全文获取类型
收费全文 | 3089篇 |
免费 | 55篇 |
国内免费 | 5篇 |
专业分类
化学 | 2160篇 |
晶体学 | 20篇 |
力学 | 36篇 |
数学 | 629篇 |
物理学 | 304篇 |
出版年
2020年 | 23篇 |
2019年 | 25篇 |
2017年 | 22篇 |
2016年 | 64篇 |
2015年 | 41篇 |
2014年 | 54篇 |
2013年 | 126篇 |
2012年 | 98篇 |
2011年 | 104篇 |
2010年 | 75篇 |
2009年 | 74篇 |
2008年 | 118篇 |
2007年 | 94篇 |
2006年 | 106篇 |
2005年 | 81篇 |
2004年 | 97篇 |
2003年 | 72篇 |
2002年 | 70篇 |
2001年 | 46篇 |
2000年 | 41篇 |
1999年 | 65篇 |
1998年 | 45篇 |
1997年 | 43篇 |
1996年 | 51篇 |
1995年 | 40篇 |
1994年 | 45篇 |
1993年 | 44篇 |
1992年 | 41篇 |
1991年 | 46篇 |
1990年 | 49篇 |
1989年 | 50篇 |
1988年 | 57篇 |
1987年 | 54篇 |
1986年 | 45篇 |
1985年 | 61篇 |
1984年 | 71篇 |
1983年 | 58篇 |
1982年 | 73篇 |
1981年 | 59篇 |
1980年 | 53篇 |
1979年 | 49篇 |
1978年 | 60篇 |
1977年 | 50篇 |
1976年 | 44篇 |
1975年 | 55篇 |
1974年 | 41篇 |
1973年 | 26篇 |
1972年 | 38篇 |
1971年 | 30篇 |
1970年 | 34篇 |
排序方式: 共有3149条查询结果,搜索用时 15 毫秒
81.
The systematic integration of the small 2π- and 4π-electron systems cyclopropenylium ion, cyclopropenone, oxo-cyclobutenylium ion, and cyclobutadiene into phane chemistry was initiated only a few years ago. [n2]Cyclopropenylophanes, [n2]cyclopropenonophanes, metal-capped [n4]cyclobutadienosuperphanes, and other new families of double-decker phane species became available from cycloalkydiynes through special methods of double [2+1]cycloaddition with carbenes and metal complex induced dimerization by double [2+2]cycloaddition. Phane-specific structural features were elucidated by X-ray structural analyses. Cyclic voltammetry and PE spectroscopy as well as MO calculations reealed considerable interactions between closely spaced π-electron systems. Decapping cyclobutadienosuperphanes formally extended the synthetic principle to threefold [2+2]cycloadditions resulting in [n4]-bridged tricyclo-[4.2.0.03, 5]octa-3,7-dienes, which represent a new type of cage compounds. Moreover, completion to fourfold [2+2]addition was achieved with the photoinduced transformation of [34]bridged tricyclo[4.2.0.03, 5]octa-3,7-diene into propella[34]cubane. 相似文献
82.
Zusammenfassung Beim Verschmelzen von Isatosäureanhydrid (1) mit Thioamiden bilden sich 2-substit.-4(3H)-Chinazolinone (3a-h).
Syntheses of heterocycles, CXXIV.: Reaction of isatoic anhydride with thioamides and amides
The reaction of isatoic anhydride with thioamides yields 2-substituted 4(3H)-quinazolinones (3a-h).相似文献
83.
Douglas A. Livingston Andreas Pfaltz Jakob Schreiber Albert Eschnmoser Dorothe Ankel-Fuchs Johanna Moll Rolf Jaenchen Rudolf K. Thauer 《Helvetica chimica acta》1984,67(1):334-351
Factor F430 from Methanogenic Bacteria: Structure of the Protein-free Factor Factor F430, the porphinoid nickel-containing coenzyme of the methylcoenzyme-M reductase of metanogenic bacteria is shown to be the 33,83,122,133,182-pentaacid derivative of the pentamethylester F430M, the structure of which had been determined previously (see structural formulae 1 and 2 ). The structure assignment rests on chromatographic, UV/VIS-, CD-, IR-, and 13C-NMR-spectroscopic as well as FAB-mass spectral comparision of F430 with F430M and the pentaacid prepared by acid-catalyzed hydrolysis of F430M. In the cells of Methanobacterium thermoautotrophicum, factor F430 is present in a ‘bound’ and also, depending on the growth conditions, in ‘free’ form, the latter being defined as the part of total F430 that can be extracted from the cells under extremely mild conditions (80% EtOH at 0–4°). From the (protein)-‘bound’ form, F430 is extracted by subsequently treating the cells at 0–4° with 80% EtOH containing (e.g.), 2m LiCi. From both sources, the extracted factor is the same pentaacid, and there is no indication for the existence of a protein-free F430 species that would contain additional (covalently bound) structural elements. 相似文献
84.
The universal voltammetric instrument, described in part I of this series, is used for comparative studies on the most important polarographic methods, direct current, derivative pulse, simple and phase-sensitive alternating current, double-tone, and harmonic-wave polarography. In general the peak-heights of these methods are influenced in the same measure by the composition of base electrolyte. In single cases characteristic differences are observed. 相似文献
85.
The reaction of tribromophosphine oxide in the superacidic systems XF/MF(5) (X = H, D; M = As, Sb) leads to tribromohydroxyphosphonium hexafluorometalates. The structure was successfully elucidated in the case of tribromohydroxyphosphonium hexafluoroarsenate. Br(3)POH(+)AsF(6)(-) crystallizes in the orthorhombic space group Pnma (No. 53 with a = 1292.5(1) pm, b = 871.6(1) pm, and c = 845.0(1) pm) with four formula units per cell. The Raman, IR, (1)H NMR, and (31)P NMR spectra of Br(3)POX(+)MF(6)(-) (X = H, D; M = As, Sb) are reported. 相似文献
86.
