A Siddiqui HL Ashraf CM Bukhari MH Akram CM 《Chemical & pharmaceutical bulletin》2007,55(7):1070-1072
Thirteen new Schiff bases derived from p-nitrobenzaldehyde were synthesized by condensation with the appropriate amines. An unusual reduction of the p-nitrobenzaldehyde to the corresponding alcohol was also observed in one of the reactions. The structures of the compounds were identified using spectroscopic techniques. Cytotoxicity for the titled compounds was studied against Brine Shrimp, used as the test animal. 相似文献
104.
Dey A Jeffrey SP Darensbourg M Hodgson KO Hedman B Solomon EI 《Inorganic chemistry》2007,46(12):4989-4996
S K-edge X-ray absorption spectroscopy data on a series of NiII complexes with thiolate (RS-) and oxidized thiolate (RSO2-) ligands are used to quantify Ni-S bond covalency and its change upon ligand oxidation. Analyses of these results using geometry-optimized density functional theory (DFT) calculations suggest that the Ni-S sigma bonds do not weaken on ligand oxidation. Molecular orbital analysis indicates that these oxidized thiolate ligands use filled high-lying S-O pi* orbitals for strong sigma donation. However, the RSO2- ligands are poor pi donors, as the orbital required for pi interaction is used in the S-O sigma-bond formation. The oxidation of the thiolate reduces the repulsion between electrons in the filled Ni t2 orbital and the thiolate out-of-plane pi-donor orbital leading to shorter Ni-S bond length relative to that of the thiolate donor. The insights obtained from these results are relevant to the active sites of Fe- and Co-type nitrile hydratases (Nhase) that also have oxidized thiolate ligands. DFT calculations on models of the active site indicate that whereas the oxidation of these thiolates has a major effect in the axial ligand-binding affinity of the Fe-type Nhase (where there is both sigma and pi donation from the S ligands), it has only a limited effect on the sixth-ligand-binding affinity of the Co-type Nhases (where there is only sigma donation). These oxidized residues may also play a role in substrate binding and proton shuttling at the active site. 相似文献
105.
We investigate the influence of the first-order correction of entropy caused by thermal quantum fluctuations on the thermodynamics of a logarithmic corrected charged black hole in massive gravity. For this black hole, we explore the thermodynamic quantities, such as entropy, Helmholtz free energy, internal energy, enthalpy, Gibbs free energy and specific heat. We discuss the influence of the topology of the event horizon, dimensions and nonlinearity parameter on the local and global stability of the black hole. As a result, it is found that the holographic dual parameter vanishes. This means that the thermal corrections have no significant role to disturb the holographic duality of the logarithmic charged black hole in massive gravity, although the thermal corrections have a substantial impact on the thermodynamic quantities in the high-energy limit and the stability conditions of black holes. 相似文献
106.
This article deals with the hitherto unexplored metal complexes of deprotonated 6,12‐di(pyridin‐2‐yl)‐5,11‐dihydroindolo[3,2‐b]carbazole (H2L). The synthesis and structural, optical, electrochemical characterization of dimeric [{RuIII(acac)2}2(μ‐L.?)]ClO4 ([ 1 ]ClO4, S=1/2), [{RuII(bpy)2}2(μ‐L.?)](ClO4)3 ([ 2 ](ClO4)3, S=1/2), [{RuII(pap)2}2(μ‐L2?)](ClO4)2 ([ 4 ](ClO4)2, S=0), and monomeric [(bpy)2RuII(HL?)]ClO4 ([ 3 ]ClO4, S=0), [(pap)2RuII(HL?)]ClO4 ([ 5 ]ClO4, S=0) (acac=σ‐donating acetylacetonate, bpy=moderately π‐accepting 2,2’‐bipyridine, pap=strongly π‐accepting 2‐phenylazopyridine) are reported. The radical and dianionic states of deprotonated L in isolated dimeric 1 +/ 2 3+ and 4 2+, respectively, could be attributed to the varying electronic features of the ancillary (acac, bpy, and pap) ligands, as was reflected in their redox potentials. Perturbation of the energy level of the deprotonated L or HL upon coordination with {Ru(acac)2}, {Ru(bpy)2}, or {Ru(pap)2} led to the smaller energy gap in the frontier molecular orbitals (FMO), resulting in bathochromically shifted NIR absorption bands (800–2000 nm) in the accessible redox states of the complexes, which varied to some extent as a function of the ancillary ligands. Spectroelectrochemical (UV/Vis/NIR, EPR) studies along with DFT/TD‐DFT calculations revealed (i) involvement of deprotonated L or HL in the oxidation processes owing to its redox non‐innocent potential and (ii) metal (RuIII/RuII) or bpy/pap dominated reduction processes in 1 + or 2 2+/ 3 +/ 4 2+/ 5 +, respectively. 相似文献
107.
