全文获取类型
收费全文 | 1090篇 |
免费 | 106篇 |
国内免费 | 6篇 |
专业分类
化学 | 877篇 |
晶体学 | 9篇 |
力学 | 20篇 |
数学 | 118篇 |
物理学 | 178篇 |
出版年
2023年 | 31篇 |
2022年 | 34篇 |
2021年 | 49篇 |
2020年 | 73篇 |
2019年 | 81篇 |
2018年 | 29篇 |
2017年 | 23篇 |
2016年 | 85篇 |
2015年 | 54篇 |
2014年 | 40篇 |
2013年 | 75篇 |
2012年 | 58篇 |
2011年 | 73篇 |
2010年 | 35篇 |
2009年 | 23篇 |
2008年 | 49篇 |
2007年 | 28篇 |
2006年 | 41篇 |
2005年 | 31篇 |
2004年 | 23篇 |
2003年 | 19篇 |
2002年 | 23篇 |
2001年 | 12篇 |
2000年 | 13篇 |
1999年 | 8篇 |
1997年 | 7篇 |
1996年 | 9篇 |
1995年 | 5篇 |
1994年 | 7篇 |
1993年 | 10篇 |
1992年 | 7篇 |
1991年 | 4篇 |
1990年 | 4篇 |
1989年 | 4篇 |
1988年 | 5篇 |
1987年 | 5篇 |
1985年 | 12篇 |
1984年 | 13篇 |
1983年 | 12篇 |
1982年 | 6篇 |
1978年 | 3篇 |
1977年 | 4篇 |
1976年 | 4篇 |
1975年 | 3篇 |
1973年 | 3篇 |
1970年 | 5篇 |
1969年 | 3篇 |
1966年 | 3篇 |
1965年 | 4篇 |
1960年 | 4篇 |
排序方式: 共有1202条查询结果,搜索用时 492 毫秒
1.
Maximilian W. Kuntze-Fechner Hendrik Verplancke Lukas Tendera Martin Diefenbach Ivo Krummenacher Holger Braunschweig Todd B. Marder Max C. Holthausen Udo Radius 《Chemical science》2020,11(40):11009
The reaction of [Ni(Mes2Im)2] (1) (Mes2Im = 1,3-dimesityl-imidazolin-2-ylidene) with polyfluorinated arenes as well as mechanistic investigations concerning the insertion of 1 and [Ni(iPr2Im)2] (1ipr) (iPr2Im = 1,3-diisopropyl-imidazolin-2-ylidene) into the C–F bond of C6F6 is reported. The reaction of 1 with different fluoroaromatics leads to formation of the nickel fluoroaryl fluoride complexes trans-[Ni(Mes2Im)2(F)(ArF)] (ArF = 4-CF3-C6F42, C6F53, 2,3,5,6-C6F4N 4, 2,3,5,6-C6F4H 5, 2,3,5-C6F3H26, 3,5-C6F2H37) in fair to good yields with the exception of the formation of the pentafluorophenyl complex 3 (less than 20%). Radical species and other diamagnetic side products were detected for the reaction of 1 with C6F6, in line with a radical pathway for the C–F bond activation step using 1. The difluoride complex trans-[Ni(Mes2Im)2(F)2] (9), the bis(aryl) complex trans-[Ni(Mes2Im)2(C6F5)2] (15), the structurally characterized nickel(i) complex trans-[NiI(Mes2Im)2(C6F5)] (11) and the metal radical trans-[NiI(Mes2Im)2(F)] (12) were identified. Complex 11, and related [NiI(Mes2Im)2(2,3,5,6-C6F4H)] (13) and [NiI(Mes2Im)2(2,3,5-C6F3H2)] (14), were synthesized independently by reaction of trans-[Ni(Mes2Im)2(F)(ArF)] with PhSiH3. Simple electron transfer from 1 to C6F6 was excluded, as the redox potentials of the reaction partners do not match and [Ni(Mes2Im)2]+, which was prepared independently, was not detected. DFT calculations were performed on the insertion of [Ni(iPr2Im)2] (1ipr) and [Ni(Mes2Im)2] (1) into the C–F bond of C6F6. For 1ipr, concerted and NHC-assisted pathways were identified as having the lowest kinetic barriers, whereas for 1, a radical mechanism with fluoride abstraction and an NHC-assisted pathway are both associated with almost the same kinetic barrier.A combined experimental and theoretical study on the mechanism of the C–F bond activation of C6F6 with [Ni(NHC)2] is provided. 相似文献
2.
