全文获取类型
收费全文 | 66821篇 |
免费 | 5364篇 |
国内免费 | 8430篇 |
专业分类
化学 | 50022篇 |
晶体学 | 1424篇 |
力学 | 1643篇 |
综合类 | 459篇 |
数学 | 11480篇 |
物理学 | 15587篇 |
出版年
2024年 | 84篇 |
2023年 | 508篇 |
2022年 | 1082篇 |
2021年 | 1068篇 |
2020年 | 1501篇 |
2019年 | 1474篇 |
2018年 | 1322篇 |
2017年 | 1851篇 |
2016年 | 1994篇 |
2015年 | 1866篇 |
2014年 | 2454篇 |
2013年 | 5338篇 |
2012年 | 4507篇 |
2011年 | 3709篇 |
2010年 | 3098篇 |
2009年 | 4108篇 |
2008年 | 4316篇 |
2007年 | 4449篇 |
2006年 | 4059篇 |
2005年 | 3579篇 |
2004年 | 3365篇 |
2003年 | 2795篇 |
2002年 | 3786篇 |
2001年 | 2397篇 |
2000年 | 2171篇 |
1999年 | 2075篇 |
1998年 | 1746篇 |
1997年 | 1401篇 |
1996年 | 1078篇 |
1995年 | 1054篇 |
1994年 | 966篇 |
1993年 | 744篇 |
1992年 | 786篇 |
1991年 | 491篇 |
1990年 | 429篇 |
1989年 | 423篇 |
1988年 | 314篇 |
1987年 | 245篇 |
1986年 | 241篇 |
1985年 | 212篇 |
1984年 | 210篇 |
1983年 | 123篇 |
1982年 | 181篇 |
1981年 | 150篇 |
1980年 | 178篇 |
1979年 | 157篇 |
1978年 | 143篇 |
1977年 | 102篇 |
1976年 | 87篇 |
1973年 | 52篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Alireza Haghiri Ilkhechi 《Journal of organometallic chemistry》2005,690(8):1971-1977
Reaction of the ferrocenyl(dimethylamino)boranes FcB(Me)NMe2, Fc2BNMe2, and 1,1′-fc[B(Me)NMe2]2 with 1:1 mixtures of pyrazole and potassium pyrazolide in refluxing THF gave the potassium salts of the ferrocene-based bis(pyrazol-1-yl)borate ligands FcB(Me)pz2K, Fc2Bpz2K, and 1,1′-fc[B(Me)pz2]2K2 in good yield (Fc: ferrocenyl, fc: ferrocenylene, pz: pyrazolyl). In the solid state, FcB(Me)pz2K and Fc2Bpz2K form centrosymmetric dimers with short K?Cp contacts suggesting an η5 coordination mode of the potassium ion. The crystal lattice of the ditopic ligand 1,1′-fc[B(Me)pz2]2K2 consists of coordination polymer strands featuring essentially the same structural motif that has been observed for the monotopic derivatives. All three scorpionate ligands are thus promising building blocks for the preparation of ferrocene-containing multiple-decker sandwich complexes. 相似文献
992.
Khuloud Takrouri Israel Goldberg Morris Srebnik 《Journal of organometallic chemistry》2005,690(18):4150-4158
The first examples of diborane (4) compounds derived from amine cyanoboranes are described. A series of monobromo derivatives of amine cyanoboranes (A:BHBrCN), and dibromo derivatives (A:BBr2CN), 1-7, were prepared. Lithiation of the monobromo derivative of trimethylamine cyanoborane, using n-BuLi, did not produce the C-lithiated intermediate Li+ [CH2NMe2BHBrCN]−, but instead the B-lithiated intermediate Li+ [Me3NBHCN]−, was obtained. This intermediate, when allowed to react for 16 h, coupled with the un-lithiated trimethylamine monobromocyanoborane (Me3NBHBrCN) and resulted in diborane (4) derivative formation as the 2LiBr complex. The same result was obtained when one equiv of the trimethylamine monobromocyanoborane was added to the reaction mixture 1 h after lithiation. Following the same procedure, novel diborane (4) derivatives of amine cyanoboranes were successfully obtained, 8-11, as their 2LiBr complexes from the monobromo derivatives of the corresponding amine cyanoboranes. Molecular structures of the trimethylamine dibromocyanoborane, 6, and the triethylamine dibromocyanoborane, 7, were determined using X-ray crystallography. 相似文献
993.
Henk Buck 《International journal of quantum chemistry》2005,101(1):73-83
With semi‐empirical MO and ab initio calculations at different levels, we investigated the π conjugation of alternating X? Y bonds with X? Y for N/B and N/C combinations in an open and cyclic arrangement. Although the intrinsic symmetry is lost for the acyclic even‐membered compounds, the alternation is still reflected in its geometry and electron‐density transfer. For the cyclic π compounds, we focused our attention on borazine N3B3H6 (D3h symmetry), which is isoelectronic with benzene (D6h symmetry). Specific attention is given to the electrophilic behavior of borazine with respect to CH and SiMe. The dynamics based on the results of FT‐ICR mass spectrometry was studied in more detail. In addition, the results of the cyclic systems with 4n and 4n+2 π electrons concerning their geometries are compared with the corresponding carbon compounds. Attention is also given to the dication of borazine, because of the corresponding triplet ground state of the benzene dication. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 相似文献
994.