Himali Sarangal Amarpal Singh Jyoteesh Malhotra Sushank Chaudhary 《Optical and Quantum Electronics》2017,49(5):184
FSO or free space optics is a familiar name used in a wide array of applications in the area of telecommunications. Due to its features of low maintenance cost and deployment time, most of the applications consider FSO as the alternative solution for appropriately replacing fiber optics. In this work, we have designed 100 Gbps FSO system by combining mode division multiplexing (MDM) and optical code multiple access scheme (OCDMA). Ten channels, each carrying 10 Gbps data, are transported over 8 km FSO link by using MDM of two Laguerre Gaussian modes and random diagonal codes. Moreover, the performance of proposed MDM–OCDMA–FSO system is also investigated under atmospheric turbulences. 相似文献
108.
109.
Hyung-Jung Pyun Kleem Chaudhary John R. Somoza X. Christopher Sheng Choung U. Kim 《Tetrahedron letters》2009,50(27):3833-1396
We herein describe an efficient synthesis of optically active diethyl 1-amino-2-vinylcyclopropane-1-phosphonate (analogous to 1-amino-2-vinylcyclopropane-1-carboxylate). The racemic phosphonate diethyl ester was obtained from an imine derived from aminomethylphosphonate diester and trans-1,4-dibromo-2-butene. Crystallizations of the dibenzoyl-l-tartaric acid salt allowed for separation of enantiomers. The enantiomerically pure material was used to synthesize an extremely potent tripeptide phosphonate inhibitor of HCV NS3 protease. X-ray crystal structure of the inhibitor bound to the HCV NS3 protease confirmed the absolute stereochemistry of the title compound. 相似文献
110.
Dey A Hocking RK Larsen P Borovik AS Hodgson KO Hedman B Solomon EI 《Journal of the American Chemical Society》2006,128(30):9825-9833
Iron L-edge, iron K-edge, and sulfur K-edge X-ray absorption spectroscopy was performed on a series of compounds [Fe(III)H(3)buea(X)](n-) (X = S(2-), O(2-), OH(-)). The experimentally determined electronic structures were used to correlate to density functional theory calculations. Calculations supported by the data were then used to compare the metal-ligand bonding and to evaluate the effects of H-bonding in Fe(III)(-)O vs Fe(III)(-)S complexes. It was found that the Fe(III)(-)O bond, while less covalent, is stronger than the Fe(III)(-)S bond. This dominantly reflects the larger ionic contribution to the Fe(III)(-)O bond. The H-bonding energy (for three H-bonds) was estimated to be -25 kcal/mol for the oxo as compared to -12 kcal/mol for the sulfide ligand. This difference is attributed to the larger charge density on the oxo ligand resulting from the lower covalency of the Fe-O bond. These results were extended to consider an Fe(IV)(-)O complex with the same ligand environment. It was found that hydrogen bonding to Fe(IV)(-)O is less energetically favorable than that to Fe(III)(-)O, which reflects the highly covalent nature of the Fe(IV)(-)O bond. 相似文献