全文获取类型
收费全文 | 5047篇 |
免费 | 296篇 |
国内免费 | 22篇 |
专业分类
化学 | 3886篇 |
晶体学 | 13篇 |
力学 | 111篇 |
数学 | 669篇 |
物理学 | 686篇 |
出版年
2023年 | 55篇 |
2022年 | 57篇 |
2021年 | 89篇 |
2020年 | 110篇 |
2019年 | 112篇 |
2018年 | 68篇 |
2017年 | 62篇 |
2016年 | 225篇 |
2015年 | 196篇 |
2014年 | 191篇 |
2013年 | 288篇 |
2012年 | 369篇 |
2011年 | 390篇 |
2010年 | 244篇 |
2009年 | 217篇 |
2008年 | 314篇 |
2007年 | 268篇 |
2006年 | 280篇 |
2005年 | 281篇 |
2004年 | 246篇 |
2003年 | 175篇 |
2002年 | 189篇 |
2001年 | 95篇 |
2000年 | 83篇 |
1999年 | 77篇 |
1998年 | 61篇 |
1997年 | 87篇 |
1996年 | 69篇 |
1995年 | 66篇 |
1994年 | 45篇 |
1993年 | 50篇 |
1992年 | 32篇 |
1991年 | 24篇 |
1990年 | 20篇 |
1989年 | 21篇 |
1988年 | 14篇 |
1987年 | 15篇 |
1986年 | 13篇 |
1985年 | 10篇 |
1984年 | 10篇 |
1983年 | 8篇 |
1982年 | 10篇 |
1981年 | 11篇 |
1979年 | 7篇 |
1978年 | 12篇 |
1977年 | 8篇 |
1976年 | 7篇 |
1975年 | 8篇 |
1974年 | 14篇 |
1973年 | 14篇 |
排序方式: 共有5365条查询结果,搜索用时 0 毫秒
101.
Herbert Schumann Sebastian Dechert Frank Girgsdies Bernd Heymer Markus Hummert Ji‐Young Hyeon Jens Kaufmann Stefan Schutte Sonja Wernik Birgit C. Wassermann 《无机化学与普通化学杂志》2006,632(2):251-263
Synthesis and Characterization of New Intramolecularly Nitrogen‐stabilized Organoaluminium‐ and Organogallium Alkoxides The intramolecularly nitrogen stabilized organoaluminium alkoxides [Me2Al{μ‐O(CH2)3NMe2}]2 ( 1a ), Me2AlOC6H2(CH2NMe2)3‐2,4,6 ( 2a ), [(S)‐Me2Al{μ‐OCH2CH(i‐Pr)NH‐i‐Pr}]2 ( 3a ) and [(S)‐Me2Al{μ‐OCH2CH(i‐Pr)NHCH2Ph}]2 ( 4 ) are formed by reacting equimolar amounts of AlMe3 and Me2N(CH2)3OH, C6H2[(CH2NMe2)3‐2,4,6]OH, (S)‐i‐PrNHCH(i‐Pr)CH2OH, or (S)‐PhCH2NHCH(i‐Pr)CH2OH, respectively. An excess of AlMe3 reacts with Me2N(CH2)2OH, Me2N(CH2)3OH, C6H2[(CH2NMe2)3‐2,4,6]OH, and (S)‐i‐PrNHCH(i‐Pr)CH2OH producing the “pick‐a‐back” complexes [Me2AlO(CH2)2NMe2](AlMe3) ( 5 ), [Me2AlO(CH2)3NMe2](AlMe3) ( 1b ), [Me2AlOC6H2(CH2NMe2)3‐2,4,6](AlMe3)2 ( 2b ), and [(S)‐Me2AlOCH2CH(i‐Pr)NH‐i‐Pr](AlMe3) ( 3b ), respectively. The mixed alkyl‐ or alkenylchloroaluminium alkoxides [Me(Cl)Al{μ‐O(CH2)2NMe2}]2 ( 6 ) and [{CH2=C(CH3)}(Cl)Al{μ‐O(CH2)2NMe2}]2 ( 8 ) are to obtain from Me2AlCl and Me2N(CH2)2OH and from [Cl2Al{μ‐O(CH2)2NMe2}]2 ( 7 ) and CH2=C(CH3)MgBr, respectively. The analogous dimethylgallium alkoxides [Me2Ga{μ‐O(CH2)3NMe2}]2 ( 9 ), [(S)‐Me2Ga{μ‐OCH2CH(i‐Pr)NH‐i‐Pr}]n ( 10 ), [(S)‐Me2Ga{μ‐OCH2CH(i‐Pr)NHCH2Ph}]n ( 11 ), [(S)‐Me2Ga{μ‐OCH2CH(i‐Pr)N(Me)CH2Ph}]n ( 12 ) and [(S)‐Me2Ga{μ‐OCH2(C4H7NHCH2Ph)}]n ( 13 ) result from the equimolar reactions of GaMe3 with the corresponding alcohols. The new compounds were characterized by elemental analyses, 1H‐, 13C‐ and 27Al‐NMR spectroscopy, and mass spectrometry. Additionally, the structures of 1a , 1b , 2a , 2b , 3a , 5 , 6 and 8 were determined by single crystal X‐ray diffraction. 相似文献
102.
