全文获取类型
收费全文 | 3076篇 |
免费 | 42篇 |
国内免费 | 3篇 |
专业分类
化学 | 2181篇 |
晶体学 | 11篇 |
力学 | 42篇 |
数学 | 477篇 |
物理学 | 410篇 |
出版年
2018年 | 26篇 |
2017年 | 26篇 |
2016年 | 33篇 |
2015年 | 32篇 |
2014年 | 30篇 |
2013年 | 86篇 |
2012年 | 82篇 |
2011年 | 106篇 |
2010年 | 76篇 |
2009年 | 77篇 |
2008年 | 104篇 |
2007年 | 120篇 |
2006年 | 100篇 |
2005年 | 111篇 |
2004年 | 112篇 |
2003年 | 88篇 |
2002年 | 93篇 |
2001年 | 53篇 |
2000年 | 76篇 |
1999年 | 66篇 |
1998年 | 50篇 |
1997年 | 52篇 |
1996年 | 41篇 |
1995年 | 40篇 |
1994年 | 34篇 |
1993年 | 60篇 |
1992年 | 48篇 |
1991年 | 38篇 |
1990年 | 27篇 |
1989年 | 33篇 |
1988年 | 39篇 |
1987年 | 34篇 |
1986年 | 50篇 |
1985年 | 61篇 |
1984年 | 39篇 |
1983年 | 31篇 |
1982年 | 40篇 |
1981年 | 50篇 |
1980年 | 38篇 |
1979年 | 52篇 |
1978年 | 40篇 |
1977年 | 41篇 |
1976年 | 37篇 |
1975年 | 30篇 |
1974年 | 35篇 |
1973年 | 23篇 |
1972年 | 25篇 |
1971年 | 23篇 |
1970年 | 24篇 |
1934年 | 21篇 |
排序方式: 共有3121条查询结果,搜索用时 317 毫秒
81.
Kurt Egger 《Fresenius' Journal of Analytical Chemistry》1961,182(3):161-166
Zusammenfassung 14 Glykoside der Aglykone Kaempferol, Quercetin und Myricetin werden auf ihr Verhalten im Perlon-Dünnschichtchromatogramm geprüft. Es wird gezeigt, daß 3-Monoside, 3-Bioside und 3,7-Glykoside scharf voneinander getrennt werden und an den stark verschiedenen Rf-Werten erkannt werden können. Der Einfluß des Aglykons ist gering; der entscheidende Faktor für den Rf-Wert ist die Glykosidierung.
Herrn Prof. Dr. L. Hörhammer, Herrn Doz. Dr. H. Wagner und Herrn Dr. W. Leeb, München, danke ich sehr für die freundlicherweise erteilte praktische Einführung in die Polyamid-Dünnschichtchromatographie, sowie für die Überlassung einer Probe von Kaempferol-3-rhamnoglucosid. Die Durchführung der hier dargestellten Versuche hielt sich eng an die in den Münchner Laboratorien entwickelten Verfahren.
Die Untersuchungen wurden von der Görres-Gesellschaft zur Pflege der Wissenschaften und von der Deutschen Forschungsgemeinschaft gefördert, wofür an dieser Stelle gedankt sei. Der BASF Ludwigshafen danke ich für die großzügige Überlassung des Polyamids. 相似文献
Summary 14 glycosides of the aglycons quercetin, kaempferol and myricetin were examined for their behaviour in chromatography with polyamide. The results show that the 3-monosides can be easily differentiated from the 3-biosides, as well as from both the 3,7-glycosides, by their sharply different Rf-values. The influence of the aglycon is small; the decisive factor for the Rf-value is the glycosidation.
Herrn Prof. Dr. L. Hörhammer, Herrn Doz. Dr. H. Wagner und Herrn Dr. W. Leeb, München, danke ich sehr für die freundlicherweise erteilte praktische Einführung in die Polyamid-Dünnschichtchromatographie, sowie für die Überlassung einer Probe von Kaempferol-3-rhamnoglucosid. Die Durchführung der hier dargestellten Versuche hielt sich eng an die in den Münchner Laboratorien entwickelten Verfahren.
Die Untersuchungen wurden von der Görres-Gesellschaft zur Pflege der Wissenschaften und von der Deutschen Forschungsgemeinschaft gefördert, wofür an dieser Stelle gedankt sei. Der BASF Ludwigshafen danke ich für die großzügige Überlassung des Polyamids. 相似文献
82.