Bringmann G Pfeifer RM Rummey C Hartner K Breuning M 《The Journal of organic chemistry》2003,68(18):6859-6863
The enantioselective synthesis of a novel-type C(3)-symmetric tripodal ligand that is composed of a central mesitylene-derived core and three functionalized, axially chiral biaryl subunits is described. The triol (M,M,M)-3 is a suitable catalyst for the enantioselective addition of dialkylzinc to various aromatic aldehydes with asymmetric inductions of up to 98% ee. 相似文献
3.
Scherer LJ Merz L Constable EC Housecroft CE Neuburger M Hermann BA 《Journal of the American Chemical Society》2005,127(11):4033-4041
We describe the synthesis and a novel approach to the conformational analysis of 2,2'-bipyridines (bpy) bearing aromatic rich Frechet-type dendritic wedges of the first and second generation as substituents. The evaporation of solutions of these new ligands on graphite surfaces under ambient conditions results in the formation of self-organized monolayers. Scanning tunneling microscopy (STM) investigations of the monolayers under ambient conditions (air, 298 K) gave images at submolecular and near-atomic resolution. The analysis of the STM images includes the following processes: (i) identification and reproduction of potential homoconformational domains, (ii) exclusion of improper data using quality criteria for drift and feedback artifacts, (iii) compilation of running averages and checking for averaging artifacts, (iv) analysis of three-dimensional and contour plots, (v) calculation of the HOMO properties of the free molecules, and (vi) final conformational assignment based on all accessible information. Following this procedure, two different conformations could be assigned to domains observed in the monolayers of the first-generation (G1) and second-generation (G2) dendritic compounds. Homoconformational domains are observed side-by-side. The different conformations arise from syn or anti arrangements at the ether substituents. An additional conformational effect is found upon treating the G1 domains with HCl gas, when a partial rearrangement of the bpy from trans to cis occurs, concomitant with protonation. 相似文献
4.
S. Pfeifer H. D. Beckey und H. -R. Schulten 《Fresenius' Journal of Analytical Chemistry》1977,284(3):193-195
Zusammenfassung Die Molekülionen von Gemischen polycyclischer Kohlenwasserstoffe wurden auf der Photoplatte eines hochauflösenden Felddesorptions-Massenspektrometers registriert. Die erhaltenen Schwärzungsintensitäten wurden mit der relativen Konzentration der eingesetzten Mischungskomponenten verglichen. Proben im Mikrogrammbereich ergaben Abweichungen von etwa ± 5%.Wir danken Herrn Dr. H. Güsten, Karlsruhe, für die uns zur Verfügung gestellten polycyclischen Kohlenwasserstoffe und Herrn Dr. W. D. Lehmann für wertvolle Diskussionen. Der Deutschen Forschungsgemeinschaft, dem Ministerium für Wissenschaft und Forschung des Landes Nordrhein-Westfalen sowie dem Fonds der chemischen Industrie danken wir für die finanzielle Unterstützung. 相似文献
5.
Wagner J Andres H Rohrbach S Wagner D Oberer L France J 《The Journal of organic chemistry》2005,70(23):9588-9590
[Reaction: see text]. Sanglifehrin A is a novel complex natural product showing strong immunosuppressive activity and remarkably high affinity for cyclophilin A. To assess its pharmacokinetic properties in vivo, an efficient synthetic route was developed to introduce a tritium label in position C35 of sangliferin A via an oxidation/reduction strategy. The synthetic approach is particularly attractive, because the C35-oxo intermediate 7 is available in good yield on large scale and the reducing agent, lithium tri-sec-butylborotritide, is readily available. An attempt to apply a similar strategy to the alcohol in position C31 led primarily to C31-epi-hydroxy sanglifehrin A under a variety of conditions. 相似文献
6.
Humic substances were analysed by atmospheric pressure chemical ionisation (APCI) and electrospray ionisation (ESI) mass spectrometry in positive and negative modes. Using APCI the average m/z range of humic substances was reduced 5-fold compared to ESI. High-resolution time-of-flight mass spectrometry revealed the formation of multiply charged molecules in the ESI mode. Moreover, it was possible to obtain daughter ion mass spectra of humic substances by nanospray tandem mass spectrometry. The size-exclusion chromatography elution profile of humic substances was highly influenced by the pH of the analyte solution. By contrast, the pH had no significant influence on the observed mass spectra of humic substances. 相似文献
7.