Köhler K Förster G Hauser A Dobner B Heiser UF Ziethe F Richter W Steiniger F Drechsler M Stettin H Blume A 《Journal of the American Chemical Society》2004,126(51):16804-16813
The temperature-dependent self-assembly of the single-chain bolaamphiphile dotriacontan-1,1'-diyl-bis[2-(trimethylammonio)ethyl phosphate] (PC-C32-PC) was investigated by transmission electron microscopy (TEM), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FT-IR), X-ray scattering, rheological measurements, and dynamic light scattering (DLS). At room temperature this compound, in which two phosphocholine headgroups are connected by a C(32) alkyl chain, proved to be capable of gelling water very efficiently by forming a dense network of nanofibers (Kohler et al. Angew. Chem., Int. Ed. 2004, 43, 245). A specific feature of this self-assembly process is that it is not driven by hydrogen bonds but solely by hydrophobic interactions of the long alkyl chains. The nanofibers have a thickness of roughly the molecular length and show a helical superstructure. A model for the molecular structure of the fibrils which considers the extreme constitution of the bolaamphiphile is proposed. Upon heating the suspensions three different phase transitions can be detected. Above 49 degrees C, the temperature of the main transition where the alkyl chains become "fluid", a clear low-viscosity solution is obtained due to a breakdown of the fibrils into smaller aggregates. Through mechanical stress the gel structure can be destroyed as well, indicating a low stability of these fibers. The gel formation is reversible, but as a drastic rearrangement of the molecules takes place, metastable states occur. 相似文献
103.
Nicole Frankenberg Christine Hager-Braun Ute Feiler Markus Fuhrmann Hans Rogl Nikolaus Schneebauer Nathan Nelson Günter Hauska 《Photochemistry and photobiology》1996,64(1):14-19
Membranes of Chlorobium tepidum contain about 35, 45 and2–10 molecules of menaquinone-7, chlorobium quinone (1′-oxo-menaquinone-7) and of the polar menaquinone (probably 1′-OH-menaquinone-7) per reaction center, respectively. None of these quinones was retained during the isolation of P840-reaction centers beyond the detection limit of about 0.2 quinones per reaction center, neither in the core complex nor in functionally intact reaction center preparations. The latter is shown to catalyze the formation of an electrochemical proton gradient in the presence of ascorbate and phenazinium methosulfate, when it is incorporated into lipid vesicles. 相似文献
104.
The organoaluminium and -gallium hydroxylamides (Me2GaONMe2)2, (tBu2AlONMe2)2, (tBu2GaONMe2)2 and (Me2AlONiPr2)2 have been prepared by the reaction of the hydroxylamines Me2NOH and iPr2NOH with the trialkylmetal compounds trimethylgallium, tri-tbutylaluminium and tri-tbutylgallium, respectively. All compounds have been characterised by NMR spectroscopy (1H, 13C, 15N, 17O and 27Al), by mass spectrometry and elemental analyses. The crystal structures of the four compounds have been determined, showing that they all form dimers but with different modes of aggregation: (Me2GaONMe2)2 has a Ga2O2N2 six-membered ring, (tBu2AlONMe2)2 and (Me2AlONiPr2)2 have Al2O2 four-membered rings, (tBu2GaONMe2)2 forms a Ga2O2N five-membered ring. 相似文献
105.
