全文获取类型
收费全文 | 3860篇 |
免费 | 57篇 |
国内免费 | 7篇 |
专业分类
化学 | 2718篇 |
晶体学 | 8篇 |
力学 | 39篇 |
数学 | 561篇 |
物理学 | 598篇 |
出版年
2021年 | 24篇 |
2016年 | 48篇 |
2015年 | 45篇 |
2014年 | 42篇 |
2013年 | 98篇 |
2012年 | 79篇 |
2011年 | 95篇 |
2010年 | 65篇 |
2009年 | 86篇 |
2008年 | 110篇 |
2007年 | 122篇 |
2006年 | 126篇 |
2005年 | 133篇 |
2004年 | 112篇 |
2003年 | 102篇 |
2002年 | 131篇 |
2001年 | 66篇 |
2000年 | 78篇 |
1999年 | 70篇 |
1998年 | 66篇 |
1997年 | 58篇 |
1996年 | 55篇 |
1995年 | 76篇 |
1994年 | 61篇 |
1993年 | 65篇 |
1992年 | 57篇 |
1991年 | 51篇 |
1990年 | 62篇 |
1989年 | 85篇 |
1988年 | 55篇 |
1987年 | 68篇 |
1986年 | 68篇 |
1985年 | 92篇 |
1984年 | 71篇 |
1983年 | 59篇 |
1982年 | 64篇 |
1981年 | 60篇 |
1980年 | 68篇 |
1979年 | 76篇 |
1978年 | 82篇 |
1977年 | 87篇 |
1976年 | 65篇 |
1975年 | 55篇 |
1974年 | 51篇 |
1973年 | 48篇 |
1972年 | 35篇 |
1971年 | 37篇 |
1970年 | 23篇 |
1968年 | 24篇 |
1960年 | 23篇 |
排序方式: 共有3924条查询结果,搜索用时 734 毫秒
991.
Tetraphenylcyclobutadienepalladium dichloride reacts with 1,2,3,4-tetraphenyl-1,4-dilithiumbutadiene or with sodium with abstraction of halide to give the sandwich compound bis(tetraphenylcyclobutadiene)palladium(0). The structure of the latter is elucidated by spectroscopic methods and its reactions with Br2, H2, K and HNO3 are described. 相似文献
992.
General valence force fields for SF(3)CN and FS(O)CN are derived from vibrational data taken from the literature and from theoretical calculations. Gas phase electron diffraction studies on both molecules yield the following geometric parameters (r(a) distances and angles with 3sigma uncertainties). SF(3)CN: r(S-F(e)) = 155.2(4) r(S-F(a)) = 165.7(3), r(S-C) = 173.6(8), r(C&tbd1;N) = 115.9(4) pm; angle(F(a)SF(e)) = 86.9(3), angle(F(a)SC) = 86.0(4) angle(F(e)SC) = 98.7(8), angle(F(a)SF(a)) = 169.0(6), angle(SCN) = 171(4) degrees. FS(O)CN: r(S-F) = 159.8(3), r(S=O) = 143.2(2), r(S-C) = 178.3(3), r(C&tbd1;N) = 115.0(3) pm; angle(FSO) = 104.9(4), angle(FSC) = 93.9(4), angle(CSO) = 105.3(5), angle(SCN) = 176(4) degrees. These experimental results are compared to ab initio values (HF/3-21G, HF/6-31G, and MP2/6-31G), and the bonding properties in these sulfur (IV) cyanides are discussed. 相似文献
993.
Photolysis of Heterocyclic Azidinium Salts The photochemistry of some azidinium salts was investigated. Their photolusis led to a large variety of products which were isolated and identified. Reaction mechanisms involving singlet and triplet nitrene intermediates are discussed to explain the product formation. 相似文献
994.
Karl H. A. Ostoja Starzewski Paul S. Pregosin Heinz Rüegger 《Helvetica chimica acta》1982,65(3):785-797
The synthesis and solution structures of new four- and five-coordinate phosphine and arsine complexes of Pt and Pd containing the trichlorostannate ligand are described. Complexes containing two and three SnCl?3-ligands have been identified from their 31P-, 119Sn- and 195Pt-NMR. spectra. The complexes trans-[M (SnCl3)2L2] (M = Pt, L-PEt3, PPr3, AsEt3; M = Pd, L = AsEt3) show unexpectedly large 2J(119Sn, 117Sn)-values (34,674–37,164 Hz) with the trans-orientation of these spins playing an important role. The heteronuclear coupling constant 2J(119Sn, 31P) in the five-coordinate cationic complexes [Pt(SnCl3)(P(o-AsPh2? C6H4)3)]+ and [Pt(SnCl3)(As(o-PPh2? C6H4)3)]+ also shows a geometric dependence. New five-coordinate anionic complexes of type [M (SnCl3)3L2]? (M = Pd, Pt; L = PEt3, AsEt3) may be prepared via addition of three mol-equiv. of SnCl2 and one mol-equiv. of (PPN)Cl to [MCl2L2] in acetone. 相似文献
995.
Crystal Structures and Phase Transformations of Cesium Trihalogenogermanates CsGeX3(X = Cl, Br, I) The compounds CsGeX3 (X ? Cl, Br, I) have been obtained by reactions of Ge(OH)2 with CsX in aquaeous HX solutions. The thermal behavior has been studied by X-ray diffraction. Raman spectroscopy, and DTA/DSC. The compounds are dimorph. The low temperature modifications L-CsGeX3 show a rhomboedric deformed perovskite type structure. The high temperature phases H-CsGeX3 form the cubic perovskite type structure. The reversible phase transitions are interpreted as a result of position changes of the Ge atoms in the H-forms (Order-Disorder transitions). The transition temperatures increase in the sequence CsGeCl3 (155°C), CsGeBr3 (238°C), CsGeI3 (277°C). 相似文献
996.
