全文获取类型
收费全文 | 3180篇 |
免费 | 139篇 |
国内免费 | 20篇 |
专业分类
化学 | 2490篇 |
晶体学 | 7篇 |
力学 | 54篇 |
数学 | 468篇 |
物理学 | 320篇 |
出版年
2023年 | 19篇 |
2022年 | 26篇 |
2021年 | 27篇 |
2020年 | 71篇 |
2019年 | 49篇 |
2018年 | 29篇 |
2017年 | 41篇 |
2016年 | 74篇 |
2015年 | 101篇 |
2014年 | 106篇 |
2013年 | 176篇 |
2012年 | 212篇 |
2011年 | 224篇 |
2010年 | 161篇 |
2009年 | 120篇 |
2008年 | 220篇 |
2007年 | 212篇 |
2006年 | 218篇 |
2005年 | 200篇 |
2004年 | 153篇 |
2003年 | 131篇 |
2002年 | 154篇 |
2001年 | 68篇 |
2000年 | 55篇 |
1999年 | 59篇 |
1998年 | 45篇 |
1997年 | 52篇 |
1996年 | 58篇 |
1995年 | 44篇 |
1994年 | 31篇 |
1993年 | 23篇 |
1992年 | 22篇 |
1991年 | 11篇 |
1990年 | 17篇 |
1989年 | 14篇 |
1988年 | 4篇 |
1987年 | 11篇 |
1986年 | 8篇 |
1985年 | 12篇 |
1983年 | 5篇 |
1982年 | 12篇 |
1981年 | 8篇 |
1979年 | 5篇 |
1978年 | 6篇 |
1977年 | 7篇 |
1975年 | 3篇 |
1974年 | 6篇 |
1973年 | 3篇 |
1972年 | 5篇 |
1971年 | 4篇 |
排序方式: 共有3339条查询结果,搜索用时 15 毫秒
41.
Pocha R Johrendt D Ni B Abd-Elmeguid MM 《Journal of the American Chemical Society》2005,127(24):8732-8740
The crystal structures of the tetrahedral cluster compounds GaNb(4)S(8) and GaTa(4)Se(8) were determined by single-crystal X-ray diffraction. They crystallize in the cubic GaMo(4)S(8) structure type (F3m), which can be derived from the spinel type by shifting the metal atoms off the centers of the chalcogen octahedra along [111]. Electrical resistivity and magnetic susceptibility measurements show that the electronic conduction originates from hopping of localized unpaired electrons (S = (1)/(2)) among widely separated Nb(4) or Ta(4) clusters, and thus these materials represent a new class of Mott insulators. Under high pressure we find that GaNb(4)S(8) undergoes a transition from the Mott insulating to a superconducting state with T(C) up to 4 K at 23 GPa, similar to GaNb(4)Se(8) and GaTa(4)Se(8). High-pressure single-crystal X-ray studies of GaTa(4)Se(8) reveal that the superconducting transition is connected with a gradual decrease of the octahedral distortion with increasing pressure. DFT band structure calculations show that weakly coupled cluster orbitals are responsible for a high density of states at the Fermi level. The correct insulating magnetic ground state for GaNb(4)S(8) with mu(eff) = 1.73 mu(B) is for the first time achieved by the LDA+U method using U = 6 eV and rhombohedral symmetry. 相似文献
42.
Alexander Chernega Dirk. L. Elend Christian A.P. Smethurst Paul M. Roberts Andrew D. Smith G. Darren Smyth 《Tetrahedron》2007,63(30):7036-7046
A three-step protocol for the asymmetric synthesis of a range of β-substituted Baylis-Hillman products has been developed. This procedure involves the diastereoselective conjugate addition of lithium (R)-N-methyl-N-(α-methylbenzyl)amide to an α,β-unsaturated ester to generate an N-protected β-amino ester in high de. Subsequent asymmetric aldol reaction via deprotonation with LDA, transmetallation with B(OMe)3 and addition of an aldehyde gives a range of syn-aldol products in moderate to high de. Purification of the syn-aldol products to homogeneity followed by tandem N-oxidation and Cope elimination gives the desired β-substituted Baylis-Hillman products in good yield and high de and ee. 相似文献
43.