Makarycheva-Mikhailova AV Kukushkin VY Nazarov AA Garnovskii DA Pombeiro AJ Haukka M Keppler BK Galanski M 《Inorganic chemistry》2003,42(8):2805-2813
The reaction between the platinum(IV) complex trans-[PtCl(4)(EtCN)(2)] and the amino alcohols NH(2)CH(2)CH(2)OH, NH(2)CH(2)CH(Me)OH-(R)-(-), NH(2)CH(Ph)CH(2)OH-(R)-(-), NH(2)CH(Et)CH(2)OH-(R)-(-), NH(2)CH(Et)CH(2)OH-(S)-(+), and NH(2)CH(Pr(n)())CH(2)OH proceeds rapidly at room temperature in CH(2)Cl(2) to furnish the amidine complexes [PtCl(4)(HN=C(Et)NH(arcraise;)OH)(2)] (1-6) in good yield (70-80%). The related reaction between the platinum(II) complex trans-[PtCl(2)(EtCN)(2)] and monoethanolamine in a molar ratio of 1:2 in CH(2)Cl(2) results in the addition of 4 equiv of NH(2)CH(2)CH(2)OH per mole of complex to give [Pt(HN=C(Et)NHCH(2)CH(2)OH)(2)(NH(2)CH(2)CH(2)OH)(2)](2+) (7). Formulation of 1-6 is based upon satisfactory C, H, N elemental analyses, electrospray mass spectrometry, IR spectroscopy, and (1)H, (13)C((1)H), (15)N, and (195)Pt NMR spectroscopies, while the structures of trans-[PtCl(4)((Z)-NH=C(Et)NHCH(2)CH(2)OH)(2)] (1), trans-[PtCl(4)((Z)-NH=C(Et)NHCH(2)CH(Me)OH-(R)-(-))(2)] (2), and trans-[PtCl(4)((Z)-NH=C(Et)NHCH(Et)CH(2)OH-(R)-(-))(2)] (4) were determined by X-ray single-crystal diffraction. The Z-amidine configuration of the ligands is preserved in CDCl(3) solutions as confirmed by gradient-enhanced (15)N,(1)H-HMQC spectroscopy and NOE experiments. The amidines, formed upon Pt(IV)-mediated nitrile-amino alcohol coupling, were liberated from their platinum(IV) complexes 1, 3, and 4 by reaction with Ph(2)PCH(2)CH(2)PPh(2) (dppe) giving free NH=C(Et)NHCHRCH(2)OH (R = H 8, Et 9, Ph 10), with the substituents R of different types, and dppe oxides; the P-containing species were identified by (31)P((1)H) NMR spectroscopy. NOESY spectroscopy indicates that the liberated amidines retained the same configuration relative to the C=N double bond, i.e., syn-(H,Et)-NH=C(Et)NHCHRCH(2)OH. The liberated hydroxo-functionalized amidines 8-10 were converted into oxazolines (11-13) in the presence of a catalytic amount of ZnCl(2). A similar catalytic effect has also been reached using anhydrous MSO(4) (M = Cu, Co, Cd), CdCl(2), and AlCl(3). 相似文献
106.
Hölscher M Leitner W Holthausen MC Frenking G 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(16):4700-4708
Quantum chemical calculations by using density functional theory at the B3LYP level have been carried out to elucidate the reaction course for the addition of ethylene to [OsO2(CH2)2] (1). The calculations predict that the kinetically most favorable reaction proceeds with an activation barrier of 8.1 kcal mol(-1) via [3+2] addition across the O=Os=CH2 moiety. This reaction is -42.4 kcal mol(-1) exothermic. Alternatively, the [3+2] addition to the H2C=Os=CH2 fragment of 1 leads to the most stable addition product 4 (-72.7 kcal mol(-1)), yet this process has a higher activation barrier (13.0 kcal mol(-1)). The [3+2] addition to the O=Os=O fragment yielding 2 is kinetically (27.5 kcal mol(-1)) and thermodynamically (-7.0 kcal mol(-1)) the least favorable [3+2] reaction. The formal [2+2] addition to the Os=O and Os=CH2 double bonds proceeds by initial rearrangement of 1 to the metallaoxirane 1 a. The rearrangement 1-->1 a and the following [2+2] additions have significantly higher activation barriers (>30 kcal mol(-1)) than the [3+2] reactions. Another isomer of 1 is the dioxoosmacyclopropane 1 b, which is 56.2 kcal mol(-1) lower in energy than 1. The activation barrier for the 1-->1 b isomerization is 15.7 kcal mol(-1). The calculations predict that there are no energetically favorable addition reactions of ethylene with 1 b. The isomeric form 1 c containing a peroxo group is too high in energy to be relevant for the reaction course. The accuracy of the B3LYP results is corroborated by high level post-HF CCSD(T) calculations for a subset of species. 相似文献
107.