Rolf Huisgen 《Angewandte Chemie (International ed. in English)》1980,19(12):947-973
The principle of orbital control of pericyclic reactions has deepened our understanding of reaction phenomena and provided an excellent classification of these one-step processes. The electrocyclic reaction of the pentadienyl anion ? cyclopentenyl anion type is relatively unimportant in all-carbon systems and has not even been verified in the case of the parent compound. In the heterocyclic series, however, where up to five C-atoms of the pentadienyl anion are replaced by heteroatoms, a multitude of ring closures and ring openings find their ordering principle in the mentioned electrocyclic reaction. The replacement of the carbon atoms by heteroatoms can take place isoinically, i. e., with retention of the anionic character, or isoelectronically. An isoelectronic replacement of CR2 in position 1 by NR2 OR, and or CR in position 3 by NR or O leads to a charge-free resonance structure for the open-chain species; the migration of the charge during the electrocyclization results in a correlation with a cyclic zwitterion. Conversely, isoelectronic exchange of CR in position 2 by NR or O produces a conjugated 1,3-dipole, which cyclizes to a charge-free unsaturated five-membered ring. Twofold isoelectronic exchange allows the whole process to take place in a cation. Selected examples are to shed light on the classification and the thermodynamics of this electrocyclic reaction. 相似文献
87.
The conversion of methylenedimethyloxonium ylide to ethyl methyl ether was investigated by the semiempirical MNDO method. An activation energy of 51 kJ/mol suggests that this rearrangement is a possible elementary reaction in the well-known MTG process.
. 51 /, , .相似文献
88.
Rolf Holderegger Luigi M. Venanzi Fiorella Bachechi Pasquale Mura Luigi Zambonelli 《Helvetica chimica acta》1979,62(7):2159-2167
The preparation of the nucleophile trans-[RuCl(NO)( 1 )], where 1 is the bidentate ligand Ph2PCH2C18CH2PPh2, and of the five-coordinate species [RuCl(CO)(NO)( 1 )], [RuCl(CO)(NO)(Ph2PCH2Ph)2] and [RuCl(NO)( 2 )( 1 )] are reported. The crystal structure of [RuCl(CO)(NO)( 1 )] shows that the coordination around the metal atom is distorted trigonal bipyramidal with the phosphorus atoms in axial positions. The Ru? N? O bond angle is 142.8°. 1H- and 31P-NMR. and \documentclass{article}\pagestyle{empty}\begin{document}$ \tilde \nu $\end{document}NO IR.-data for the above complexes are reported and related to the coordination geometry. 相似文献
89.
Bruggink C Maurer R Herrmann H Cavalli S Hoefler F 《Journal of chromatography. A》2005,1085(1):104-109
A versatile liquid chromatographic platform has been developed for analysing underivatized carbohydrates using high performance anion exchange chromatography (HPAEC) followed by an inert PEEK splitter that splits the effluent to the integrated pulsed amperometric detector (IPAD) and to an on-line single quadrupole mass spectrometer (MS). Common eluents for HPAEC such as sodium hydroxide and sodium acetate are beneficial for the amperometric detection but not compatible with electrospray ionisation (ESI). Therefore a membrane-desalting device was installed after the splitter and prior to the ESI interface converting sodium hydroxide into water and sodium acetate into acetic acid. To enhance the sensitivity for the MS detection, 0.5 mmol/l lithium chloride was added after the membrane desalter to form lithium adducts of the carbohydrates. To compare sensitivity of IPAD and MS detection glucose, fructose, and sucrose were used as analytes. A calibration with external standards from 2.5 to 1000 pmole was performed showing a linear range over three orders of magnitude. Minimum detection limits (MDL) with IPAD were determined at 5 pmole levels for glucose to be 0.12 pmole, fructose 0.22 pmole and sucrose 0.11 pmole. With MS detection in the selected ion mode (SIM) the lithium adducts of the carbohydrates were detected obtaining MDL's for glucose of 1.49 pmole, fructose 1.19 pmole, and sucrose 0.36 pmole showing that under these conditions IPAD is 3-10 times more sensitive for those carbohydrates. The applicability of the method was demonstrated analysing carbohydrates in real world samples such as chicory inulin where polyfructans up to a molecular mass of 7000 g/mol were detected as quadrupoly charged lithium adducts. Furthermore mono-, di-, tri-, and oligosaccharides were detected in chicory coffee, honey and beer samples. 相似文献
90.
Rolf Bombach Josef Dannacher J.-P. Stadelmann Reinhard Neier 《Helvetica chimica acta》1983,66(2):701-717
A most recently developed method to quantify the fragmentation pathways of excited radical cations is presented. Using bicyclobutane cation as an illustrative example, the RRKM analysis of the breakdown diagram determined by He-Iα photoelectron-photoion coincidence spectroscopy is outlined. The results imply complete isomerization to 1,3-butadiene cation preceding the dissociative processes. The rate-energy functions of four competitive primary fragmentation reactions, leading to C3H, C4H, C4H and C2H are established. There is compelling evidence that the production of C2H fragment ions does not compete effectively with these four reactions. The extent of kinetic and competitive shift effects is determined. The derived enthalpies of formation are in excellent accord with the available high quality reference data. The relative importance of different fragmentation pathways which ultimately lead to fragment ions of identical mass to charge ratio is assessed. 相似文献