Viktor Pfeifer 《Mikrochimica acta》1960,48(4):518-533
Zusammenfassung Die Anwendbarkeit des Wolfram(VI)-komplexes mit Rhodanid für die analytische Praxis wurde untersucht; durch seine Löslichkeit in organischen Lösungsmitteln eignet sich der Komplex gut zur Trennung von anderen Elementen.Unter Zuhilfenahme des radioaktiven Wolframisotopes185W konnten die optimalen Extraktionsbedingungen, wie Säure- und Rhodanidkonzentration, rasch ermittelt werden. Bei der Untersuchung verschiedener organischer Lösungsmittel zeigte Amylalkohol sowohl in reiner Form als auch in Mischungen mit Chloroform die besten Extraktionseigenschaften.Da der Komplex zu den stärksten des Wolframs gehört, stören bei der Durchführung von Analysen nur wenige Anionen, am meisten Fluorid. Von den Kationen werden nach Reduktion des Eisens, wozu Ascorbinsäure empfohlen wird, nur wenige andere mitextrahiert; am störendsten wirkt Molybdän, das mitausgeschüttelt wird und anschließend entfernt werden muß. Zur Bestimmung des Wolframs eignet sich dann am besten die Hydrochinonmethode.
Summary A study was made of the use of tungsten(VI) complex with thiocyanate in analytical practice. Because of its solubility in organic liquids the complex is well suited for separation from other elements.By using the radioactive isotope185W, the optimal extraction conditions, such as the acid- and thiocyanate concentration, could be rapidly determined. When various organic solvents were tried, it was found that amyl alcohol, either by itself or mixed with chloroform, had the best extraction characteristics.Since the complex is one of the most stable formed by tungsten, only a few anions interfere in the analysis; fluoride causes the most trouble. Among the cations, only a few interfere after the iron has been reduced (ascorbic acid is recommended for this purpose). The greatest interference comes from molybdenum, which is co-extracted and has to be removed subsequently. The hydroquinone method is then the best procedure for determining the tungsten.
Résumé On a cherché à appliquer le complexe tungstène-VI-thiocyanate à l'analyse. Grâce à sa solubilité dans les solvants organiques, le complexe se prête bien à la séparation d'avec les autres éléments.Au moyen de l'isotope radioactif du tungstène185W, on a pu trouver rapidement les conditions optimales d'extraction, ainsi que la concentration en acide et en thiocyanate. Après essai de divers agents de dissolution organiques, l'alcool amylique a montré les meilleures propriétés extractives, aussi bien sous sa forme pure qu'en mélange avec le chloroforme.Etant donné que le complexe est l'un des plus robustes du tungstène, seuls quelques anions gênent pour effectuer les analyses, et surtout les fluorures. Parmi les cations, après réduction du fer pour laquelle l'acide ascorbique est recommandé, seuls quelques autres sont extraits avec lui; le molybdène présente l'action la plus gênante, il doit être masqué et séparé complètement. Pour le dosage du tungstène, la méthode à l'hydroquinone convient alors le mieux.相似文献
8.
Karl Krmer Hans U. Güdel Gerd Meyer Thorsten Heuer Norman N. Edelstein Bernd Jung Lukas Keller Peter Fischer Eugeniusz Zych Janusz Drozdzynski 《无机化学与普通化学杂志》1994,620(8):1339-1345
The ternary uranium(III) halides A2UX5 (A = K, Rb; X = Cl, Br, I) have been prepared from the binary components AX and UX3 in sealed tantalum containers. According to their Guinier X-ray powder patterns, they all crystallize with the K2PrCl5/Y2HfS5 type of structure. Lattice constants for ambient temperature are reported. Single-crystal structure refinemens were undertaken for K2UI5 and Rb2UCl5. Magnetic susceptibility data were recorded with a SQUID magnetometer from liquid helium to room temperature. One-dimensional (intrachain) and three-dimensional antiferromagnetic ordering occur at low temperatures dependent upon the U3+? U3+ distance. Absorption spectra were recorded between 4 000 and 28 000 cm?1. They show f—f transitions typical for U3+ and, depending on the halide, very strong f—d transitions above 14 000 to 15 000 cm?1, respectively. 相似文献
9.