The apparent molar volumes V of KCl, BaCl2, K2SO4, LaCl3, Co(en)3Cl3 [Tris(ethylenediamine)cobalt(III) chloride], K3Co(CN)6, K3Fe(CN)6, K4Fe(CN)6, and Ba3[Co(CN)6]2 have been determined at 25°C in both light and heavy water. The V values in D2O are systematically lower and increase more rapidly with salt concentration than the V in H2O. The volume of transfer from H2O to D2O as well as the partial molar volume at infinite dilution in both solvents have also been calculated. These results together with literature values for other electrolytes were used to estimate both of these quantities for D2O solutions of individual ions. The predictions of ion hydration models and ion-ion interactions are compared with experimental observations. 相似文献
995.
A hydrazide-derived bis(meta-phenylene)-32-crown-10 host showed a dimeric structure via quadruple N-H?O hydrogen bonds, but a polymeric structure via two N-H?O hydrogen bonds and two C-H?O hydrogen bonds at each knot in the presence of paraquat in the solid state, which led to a novel poly(taco complex) and ordering arrangement of the guest molecules indirectly. 相似文献
996.
A number of aldehydes and ketones were prepared by oxidation of alcohol by N,N‐dichloro‐4‐methylbenzenesulfonamide under mild and neutral conditions in good to high yield in dichloromethane at room temperature. 相似文献
997.
998.
Vukadin M. Leovac Vladimir Divjaković Ana F. Petrović Nikolai V. Gerbeleu 《Monatshefte für Chemie / Chemical Monthly》1989,120(3):181-186
Summary The reaction of warmDMF solutions of Cu(II) perchlorate, 5-nitrosalicylaldehyde S-methylisothiosemicarbazone (H2
L) and piperidine (Pip) yielded the paramagnetic (eff=2.19 B.M.) Cu(L)Pip complex, whereL is the dianionic form of H2
L, formed as a result of deprotonation of the phenolic hydroxyl and NH2 group. Crystal data of the complex are: monoclinic P21,a=11.902(4),b=6.765(3),c=11.343(4)Å; =112.92°,V=841.20(8)Å3,M=400.9,Z=2,d
0=1.60 gcm–3,d
c
=1.58 gcm–3,F(000)=424. The structure was refined to a residualR=0.047. The copper(II) ion is coordinated in a square-planar arrangement by the piperidine nitrogen and the NNO set of donor atoms ofL.
Komplexe der Übergangsmetalle mit Thiosemicarbazid-Liganden, IV. Synthese und molekulare Struktur von 5-Nitrosalicylaldehyd-S-methylisothiosemicarbazonato-piperidin-kupfer(II)
Zusammenfassung Durch die Reaktion in warmenDMF-Lösungen von Cu(II) Perchlorat, 5-Nitrosalicylaldehyd-S-methylisothiosemicarbazon (H2 L) und Piperidin (Pip) wurde ein paramagnetischer Komplex (eff=2.19 B.M.) Cu(L)Pip erhalten, wobeiL nach doppelter Deprotonierung (NH2- und Phenolhydroxyl-Gruppe) als dianionische Form von H2 L vorliegt. Die kristallographischen Daten des Komplexes sind: monokline Raumgruppe P21,a=11.902(4),b=6.765(3),c=11.343(4)Å, =112.92°;V=841.20Å3,M=400.9,Z=2,d 0=1.60 gcm–3,d c =1.58 gcm–3,F(000)=424. Die struktur wurde bis zu einemR-Wert von 0.047 verfeinert. Die Koordination des Kupfers wird quadratisch-planar über den Piperidin-Stickstoff und die NNO-Donorhülle der Chelatliganden gebildet.相似文献
999.
The determination of nanometer thick layers of poly(methyl methacrylate) coated on to the surface of poly(ethylene terephthalate) film has been investigated by high resolution pyrolysis gas chromatography without sample pretreatment or modification of the instrumentation used. A good linear relationship was observed between the quantity of the characteristic pyrolysate and the thickness of the poly(methyl methacrylate) layer; the detection limit was sufficient to enable the quantitation of poly(methyl methacrylate)-to-poly(ethylene terephthalate) film thickness ratios of 1:20000 in composite materials. 相似文献
1000.
Pseudoelement Compounds. VIII. [1]. On the Coordination Behaviour of Potential Ambidentate Aryl-cyanamidosulfonate Ligands Aryl-cyanamidosulfonates form with metal(II) ions (Co, Ni, Cu, Zn, Cd, Hg, Pd, Pt) complexes of the types MX2, [MX2L2], [MX2L4] and [MX4]2?. The ionic, potentially ambidentate ligand [RSO2NCN]? is monodentately coordinated via the terminal nitrogen atom and bidentately in a bridging function through the nitrile group end-on (equatorial) and one of the two sulfonyl oxygen atoms (axial). The new compounds are characterized by means of ir and uv-vis spectroscopy. In the case of [Ni(NCNSO2C6H4Me-4)2(py)4] the molecular and crystal structure was determined. 相似文献