全文获取类型
收费全文 | 3179篇 |
免费 | 111篇 |
国内免费 | 18篇 |
专业分类
化学 | 2059篇 |
晶体学 | 12篇 |
力学 | 141篇 |
数学 | 638篇 |
物理学 | 458篇 |
出版年
2023年 | 15篇 |
2022年 | 46篇 |
2021年 | 72篇 |
2020年 | 74篇 |
2019年 | 56篇 |
2018年 | 58篇 |
2017年 | 37篇 |
2016年 | 121篇 |
2015年 | 87篇 |
2014年 | 114篇 |
2013年 | 156篇 |
2012年 | 175篇 |
2011年 | 215篇 |
2010年 | 136篇 |
2009年 | 128篇 |
2008年 | 178篇 |
2007年 | 184篇 |
2006年 | 150篇 |
2005年 | 158篇 |
2004年 | 133篇 |
2003年 | 109篇 |
2002年 | 79篇 |
2001年 | 40篇 |
2000年 | 39篇 |
1999年 | 46篇 |
1998年 | 28篇 |
1997年 | 40篇 |
1996年 | 36篇 |
1995年 | 28篇 |
1994年 | 28篇 |
1993年 | 20篇 |
1992年 | 27篇 |
1991年 | 19篇 |
1990年 | 23篇 |
1989年 | 19篇 |
1988年 | 23篇 |
1987年 | 14篇 |
1986年 | 21篇 |
1985年 | 35篇 |
1984年 | 28篇 |
1983年 | 21篇 |
1982年 | 16篇 |
1981年 | 13篇 |
1979年 | 10篇 |
1978年 | 10篇 |
1977年 | 11篇 |
1976年 | 8篇 |
1975年 | 7篇 |
1974年 | 8篇 |
1888年 | 8篇 |
排序方式: 共有3308条查询结果,搜索用时 15 毫秒
91.
92.
93.
Di Natale G Grasso G Impellizzeri G La Mendola D Micera G Mihala N Nagy Z Osz K Pappalardo G Rigó V Rizzarelli E Sanna D Sóvágó I 《Inorganic chemistry》2005,44(20):7214-7225
Copper(II) complexes of the neurotoxic peptide fragments of human and chicken prion proteins were studied by potentiometric, UV-vis, CD, and EPR spectroscopic and ESI-MS methods. The peptides included the terminally blocked native and scrambled sequences of HuPrP106-126 (HuPrPAc106-126NH2 and ScrHuPrPAc106-126NH2) and also the nona- and tetrapeptide fragments of both the human and chicken prion proteins (HuPrPAc106-114NH2, ChPrPAc119-127NH2, HuPrPAc109-112NH2, and ChPrPAc122-125NH2). The histidyl imidazole-N donor atoms were found to be the major copper(II) binding sites of all peptides; 3N and 4N complexes containing additional 2 and 3 deprotonated amide-N donors, respectively, are the major species in the physiological pH range. The complex formation processes for nona- and tetrapeptides are very similar, supporting the fact that successive deprotonation and metal ion coordination of amide functions go toward the N-termini in the form of joined six- and five-membered chelates. As a consequence, the peptide sequences investigated here, related to the neurotoxic region of the human PrP106-126 sequence, show a higher metal-binding affinity than the octarepeat fragments. In the case of the HuPrP peptide sequences, a weak pH-dependent binding of the Met109 residue was also detected in the 3N-coordinated complexes. 相似文献
94.
Both single crystals of VCl3(THF)3 as well as isotypic cocrystals of the composition MCl3(THF)3, M=Ti/V 1/3, undergo a topotactic reversible phase transition to a hitherto unknown low-temperature modification. The close relationship between this new structure and the room-temperature phase determined by Cottonet al. is discussed from the molecular and the intermolecular point of view: Both modifications are built up by conformationally very similar molecules which change their arrangement during the phase transition. Lattice energy calculations confirm that these two alternative arrangements correspond to minima of almost the same packing energy.Dedicated to Professor Dr. G. E. Herberich on the occasion of his 60th birthday. 相似文献
95.