In this study we have compared different ligands containing three or more hetero-atoms (N, O and/or S) with respect to their ability to form tridentate complexes with a Tc-tricarbonyl moiety. Comparison of each ligand in a competition reaction with histidine first and then with each other compound allowed to rank the ligands according to their ability of complex formation with the [99mTc(CO)3]+ precursor from diethylenetriamine (most efficient of the studied ligands) to nitrilotriacetic acid (weakest complexing properties). The results provide insight in the structural requirements for the formation of stable Tc-tricarbonyl complexes and suggest preferred combinations and arrangements of the hetero-atoms involved in the complex formation. They also give a good indication which type of ligand is most appropriate to modify biomolecules for an efficient and stable labelling with a Tc-tricarbonyl moiety. 相似文献
44.
Using ring closing metathesis (RCM) as the key operation, a rapid access to β-lactams fused to a sultam moiety of variable ring size was developed from low cost, commercially available starting materials. An efficient RCM of 4-vinyl-azetidin-2-ones to give 1-aza-bicyclo[4.2.0]oct-4-en-8-ones is also reported. 相似文献
45.
Sandra Hübner Dirk Michalik Dr. Haijun Jiao Dr. Helfried Neumann Dr. Stefan Klaus Dirk Strübing Dr. Anke Spannenberg Dr. Matthias Beller Prof. Dr. 《化学:亚洲杂志》2007,2(6):734-746
Chiral N‐dienyl lactams are crucial building blocks for the synthesis of complex organic compounds. However, their generation is rather challenging. This paper reports the novel one‐pot reaction of (S)‐methyl pyroglutamate as the a mide component with different a ldehydes and d ienophiles (AAD reaction) to give novel chiral 1‐amido‐2‐cyclohexenes. The corresponding N‐dienyl lactams generated in situ undergo subsequent Diels–Alder reactions in good yield and diastereoselectivity. The scope and limitations of the three‐component protocol were investigated. X‐ray and NMR spectroscopic analysis of the products as well as DFT calculations of the intermediates were also performed to explain the observed stereoselectivity and structural features. 相似文献
46.
Substituted 2-aminoindenes have been synthesized in almost quantitative yields by reactions of amines such as methylpiperazine, trimethylethylenediamine, 1,4-diaza-cycloheptane and N,N′-dimethylethylenediamine with 2-indanone. The 2-aminoindenes can be deprotonated and reacted with BrMn(CO)3(Py)2 to produce the respective aminoindenyl-cymantrenes in yields between 55–70%. The X-ray crystal structures of 2-(methylpiperazine)indenyl-cymantrene 5 (P1 , a = 12.667(3) Å, b = 16.630(3) Å, c = 17.382(3) Å, α = 72.70(3)°, β = 74.59(3)°, γ = 88.66(3)°, V = 3364.1(12) Å 3, Z = 8, R1(2σ(I)) = 4.02%, wR2(2σ(I)) = 10.30%) and the HClO4 adduct of 2-(trimethylethylenediamine)-indenyl-cymantrene 6 (Cc, a = 23.722(5) Å, b = 6.9080 Å, c = 13.264 Å, β = 111.77(3)°, V = 2018.6(7) Å 3, Z = 4, R1(2σ(I)) = 2.94%, wR2(2σ(I)) = 7.90%) were determined. In both complexes the indenyl-carbon bonded to nitrogen displays significantly longer bonds to manganese [223.5(3)–225.8(3) pm] than the other four carbon atoms [213.3(3)–219.1(3) pm]. The short indenyl-nitrogen bonds of 136.2(4) and 137.8(4) pm are indicative of a substantial multiple bond character. The complexation of Zn2+ by the nitrogen atoms of 6 results in significant shifts of the CO stretching frequencies. 相似文献
47.