Markus Neuenschwander Peter Bigler Klaus Christen Rudolf Iseli Rolf Kyburz Hansueli Mühle 《Helvetica chimica acta》1978,61(6):2047-2058
Chloroacylation and bromoacylation of carbonyl compounds: A forgotten carbonyl reaction. I. Scope of the reaction Aliphatic, α,β-unsaturated and aromatic aldehydes as well as aliphatic ketones react with acyl halides to (α-haloalkyl)esters. These bifunctional derivates contain two leaving groups of different reactivity. The scope of this scarcely of this scarcely known carbonyl reaction is discussed. 相似文献
108.
Constable EC Housecroft CE Neuburger M Schaffner S Shardlow EJ 《Dalton transactions (Cambridge, England : 2003)》2005,(2):234-235
Reaction of 4'-(2-propyn-1-oxy)-2,2':6',2'-terpyridine (HC triple bond CCH2Oterpy) with trans-[PtI2(PEt3)2] regioselectively metallates the alkyne to give trans-[Pt(C triple bond CCH2Oterpy)2(PEt3)2] which, when treated with Fe(II), gives a [2 + 2]-metallocycle. 相似文献
109.
Fabian Gerson Axel Lamprecht Markus Scholz Heinz Troxler Dieter Lenoir 《Helvetica chimica acta》1996,79(1):307-318
The radical anions of 5H-dibenzo[a,d]cycloheptene ( 9 ), 5 H-dibenzo[c,f][1,2]diazepine ( 10 ), 5,6-dihydrodibenzo[a,e]cyclooctene ( 11 ), 5,6-dihydrodibenzo[c,g][1,2]diazocine ( 12 ), and (E)-2,2,5,5-tetramethyl-3,4-diphenylhex-3-ene ( 13 ) were characterized by ESR and ENDOR spectroscopy. Their hyperfine data were compared with those previously reported for radical anions also containing the stilbene or the azobenzene π-system. Whereas the π-spin distribution in the radical anions of the stilbene series is only moderately sensitive to deviations of the π-system from planarity, the radical anions of the azobenzene series respond to steric strain by shifting the π-spin population from the benzene rings to the azo group. This finding is impressively demonstrated by the similar hyperfine data for 9 · ? and 11 · ? which contrast with the strongly highly hindered 13 · ?. A. plausible interpretation is readily provided by the electron affinities of the constituent π-moieties in stilbene and azobenzene. While those of benzene and ethene are both comparatively low, the azo group has a considerably higher electron affinity. 相似文献
110.
Reactions of MoNCl3 and WNCl3 with Elemental Fluorine. Crystal Structures of [MoO2F2(THF)2] and [WF4(NCl)(CH3CN)] The nitrido chlorides MoNCl3 and WNCl3 as well as WCl4(NCl) react with elemental fluorine forming the N-chloro imido complexes MoF4(NCl) and WF4(NCl), which were characterized by IR spectroscopy. With tetrahydrofurane MoF4(NCl) reacts to give [MoF4(NCl)(THF)], which in THF solution slowly converts into [MoO2F2(THF)2]. From WF4(NCl) with acetonitrile the complex [WF4(NCl)(CH3CN)] is obtained. Both donor acceptor complexes were characterized by crystal structure determinations. [MoO2F2(THF)2] : Space group P21/n, Z = 4, structure solution with 1823 unique reflections, R = 0.033 for reflections with I > 2σ(I). Lattice dimensions at ?40°C: a = 636.2, b = 1119.5, c = 1625.2 pm; β = 93.92(1)º. The compound has a monomeric molecular structure with the fluorine atoms in trans-position to one another and with the oxygen atoms of the THF molecules in trans to the oxo ligands. [WF4(NCl)(CH3CN)] : Space group P21/m, Z = 2, structure solution with 1119 unique reflections, R = 0.038 for reflections with I > 2σ(I). Lattice dimensions at 20°C: a = 511.7, b = 714.9, c = 1002.5 pm; β = 102.59(10)º. The compound has a monomeric molecular structure in which the nitrogen atom of the acetonitrile molecule coordinates in trans-position to the N-chloro imido group W?N? Cl. The structural parameters of this group are WN = 172.2 pm, NCl = 161.1 pm, WNCl = 178.6º. 相似文献