Waldemar Adam Hector Rebollo Heinz Dürr Karl-Heinz Pauly Karl Peters Eva-Maria Peters Hans-Georg von Schnering 《Tetrahedron letters》1982,23(9):923-926
4-Methyl-1,2,4-triazoline-3,5-dione (MTAD) gives with 7-spiro-fluorenyl-1,3,5-cycloheptatriene () initially he norcaradine-type urazole () which at ambient temperatures rearranges into (), while 102 gives the tropilidene-type endoperoxide (). 相似文献
997.
Binuclear redox sites consisting of a Zr oxo-bridged to a Cu(I) center have been obtained on the pore surface of MCM-41 silicate sieve by a stepwise grafting procedure, along with isolated metal centers. The bimetallic site features a Zr(IV)-O-Cu(I) to Zr(III)-O-Cu(II) metal-to-metal charge-transfer (MMCT) absorption extending from the UV region to about 500 nm. The Zr-O-Cu linkage is revealed by a Cu(I)-O infrared stretch mode at 643 cm-1. Irradiation of the MMCT chromophore of ZrCu(I)-MCM-41 loaded with 1 atm of CO2 gas at room temperature resulted in growth of CO (2150 cm-1) and H2O (1600 cm-1). Photolysis experiments using 13CO2 and C18O2 demonstrate that carbon monoxide and the oxygen atom of the water product originate from CO2. This indicates splitting of the CO2 by the excited MMCT moiety to CO and a surface OH radical, followed by trapping of the products at Cu(I) centers (OH is reduced to H2O). This is the first observation of CO2 photoreduction at a binuclear MMCT site at the gas-solid interface. 相似文献
998.
Anthony Linden Enas M. Awad Nehal M. Elwan Hamdi M. Hassaneen Heinz Heimgartner 《Acta Crystallographica. Section C, Structural Chemistry》2002,58(2):o122-o124
Crystals of the title compound, C23H17ClN4O2·2.5H2O, contain channels filled with highly disordered water molecules. The best structure refinement was obtained by removing the solvent contribution from the intensity data and refining against a solvent‐free model. The central six‐membered ring of the quinolizine molecule has a slightly distorted screw‐boat conformation. 相似文献
999.
Heinz Löster 《Monatshefte für Chemie / Chemical Monthly》2005,136(8):1443-1466
Summary. The metabolism in the heart prefers long-chain fatty acids to other substrates. L-Carnitine, a co-factor of coenzyme A, plays an essential role in the transport of long-chain fatty acids through the inner mitochondrial membrane. Without carnitine, metabolisation of long-chain fatty acids in the mitochondria is not possible. In addition, acyl groups from acyl-CoA compounds can be transferred to L-carnitine, thus influencing the enzymatic activities of important mitochondrial enzymes.The isolated heart model developed by Langendorff was used to investigate the effects of L-carnitine on the heart. During aerobic perfusion, the hemodynamic parameters of isolated hearts reacted in a very sensitive way to alterations in the external conditions (temperature, preload, composition of the perfusion solution). During postischemic perfusion, recovery of the hearts was also influenced by the composition of the perfusion. The hemodynamic parameters of the reperfused hearts increased markedly if there was a sufficiently high supply of long-chain fatty acids and/or glucose. The insufficient recovery of hearts perfused without glucose and at low fatty acid concentrations could be improved by adding L-carnitine. Determination of carnitine levels in heart tissue found that the heart loses about 30% of its carnitine content during ischemia, and that exogenous carnitine is taken up by the heart during reperfusion. There it effects the restoration of sufficient concentrations of creatine phosphate and ATP, a fact that was confirmed by 31P NMR spectroscopy. NMR spectroscopy also established that L-carnitine lessens the harmful effects of ischemia-induced metabolic acidosis.The favourable influence of L-carnitine on the heart in the reperfusion period could be due to a reduction in oxygen radicals (lowering of MDA concentrations during reperfusion, raising of GPx and SOD activities).The findings of these experiments on isolated hearts as well as the favourable results of two placebo-controlled and double-blind clinical studies (investigating the effects of carnitine in cardiomyopathy patients and the effects of L-carnitine in hemodialysis patients) demonstrate that L-carnitine produces positive therapeutic effects, particularly in heart and circulatory diseases. 相似文献
1000.
Ren Traber Max Kuhn Artur Rüegger Heinz Lichti Hans-Rudolf Loosli Albert von Wartburg 《Helvetica chimica acta》1977,60(4):1247-1255
The Structure of Cyclosporin C The structure of cyclosporin C ( 2 ), a minor antifungal metabolite from Trichoderma polysporum (Link ex Pers.) RIFAI has been elucidated. Hydrolytic cleavage and spectroscopic evidence show that cyclosporin C is a neutral oligopeptide of 11 amino acids linked together in a 33-membered ring. Cyclosporin C ( 2 ) differs from the main metabolite cyclosporin A ( 1 ) [2] [4] only by containing L-threonine in the place of L-α-aminobutyric acid as has been shown by the conversion of 2 into 1 . 13C-NMR. spectra and study of molecular models suggest that cyclosporin C ( 2 ) has the same molecular conformation as 1 , which is best described as a twisted β-pleated sheet held together in a conformationally stable form by intramolecular hydrogen bonding. 相似文献