Bouchoux G Defaye D McMahon T Likholyot A Mó O Yáñez M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(13):2900-2909
The various protonated forms of phenol (1), catechol (2), resorcinol (3), and hydroquinone (4) were explored by ab initio quantum chemical calculations at the MP2/6-31G(d) and B3LYP/6-31G(d) levels. Proton affinities (PA) of 1-4 were calculated by the combined G2(MP2,SVP) method, and their gas-phase basicities were estimated after calculation of the change in entropy on protonation. These theoretical data were compared with the corresponding experimental values determined in a high-pressure mass spectrometer. This comparison confirmed that phenols are essentially carbon bases and that protonation generally occurs in a position para to the hydroxyl group. Resorcinol is the most effective base (PA = 856 kJ mol-1) due to the participation of both oxygen atoms in the stabilization of the protonated form. Since protonation is accompanied by a freezing of the two internal rotations, a significant decrease in entropy is observed. The basicity of catechol (PA = 823 kJ mol-1) is due to the existence of an intramolecular hydrogen bond, which is strengthened upon protonation. The lower basicity of hydroquinone (PA = 808 kJ mol-1) is a consequence of the fact that protonation necessarily occurs in a position ortho to the hydroxyl group. When the previously published data are reconsidered and a corrected protonation entropy is used, a proton affinity value of 820 kJ mol-1 is obtained for phenol. 相似文献
48.
Dirk Reinen 《Monatshefte für Chemie / Chemical Monthly》1965,96(2):730-739
Zusammenfassung Für das oktaedrisch koordinierte Co2+-Ion werden ein Termschema und Energiegleichungen für zwei charakteristischeDq/B-Werte angegeben und mit deren Hilfe aus den Spektren der Co2+-haltigen Ilmenite MgTiO3 und CdTiO3
1 die Zuordnung des intensitätsstärksten Quartett-Dublett-Überganges (
a
4
T1g
b
2
T1g) auf eindeutige Weise ermöglicht.Weiter wird das spektrale Verhalten der Mg1–x
CO
x
O-Mischkristalle (0,05x1,0) in Abhängigkeit von der Co2+-Konzentration untersucht. Die Änderungen der Bandenintensitäten und desRacah-ParametersB werden wie in der analogen MgO-NiO-Mischkristallreihe2 auf Spin-Spin-Wechselwirkungen der über Sauerstoff benachbarten Co2+-Ionen zurückgeführt.
Mit 3 Abbildungen 相似文献
The energy levels of the octahedrally coordinated Co2+-ion and energy equations for two characteristicB-values are given, which allow to identify the quartet-doublet-transition ( a 4 T1g b 2 T1g) of the highest intensity from the spectrum of the Co2+ containing ilmenites MgTiO3 and CdTiO3 1.In addition, the variation of the spectra of the Mg1–x Co x O-mixed crystals (0.05x1.0) with the Co2+ concentration is investigated. The changes in the intensities of the absorption bands and of theRacah-parameterB are (as for the corresponding MgO-NiO-mixed crystals2 interpreted as being caused by spin-spin-interactions between Co2+ ions bonded to the same oxygen.
Mit 3 Abbildungen 相似文献
49.
Austen K FlattShawn M Dirk Jay C HendersonDwanleen E Shen Jie SuMark A Reed James M Tour 《Tetrahedron》2003,59(43):8555-8570
Several new classes of oligomers have been synthesized with functionalities designed to aid in the understanding of molecular device behavior, specifically when molecules are interfaced between proximal electronic probes. The compounds synthesized are series of azobenzenes, bipyridines and oligo(phenylene vinylene)s that bear acetyl-protected thiols for ultimate attachment to metallic surfaces. Some initial electrochemical and solid-state test results are also reported. 相似文献
50.
Two-dimensional (2-D) Blue Native/SDS gel electrophoresis combines a first-dimensional separation of monomeric and multimeric proteins in their native state with a second denaturing dimension. These high-resolution 2-D gels aim at identifying multiprotein complexes with respect to their subunit composition. We applied this method for the first time to analyze two human platelet subproteomes: the cytosolic and the microsomal membrane protein fraction. Solubilization of platelet membrane proteins was achieved with the nondenaturing detergent n-dodecyl-beta-D-maltoside. To validate native solubilization conditions, we demonstrated the correct assembly of the Na,K-ATPase, a functional multimeric transmembrane protein, when expressed in Xenopus oocytes. We identified 63 platelet proteins after in-gel tryptic digestion of 58 selected protein spots and liquid chromatography-coupled tandem mass spectrometry. Nine proteins were detected for the first time in platelets by a proteomic approach. We also show that this technology efficiently resolves several known membrane and cytosolic multiprotein complexes. Blue Native/SDS gel electrophoresis is thus a valuable procedure to analyze specific platelet subproteomes, like the membrane(-bound) protein fraction, by mass spectrometry and immunoblotting and could be relevant for the study of protein-protein interactions generated following platelet activation. 相似文献