When α,β-unsaturated γ-dimethoxymethyl cyclohexenones are excited to the S2(π,π*) state, certain unimolecular reactions can be observed to compete with S2 → S1 internal conversion. These reactions do not occur from the S1(n,π*) or the lowest T(π,π* and n,π*) states. They comprise the radical elimination of the formylacetal substituent (cf. 8 , 9 → 32 + 33 ), γ → α formylacetal migration (cf. 6 → 27 , 8 → 30 , 9 → 34 , 12 → 37 ), and a cyclization process involving the transfer of a methoxyl hydrogen to the α carbon and ring closure at the β position (cf. 6 → 28 , 8 → 31 , 12 → 38 , 20 → 40 + 41 ). The quantum yield of the ring closure 20a → 40a + 41a is 0.016 at ≤ 0.05M concentration. It is independent of the excitation wavelength within the π→π* absorption band (238–254 nm), but Φ ( 40a + 41a ) decreases at higher concentrations. According to the experimental data the reactive species of these specifically π→π*-induced transformations is placed energetically higher than the S1(n,π*) state, and it is either identical with the thermally equilibrated S2(n,π*) state, or reached via this latter state. The linear dienone 14 undergoes a similar π→π*-induced cyclization (→ 42 ) whereas the benzohomologue 26 proved unreactive, and the dienone 22 at both n → π and π→π* excitation only gives rise to rearrangements generally characteristic of cross-conjugated cyclohexadienones. 相似文献
83.
Hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid, decanoic acid, and dodecanoic acid are components of the trail pheromone of the ant species Lasius fuliginosus. The acids were extracted from the contents of the rectal ampullae and identified by the mass spectra and gas chromatographic retention times of the corresponding methyl esters. 相似文献
84.
Jørgensen JM Erlacher K Pedersen JS Gothelf KV 《Langmuir : the ACS journal of surfaces and colloids》2005,21(23):10320-10323
The influence of preparation temperature on the size and size distribution of dodecylthiol monolayer protected gold clusters was studied. The monolayer protected clusters (MPCs) were synthesized by two different variations of the Brust-Schiffrin procedure. In all of the experiments, the stoichiometry of the reactants dodecylthiol, HAuCl(4), and sodium borohydride was kept constant, while the temperature was varied in the range of -18 to +90 degrees C. Two series were performed in which an aqueous solution of NaBH(4) was either added over 30 s or all in one portion. The size and size distribution of the MPCs were determined by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). It has been demonstrated that in general the MPC size increases with elevated preparation temperatures. 相似文献
85.
Mono- and Binuclear Dinitrosyl Complexes of Molybdenum and Tungsten. Crystal Structures of (PPh3Me)2[WCl4(NO)2], (PPh3Me)2[MoCl3(NO)2]2, and (PPh3Me)2[WCl3(NO)2]2 The complexes (PPh3Me)2[MCl4(NO)2] (M = Mo, W), and (PPh3Me)2[MCl3(NO)2]2, respectively, are prepared by reactions of the polymeric compounds MCl2(NO)2 with triphenylmethylphosphonium chloride in CH2Cl2, forming green crystals. According to the IR spectra the nitrosyl groups are in cis-position in all cases. The tungsten compounds as well as (PPh3Me)2[MoCl3(NO)2]2 were characterized by structure determinations with X-ray methods. (PPh3Me)2[WCl4(NO)2]: space group C2/c, Z = 4. a = 1874, b = 1046, c = 2263 pm, β = 119.99°. Structure determination with 3492 independent reflexions, R = 0.057. The compound consists of PPh3Me⊕ ions, and anions [WCl4(NO)2]2? with the nitrosyl groups in cis-position (symmetry C2v). (PPh3Me)2[WCl3(NO)2]2: Space group C2/c, Z = 4. Structure determination with 2947 independent reflexions, R = 0.059. (PPH3Me)2[MoCl3(NO)2]2: Space group P1 , Z = 1. a = 989, b = 1134, c = 1186 pm; α = 63.25°, β = 80.69°, γ = 69.94°. Structure determination with 3326 independent reflexions, R = 0.046. The compounds consist of PPh3Me⊕ ions, and centrosymmetric anions [MCl3(NO)2]22?, in which the metal atoms are associated via MCl2M bridges of slightly different lengths. One of the NO groups is in an axial position, the other one in equatorial position (symmetry C2h). 相似文献
86.
Ten tetraoxygenated xanthones (1-hydroxy-3, 7, 8-trimethoxyxanthone I; 1, 7-dihydroxy-3, 8-dimethoxyxanthone II; 1, 7, 8-trihydroxy-3-methoxyxanthone III; 1, 3, 7, 8-tetrahydroxyxanthone IV; 3, 7, 8-trimethoxyxanthone-1-O-primeveroside V; 7-hydroxy-3, 8-dimethoxyxanthone-1-O-primeveroside VI; 1, 8-dihydroxy-3-methoxyxanthone-7-O-acetylrutinoside VII; 7, 8-dihydroxy-3-methoxyxanthone-1-O-primeveroside VIII; 3, 7, 8-trihydroxyxanthone-1-O-primeveroside IX; 3, 7, 8-trihydroxyxanthone-1-O-glucoside X) have been isolated from leaves of Gentiana bavarica L . by means of column chromatography on polyamid. Among these xanthones, VI, VII, VIII and IX were not encountered before in nature. 相似文献
87.
