首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   6026篇
  免费   204篇
  国内免费   33篇
化学   4325篇
晶体学   17篇
力学   192篇
数学   893篇
物理学   836篇
  2023年   31篇
  2022年   60篇
  2021年   76篇
  2020年   101篇
  2019年   112篇
  2018年   75篇
  2017年   44篇
  2016年   148篇
  2015年   155篇
  2014年   156篇
  2013年   367篇
  2012年   359篇
  2011年   386篇
  2010年   256篇
  2009年   232篇
  2008年   320篇
  2007年   359篇
  2006年   316篇
  2005年   294篇
  2004年   272篇
  2003年   231篇
  2002年   217篇
  2001年   92篇
  2000年   92篇
  1999年   61篇
  1998年   52篇
  1997年   56篇
  1996年   88篇
  1995年   51篇
  1994年   66篇
  1993年   42篇
  1992年   47篇
  1991年   48篇
  1990年   33篇
  1989年   43篇
  1988年   33篇
  1987年   30篇
  1985年   60篇
  1984年   62篇
  1983年   46篇
  1982年   58篇
  1981年   56篇
  1980年   65篇
  1979年   58篇
  1978年   64篇
  1977年   51篇
  1976年   46篇
  1975年   49篇
  1974年   53篇
  1973年   57篇
排序方式: 共有6263条查询结果,搜索用时 15 毫秒
101.
DNA triplex and quadruplex structures have been successfully detected by electrospray ionization mass spectrometry (ESI-MS). Circular dichroism and UV-melting experiments show that these structures are stable in 150 mM ammonium acetate at pH 7 for the quadruplexes and pH 5.5 for the triplexes. The studied quadruplexes were the tetramer [d(TGGGGT)](4), the dimer [d(GGGGTTTTGGGG)](2), and the intramolecular folded strand dGGG(TTAGGG)(3), which is an analog of the human telomeric sequence. The absence of sodium contamination allowed demonstration of the specific inclusion of n - 1 ammonium cations in the quadruplex structures, where n is the number of consecutive G-tetrads. We also detected the complexes between the quadruplexes and the quadruplex-specific drug mesoporphyrin IX. MS/MS spectra of [d(TGGGGT)](4) and the complex with the drug are also reported. As the drug does not displace the ammonium cations, one can conclude that the drug binds at the exterior of the tetrads, and not between them. For the triplex structure the ESI-MS spectra show the detection of the specific triplex, at m/z values typically higher than those typically observed for duplex species. Upon MS/MS the antigene strand, which is bound into the major groove of the duplex, separates from the triplex. This is the same dissociation pathway as in solution. To our knowledge this is the first report of a triplex DNA structure by electrospray mass spectrometry.  相似文献   
102.
Résumé La structure et le spectre électronique du fluoroacétylène et du chloroacétylène ont été étudiés à l'aide d'une méthode matricielle du type LCAO SCF MO où les fonctions monoélectroniques sont des orbitales orthogonalisées de Löwdin. Le transfert de charge de l'halogène au système est d'environ 0,04 électron 2p pour le fluor et 0,08 électron 3p pour le chlore. L'étude des transitions électroniques indique pour la transition du type - située vers 1850 Å un effet bathochrome et une augmentation de la force oscillatrice lorsque l'électronégativité de l'halogène décroît. Ceci permet d'identifier la transition située vers de plus grandes longueurs d'onde avec une transition - dont la force oscillatrice varie en sens inverse de celle de la transition -.
The electronic structure and spectra of fluoroacetylene and chloroacétylène have been studied by a matricial method of LCAO SCF MO type where the monoelectronic functions are Löwdin'S orthogonalized orbitals. The charge transfer from the halogen to the system is equal to about 0,04 2p electron for fluorine and to 0,08 3p electron for chlorine. A bathochromic effect and an increase of the oscillator strength with the halogen electronegativity decreasing have been found for the - transition at 1850 Å; from the theoretical results it is possible to identify the transition of lower energy as a - transition whose oscillator strength varies the other way than that of the - transition.

