全文获取类型
收费全文 | 1267篇 |
免费 | 55篇 |
国内免费 | 4篇 |
专业分类
化学 | 1014篇 |
晶体学 | 3篇 |
力学 | 23篇 |
数学 | 185篇 |
物理学 | 101篇 |
出版年
2021年 | 5篇 |
2020年 | 11篇 |
2019年 | 14篇 |
2018年 | 10篇 |
2016年 | 23篇 |
2015年 | 44篇 |
2014年 | 29篇 |
2013年 | 77篇 |
2012年 | 73篇 |
2011年 | 70篇 |
2010年 | 33篇 |
2009年 | 35篇 |
2008年 | 60篇 |
2007年 | 45篇 |
2006年 | 33篇 |
2005年 | 49篇 |
2004年 | 51篇 |
2003年 | 26篇 |
2002年 | 33篇 |
2001年 | 16篇 |
2000年 | 22篇 |
1999年 | 19篇 |
1998年 | 23篇 |
1997年 | 32篇 |
1996年 | 27篇 |
1995年 | 25篇 |
1994年 | 25篇 |
1993年 | 36篇 |
1992年 | 19篇 |
1991年 | 21篇 |
1990年 | 13篇 |
1989年 | 15篇 |
1988年 | 20篇 |
1987年 | 10篇 |
1986年 | 15篇 |
1985年 | 18篇 |
1984年 | 17篇 |
1983年 | 21篇 |
1982年 | 14篇 |
1981年 | 13篇 |
1980年 | 14篇 |
1979年 | 22篇 |
1978年 | 33篇 |
1977年 | 21篇 |
1976年 | 16篇 |
1975年 | 14篇 |
1974年 | 11篇 |
1973年 | 18篇 |
1972年 | 10篇 |
1967年 | 4篇 |
排序方式: 共有1326条查询结果,搜索用时 15 毫秒
21.
Karl Heinz Dötz Barbara Fügen-Köster Dietmar Neugebauer 《Journal of organometallic chemistry》1979,182(4):489-498
1-Diethylaminopropyne reacts with pentacarbonyl[methoxy(2,2-diphenylethenyl)carbene]chromium in a stereoselective way to give pentacarbonyl-[diethylamino-E-(2-methoxy-1-methyl-4,4-diphenyl-1,3-butadienyl)carbene]-chromium via insertion of the alkyne into the metal-carbene bond. The reaction of the ynamine with pentacarbonyl[methylamino(phenyl)carbene]chromium results in insertion of the alkyne and loss of carbon monoxide to give cis-tetracarbonyl[3-aza-1-methyl-2-phenyl-2-butenyl)diethylaminocarbene]-chromium. Its structure was established by oxidative degradation in an aqueous medium to give 2-benzoyl-N,N-diethylpropanamide and finally confirmed by an X-ray analysis. The new compounds are characterized spectroscopically. 相似文献
22.
In addition to the sesquiterpenoid compounds reported in [1] the in-depth analysis of lavender oil also led to the identification of new constituents with caryophyllane ( 1 – 6 ), cedrane ( 7 and 8 ) and other skeletons ( 9 – 21 ). Spectroscopic properties as well as partial syntheses of these compounds are discussed. 相似文献
23.