Francesca Giuntini Daniele NistriGiacomo Chiti Lia FantettiGiulio Jori Gabrio Roncucci 《Tetrahedron letters》2003,44(3):515-517
The base-catalyzed cyclotetramerization of 3-, 4- and 4,5-dimethylaminophenylthio phthalonitriles with zinc(II)acetate afforded 1(4),8(11),15(18),22(25)- and 2(3),9(10),16(17),23(24)-tetrasubstituted and 2,3,9,10,16,17,23,24-octasubstituted Zn(II)phthalocyanines, respectively. The statistical mixed condensation of the same phthalonitriles with 1,2-dicyanobenzene gave the corresponding mono- and disubstituted derivatives. Methylation of such products afforded a series of cationic Zn(II)phthalocyanines with different pattern of substitution, with potential use as photodynamic agents in microbial infections. 相似文献
96.
The orthogonal syn and anti isomers, originated by the restricted rotation about the Ar-C(O)Bu(t) single bonds in 1,4-bis(2,2-dimethylpropanoyl)durene (2e), have been separated by preparative thin layer chromatography. In solution they reach an equilibrium where the syn-anti ratio depends upon the polarity of the solvent. This allowed us to assign the anti structure, which has a null dipole moment, to the least retained isomer. The free energy of activation (DeltaG) for the interconversion was found to be 22.5 kcal mol(-)(1), a value high enough for identifying these species as configurational isomers. When less hindered derivatives, also having two RCO (R = Pr(i), Et, Me) substituents in the positions 1,4 of the durene moiety, were examined, the syn and anti forms could be detected only at low temperature by means of NMR spectroscopy. The corresponding interconversion barriers (DeltaG = 13.4, 11.7, 10.9 kcal mol(-)(1), respectively) are, in fact, much lower than for R = Bu(t), indicating that in these cases we are dealing with conformational rather than with configurational isomers. 相似文献
97.
98.
Foltz C Stecker B Marconi G Bellemin-Laponnaz S Wadepohl H Gade LH 《Chemical communications (Cambridge, England)》2005,(40):5115-5117
Chiral C3-symmetric trisoxazolines are highly efficient stereodirecting ligands in enantioselective Cu(II) Lewis acid catalysis which is based on the concept of a stereoelectronic hemilability of the divalent copper; in direct comparison with the analogous bisoxazoline systems they are more efficient in the enantioselective alpha-amination as well as the enantioselective Mannich reaction of prochiral beta-ketoesters. 相似文献
99.
Matthias Bauer Christoph Gastl Christoph Köppl Guido Kickelbick Helmut Bertagnolli 《Monatshefte für Chemie / Chemical Monthly》2006,137(5):567-581
Summary. The molecular structure of the transition metal alkoxide Zr(OnBu)4 in toluene and its modification by addition of i-propanol, tetrahydrofurane, and the coordinating ligand pentane-1,3-dione (Hacac) were investigated by extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopy.
Zr(OnBu)4 dissolved in toluene forms dimers. It was proved that cluster size is a function of the number of added equivalents ligand.
In contrast, the addition of i-propanol or tetrahydrofurane caused no structural changes observable by EXAFS spectroscopy. A detailed discussion of the
structural models is given in terms of possible alternatives and errors within the EXAFS analysis. 相似文献
100.
Najoua Ouerfelli Daniele Mazza Mohamed Faouzi Zid 《Journal of solid state chemistry》2007,180(4):1224-1229
A new solid solution TlFe0.22Al0.78As2O7 has been synthesized by a solid-state reaction. The structure of the title compound has been determined from a single-crystal X-ray diffraction and refined to final values of the reliability factors: R(F2)=0.030 and wR(F2)=0.081 for 1343 independent reflections with I>2σ(I). It crystallizes in the triclinic space group P-1, with a=6.296(2) Å, b=6.397(2) Å, c=8.242(2) Å, α=96.74(2)°, β=103.78(2)°, γ=102.99(3)°, V=309.0(2) Å3 and Z=2. The structure can be described as a three-dimensional framework containing (Fe/Al)O6 octahedra connected through As2O7 groups. The metallic units and diarsenate groups share oxygen corners to form a three-dimensional framework with interconnected tunnels parallel to the a, b and c directions, where Tl+ cations are located. The ionic conductivity measurements are performed on pellets of the polycrystalline powder. At 683 K, The conductivity value is 5.23×10−6 S cm−1 and the ionic jump activation energy is 0.656 eV. The bond valence analysis reveals that the ionic conductivity is ensured by Tl+ along the [001] direction. 相似文献