Dieter Seebach Paola E. Ciceri Mark Overhand Bernhard Jaun Dario Rigo Lukas Oberer Ulrich Hommel Ren Amstutz Hans Widmer 《Helvetica chimica acta》1996,79(8):2043-2066
Structural prerequisites for the stability of the 31 helix of β-peptides can be defined from inspection of models (Figs. 1 and 2): lateral non-H-substituents in 2- and 3-position on the 3-amino-acid residues of the helix are allowed, axial ones are forbidden. To be able to test this prediction, we synthesized a series of heptapeptide derivatives Boc-(β-HVal-β-HAla-β-HLeu-Xaa-β-HVal-β-HAla-β-HLeu)-OMe 13–22 (Xaa = α- or β-amino-acid residue) and a β-depsipeptide 25 with a central (S)-3-hydroxybutanoic-acid residue (Xaa = –OCH(Me)CH2C(O)–) (Schemes 1 3). Detailed NMR analysis (DQF-COSY, HSQC, HMBC, ROESY, and TOCSY experiments) in methanol solution of the β-hexapeptide H(-β-HVal-β-HAla-β-HLeu)2-OH ( 1 ) and of the β-heptapeptide H-β-HVal-β-HAla-β-HLeu-(S,S)-β-HAla(αMe)-β-HVal-β-HAla- β-HLeu-OH ( 22 ), with a central (2S,3S)-3-amino-2-methylbutanoic-acid residue, confirm the helical structure of such β-peptides (previously discovered in pyridine solution) (Fig.3 and Tables 1–5). The CD spectra of helical β-peptides, the residues of which were prepared by (retentive) Arndt-Eistert homologation of the (S)- or L -α-amino acids, show a trough at 215 nm. Thus, this characteristic pattern of the CD spectra was taken as an indicator for the presence of a helix in methanol solutions of compounds 13–22 and 25 (including partially and fully deprotected forms) (Figs.4–6). The results fully confirm predicted structural effects: incorporation of a single ‘wrong’ residue ((R)-β-HAla, β-HAib, (R,S)-β-HAla(α Me), or N-Me-β-HAla) in the central position of the β-heptapeptide derivatives A (see 17, 18, 20 , or 21 , resp.) causes the CD minimum to disappear. Also, the β-heptadepsipetide 25 (missing H-bond) and the β-heptapeptide analogs with a single α-amino-acid moiety in the middle ( 13 and 14 ) are not helical, according to this analysis. An interesting case is the heptapeptide 15 with the central achiral, unsubstituted 3-aminopropanoic-acid moiety: helical conformation appears to depend upon the presence or absence of terminal protection and upon the solvent (MeOH vs. MeOH/H2O). 相似文献
10.
Lukas AS Bushard PJ Weiss EA Wasielewski MR 《Journal of the American Chemical Society》2003,125(13):3921-3930
The spin-spin exchange interaction, 2J, in a radical ion pair produced by a photoinduced electron transfer reaction can provide a direct measure of the electronic coupling matrix element, V, for the subsequent charge recombination reaction. We have developed a series of dyad and triad donor-acceptor molecules in which 2J is measured directly as a function of incremental changes in their structures. In the dyads the chromophoric electron donors 4-(N-pyrrolidinyl)- and 4-(N-piperidinyl)naphthalene-1,8-dicarboximide, 5ANI and 6ANI, respectively, and a naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptor are linked to the meta positions of a phenyl spacer to yield 5ANI-Ph-NI and 6ANI-Ph-NI. In the triads the same structure is used, except that the piperidine in 6ANI is replaced by a piperazine in which a para-X-phenyl, where X = H, F, Cl, MeO, and Me(2)N, is attached to the N' nitrogen to form a para-X-aniline (XAn) donor to give XAn-6ANI-Ph-NI. Photoexcitation yields the respective 5ANI(+)-Ph-NI(-), 6ANI(+)-Ph-NI(-), and XAn(+)-6ANI-Ph-NI(-) singlet radical ion pair states, which undergo subsequent radical pair intersystem crossing followed by charge recombination to yield (3)NI. The radical ion pair distances within the dyads are about 11-12 A, whereas those in the triads are about approximately 16-19 A. The degree of delocalization of charge (and spin) density onto the aniline, and therefore the average distance between the radical ion pairs, is modulated by the para substituent. The (3)NI yields monitored spectroscopically exhibit resonances as a function of magnetic field, which directly yield 2J for the radical ion pairs. A plot of ln 2J versus r(DA), the distance between the centroids of the spin distributions of the two radicals that comprise the pair, yields a slope of -0.5 +/- 0.1. Since both 2J and k(CR), the rate of radical ion pair recombination, are directly proportional to V(2), the observed distance dependence of 2J shows directly that the recombination rates in these molecules obey an exponential distance dependence with beta = 0.5 +/- 0.1 A(-)(1). This technique is very sensitive to small changes in the electronic interaction between the two radicals and can be used to probe subtle structural differences between radical ion pairs produced from photoinduced electron transfer reactions. 相似文献