Fries G Wolf J Ilg K Walfort B Stalke D Werner H 《Dalton transactions (Cambridge, England : 2003)》2004,(12):1873-1881
A series of unsymmetrical 1,2-bis(phosphino)ethanes R(2)PCH(2)CH(2)PR'(2) and 1-arsino-2-phosphinoethanes R(2)AsCH(2)CH(2)PR'(2) mainly with bulky substituents R and R' were prepared from the cyclic sulfate by stepwise cleavage of the carbon-oxygen bonds by LiPR(2) and LiPR'(2) or LiAsR(2) and LiPR'(2), respectively. Analogously, racemic mixtures of R(2)PCH(2)CH(Me)PPh(2)(R =iPr, Cy ) as well as the enantiomers (R)-, (R)- and (R)-tBu(2)PCH(2)CH(Me)PPh(2)(R)- were obtained from the corresponding unsymmetrical cyclic sulfates and (S)-. On a similar route, the racemates of the 1,3-bis(phosphino)propanes R(2)PCH(2)CH(2)CH(Me)PPh(2)(R =iPr, tBu ), optically pure (R)- and (S,S)-iPr(2)PCH(Me)CH(2)CH(Me)PPh(2)(S,S)- were prepared. The reaction of [[RhCl([small eta](4)-C(8)H(12))](2)] with chelating ligands L-L, where L-L is R(2)PCH(2)P(men)(2)(R =iPr, Ph; men =(1S,2R,5S)-menthyl), Cy(2)AsCH(2)P(men)(2), or (R)-, (R)-, (R)-, (R)- and (S,S)-, in the presence of AgPF(6), gave the complexes [Rh(eta(4)-C(8)H(12))(L-L)]PF(6) which were used as pre-catalysts in the hydrogenation of the methyl ester of alpha-acetamidocinnamic acid (ACM). Depending on L-L, the solvent, the temperature and the pressure of H(2), optical yields of up to 69% ee were achieved. For two of the rhodium complexes, and, the molecular structures were determined by X-ray crystallography. 相似文献
24.
Wolfgang?Gutleben Verena?Unterholzner Dietmar?Volgger Andreas?ZemannEmail author 《Mikrochimica acta》2004,146(2):111-117
In the course of this investigation, a method for the characterization and differentiation of paper pulps was developed. After hydrolyzation of the papers and the respective raw pulps with trifluoroacetic acid, the hydrolyzates were analyzed with respect to their carbohydrate composition using ion exchange chromatography. The variations in carbohydrate composition mainly arise from the various hemicelluloses of the papers and pulps. The chromatographic results were then further processed using principal component analysis which allows correlating the various papers to different pulp materials. 相似文献
25.
Dietmar Seyferth 《Journal of organometallic chemistry》1979,165(1):C21-C22
The transition metal cluster hydride complex H2Os3(CO)10 reacts with aryl isocyanates and isothiocyanates to yield products containing N-aryl-formamido and -thioformamido ligands. The compound (μ-p-CH3C6H4NC(H)O) (μ-H)Os3(CO)10 has been analyzed by x-ray crystallographic methods and is shown to contain a formamido ligand bridging the edge of the triosmium cluster. 相似文献
26.
Dicobalt octacarbonyl and some of its derivatives (NaCo(CO)4, Co4(CO)12, Hg[Co(CO)4]2, [Co(CO)3PPh3]2, NaCo(CO)3PPh3) react with activated gem-dihalides, R2CX2, such as dichlorodiphenylmethane, 9,9-dihalofluorenes and dimethyl dibromomalonate, to give the ‘dimer’ olefin, R2CCR2. The course of this conversion involves formation of the coupling product, R2XCCXR2, followed by dehalogenation of the latter. These separate steps have been confirmed for activated monohalides (bromodiphenylmethane, 9-bromofluorene, dimethyl bromomalonate) which were readily coupled by cobalt carbonyls, and for activated vicinal dihalides (D,L and meso-dibromostilbene, 9,9′-dichlorobisfluorenyl) which cobalt carbonyls readily dehalogenated. A radical mechanism is favored for these processes, and indirect evidence in its favor is presented. 相似文献
27.