Ralf Garbe Jürgen Pebler Kurt Dehnicke Dieter Fenske Helmut Goesmann Gerhard Baum 《无机化学与普通化学杂志》1994,620(4):592-598
Phosphorane Iminato Complexes of Antimony. The Crystal Structures of [Sb2Cl5(NPMe3)2][SbCl6] · CH3CN and [SbCl(NPPh3)]2[SbCl6]2 · 6 CH3CN The title compounds are formed by reaction of antimony pentachloride in acetonitrile solution with the phosphorane iminato complexes SbCl2(NPMe3) and SbCl2(NPPh3), respectively, which themselves are synthesized by reaction of antimony trichloride with Me3SiNPR3 (R = Me, Ph). The complexionic compounds are characterized by 121Sb Mössbauer spectroscopy and by crystal structure determinations. [Sb2Cl5(NPMe3)2][SbCl6] · CH3CN: Space group P41, Z = 4, 3 698 observed unique reflections, R = 0.022. Lattice dimensions at ?60°C: a = b = 1 056.0(1), c = 2 709.6(2) pm. The structure consists of SbCl6? ions and cations [Sb2Cl5(NPMe3)2(CH3CN)]+, in which one SbIII atom and one SbV atom are bridged by the N atoms of the phosphorane iminato ligands. [SbCl(NPPh3)]2[SbCl6]2 · 6 CH3CN: Space group P1 , Z = 2, 5 958 observed unique reflections, R = 0.033. Lattice dimensions at ?60°C: a = 989.4(11), b = 1 273(1), c = 1 396(1) pm, α = 78.33(7), β = 77.27(8)°, γ = 86.62(8)°. The structure consists of SbCl6? ions and centrosymmetric cations [SbCl(NPPh3)(CH3CN)2]22+, in which the antimony atoms are bridged by the N atoms of the phosphorane iminato ligands. 相似文献
88.
FLUORESCENCE QUANTUM YIELDS OF 124-kDa PHYTOCHROME FROM OAT UPON EXCITATION WITHIN DIFFERENT ABSORPTION BANDS 总被引:1,自引:0,他引:1
Claudio G. Colombano Silvia E. Braslavsky Alfred R. Holzwarth Kurt Schaffner 《Photochemistry and photobiology》1990,52(1):19-22
Abstract— A fluorescence quantum yield (emission at650–850 nm) of π= (2.3 ± 0.3)10−3 was measured for the red-absorbing form (Pr) of 124-kDa phytochrome from etiolated oat seedlings ( Avena sativa ) upon excitation in the Soret band at Λexc = 380 nm. The small difference between this value and the previously determined quantum yield with Λexc = 640 nm, π= (3.5 ± 0.4)10−3 is attributed to a blue-absorbing emitter responsible for the "anomalous" or "blue" emission of the chromoprotein in the region from ca. 400 to 550 nm. The absorption of Pr at 380 nm is consequently somewhat lower than that measured directly from the spectrum. Processes from upper excited states of the Pr phytochromobilin-derived chromophore other than rapid relaxation to the emitting state are not important. A quantum yield of Φ ' 1.2 times 10−3 is estimated for the blue fluorescence. The proportion of the blue emitters relative to Pr appears to be relatively high. 相似文献
89.
Synthesis and Crystal Structure of the Nitrido Complex [Na-15-crown-5]2[MoNF4]2 · 2 CH3CN The title compound is synthesized by the reaction of [MoCl4(NSCl)]2 with excess NaF in boiling acetonitrile in the presence of the crown ether 15-crown-5. [Na-15-crown-5]2[MoNF4]2 · 2 CH3CN forms yellow crystals, which were characterized by an X-ray structure determination. Space group P1 , Z = 1. Lattice dimensions at ?90°C: a = 855.5, b = 1 069.9, C = 1 143.5 pm, α = 105.71°, β = 95.29°, γ = 102.25° (4 096 independent observed reflexions, R = 0.039). Short Na…?F contacts of 234 pm with the four axial fluoro ligands of the dimeric anion [MoNF4]22? allow formulation of a triple ion. The centrosymmetric anion is dimerized by bent fluoro bridges with Mo? F distances of 198 and 245 pm. The long Mo? F distances of the MoF2Mo ring are in transposition to the nitrido ligands, the bond lengths of which (165 pm) correspond to triple bonds. 相似文献
90.
[(Ph3Sn)3VO4]·CH3CN and [(Ph3Sn)3VO4]·2 DMF, Triphenyltin Vanadates with Novel Chain Structures The reaction of Na3VO4 with Ph3SnCl in a water/CH2Cl2 mixture leads to the formation of [(Ph3Sn)3VO4] ( 1 ). Recrystallization of 1 from toluene/CH3CN gives pale yellow crystals of [(Ph3Sn)3VO4]·CH3CN ( 2 ). 2 crystallizes as coordination polymer which consists of infinite chains composed of corner‐sharing VO4 tetrahedra and Ph3SnO2 trigonal bipyramides. Additionally the VO4 groups are connected to two terminal SnPh3‐Groups containing tin atoms in a tetrahedral environment. [(Ph3Sn)3VO4]·2 DMF ( 3 ) which is obtained from Na3VO4 and Ph3SnCl in a water/DMF mixture contains a polymeric chain structure similar to 2 and additionally one of the terminal SnPh3 groups is coordinated to a DMF solvent molecule. 相似文献