Luesch H Hoffmann D Hevel JM Becker JE Golakoti T Moore RE 《The Journal of organic chemistry》2003,68(1):83-91
The biosynthesis of the unusual amino acid 4-methylproline in the Nostoc genus of cyanobacteria was investigated on the genetic and enzymatic level. Two genes involved in the biosynthesis were cloned and the corresponding enzymes, a zinc-dependent long-chain dehydrogenase and a Delta(1)-pyrroline-5-carboxylic acid (P5C) reductase homologue, were overexpressed in Escherichia coli and biochemically characterized. Putative substrates were synthesized to test enzyme substrate specificities, and deuterium labeling studies were carried out to reveal the stereospecificities of the enzymatic reactions with respect to the substrates as well as to the coenzymes. 相似文献
28.
Dietmar Brandes 《Journal of organometallic chemistry》1977,136(1):25-31
Novel organylthio(alkoxy)silanes (I, II, III and XII) and organylthio(diethylamino)silanes (IV, V) are described. They were prepared by treating lithium or lead thiolates with the corresponding chlorosilanes or by cleavage of dimethylbis(diethylamino)silane with thiols. Phenylthiosilanes (Me3SiSPh, III and XIII) furthermore can be obtained by reaction of chlorosilanes with benzenethiol in the presence of tertiary amines. The SiS bond of Me3SiSPh is cleaved by chlorosilanes like Me2Si(NEt2)Cl or Me2Si(OPr)Cl. This reaction is a convenient route to prepare compounds I and IV. The physical and chemical properties of the novel compounds were investigated. 相似文献
29.
A new polymorph of the iminophosphorane Ph2P(CH2Py)(NSiMe3), ( 1 ), is compared to a just recently published. The reaction of the starting material, the phosphane Ph2P(CH2Py) with N3SiMe3 in the presence of water gives [Ph2P(CH2Py)(NH2)][N3], ( 2 ). A comparison of the structural and NMR parameters of 2 with previously reported derivatives of 1 , suggests that 2 is best described as a phosphonium salt in which the negatively charged imino nitrogen atom is protonated, according to [Ph2(CH2Py)P+—NH2][N3]—, rather than as an iminiumphosphane salt [Ph2(CH2Py)P=+NH2][N3]—. 相似文献
30.
Volkmer D Bredenkötter B Tellenbröker J Kögerler P Kurth DG Lehmann P Schnablegger H Schwahn D Piepenbrink M Krebs B 《Journal of the American Chemical Society》2002,124(35):10489-10496
Combining analytical and theoretical methods, we present a detailed study of a heteropolytungstate cluster encapsulated in a shell of dendritically branching surfactants, namely (C(52)H(60)NO(12))(12)[(Mn(H(2)O))(3)(SbW(9)O(33))(2)], 3. This novel surfactant-encapsulated cluster (SEC) self-assembles spontaneously from polyoxometalate-containing solutions treated with a stoichiometric amount of dendrons. Compound 3 exhibits a discrete supramolecular architecture in which a single polyoxometalate anion resides in a compact shell of dendrons. Our approach attempts to combine the catalytic activity of polyoxometalates with the steric properties of tailored dendritic surfactants into size-selective catalytic systems. The structural characterization of the SEC is based on analytical ultracentrifugation (AUC) and small-angle neutron scattering (SANS). The packing arrangement of dendrons at the cluster surface is gleaned from molecular dynamics (MD) simulations, which suggests a highly porous shell structure due to the dynamic formation of internal clefts and cavities. From analysis of the MD trajectory of 3, a theoretical neutron-scattering function is derived that is in good agreement with experimental SANS data. Force field parameters used in MD simulations are partially derived from a quantum mechanical geometry optimization of [(Zn(H(2)O))(3)(SbW(9)O(33))(2)](12)(-), 2b, at the density functional theory (DFT) level. DFT calculations are corroborated by X-ray structure analysis of Na(6)K(6)[(Zn(H(2)O))(3)(SbW(9)O(33))(2)].23H(2)O, which is isostructural with the catalytically active Mn derivative 2a. The combined use of theoretical and analytical methods aims at rapidly prototyping smart catalysts ("dendrizymes"), which are structurally related to naturally occurring metalloproteins. 相似文献