Zusammenfassung Struktur und Elektronenspektren von Fluor- und Chloracetylen wurden mit Hilfe einer Matrizenmethode vom LCAO-SCF-MO-Typ untersucht, wobei die Einelektronenfunktionen nach Löwdin orthogonalisiert wurden. Der Ladungsübergang vom Halogen zum -System entspricht etwa 0,04 2p-Elektronen bei Fluor und 0,08 3p-Elektronen bei Chlor. Für den --Übergang um 1850 Å wird für abnehmende Elektronegativität des Halogens eine bathochrome Verschiebung und eine Zunahme der Oscillatorenstärke gefunden. Das erlaubt, die längerwellige Bande einem - -Übergang zuzuordnen, dessen Oscillatorenstärke sich in entgegengesetzter Richtung ändert.


Nous tenons à remercier Monsieur Berthier pour de nombreuses discussions sur ce travail, Messieurs Romanet et Wojtkowiak pour avoir attiré notre attention sur les problèmes posés par les spectres ultraviolets des halogéno-acétyléniques et Monsieur H. v. Hirschhausen pour nous avoir signalé une erreur numérique dans la fin de cet article.  相似文献   
103.
Summary We consider here the Dirichlet's problem for the Lamé's equations without body force in a semi-infinite cylinder (0z<) with cross-sectionS z , the displacementsu i vanishing on the cylindrical surface. We give in this paper an explicit decay estimate for aL 2-norm ofu i . The evaluation is of the type wherek 1 andk 2 are determined constants andI(f i ) an integral depending only on the displacementsf i prescribed onS 0. Our results consequently reinforce the Saint-Venant's principle.
Résumé On considère ici le problème de Dirichlet pour les équations de Lamé sans force volumique dans un cylindre semi-infini (0z<) de section droiteS z , les déplacementsu i étant nuls sur la surface cylindrique. On donne dans ce travail une estimation explicite de la décroissance pour une norme (dansL 2) desu i . L'évaluation est du type oùk 1 etk 2 sont des constantes déterminées etI(f i ) une intégrale ne dépendant que des déplacementsf i imposés surS 0. Notre résultat, par conséquent, renforce le principe de Saint-Venant.
  相似文献   
104.
As metalloproteins exemplify, the chemical and physical properties of metal centers depend not only on their first but also on their second coordination sphere. Installing arrays of functional groups around the first coordination sphere of synthetic metal complexes is thus highly desirable, but it remains a challenging objective. Here we introduce a novel approach to produce tailored second coordination spheres. We used bioinspired artificial architectures based on aromatic oligoamide foldamers to construct a rigid, modular and well-defined environment around a metal complex. Specifically, aza-aromatic monomers having a tethered [2Fe–2S] cluster have been synthesized and incorporated in conical helical foldamer sequences. Exploiting the modularity and predictability of aromatic oligoamide structures allowed for the straightforward design of a conical architecture able to sequester the metal complex in a confined environment. Even though no direct metal complex–foldamer interactions were purposely designed in this first generation model, crystallography, NMR and IR spectroscopy concurred to show that the aromatic oligoamide backbone alters the structure and fluxional processes of the metal cluster.

Wrapping a [2Fe–2S] metal complex in an aromatic foldamer helix is introduced as a new approach to tailor a second coordination sphere.  相似文献   
105.
In the title compound, (1,4,7,10,13,16‐hexa­oxacyclo­octa­decane‐1κ6O)‐μ‐oxo‐1:2κ2O:O‐hexa­kis(tetra­hydro­borato)‐1κ3H;2κ2H;2κ2H;2κ3H;2κ3H;2κ3H‐diuranium(IV), [U2(BH4)6O(C12H24O6)], one of the U atoms (U1), located at the centre of the crown ether moiety, is bound to the six ether O atoms, and also to a tridentate tetra­hydro­borate group and a μ‐oxo atom in axial positions. The other U atom (U2) is bound to the same oxo group and to five tetra­hydro­borate moieties, three of them tridentate and the other two bidentate. The two metal centres are bridged by the μ‐oxo atom in an asymmetric fashion, thus giving the species (18‐crown‐6)(κ3‐BH4)U=(μ‐O)—U(κ3‐BH4)32‐BH4)2, in which the U1=O and U2—O bond lengths to the μ‐O atom [1.979 (5) and 2.187 (5) Å, respectively] are indicative of the presence of positive and negative partial charges on U1 and U2, respectively.  相似文献   
106.
Recently, sample preparation has been considered to be the major cause of bottlenecks during high-throughput analysis. With the assistance of robotic liquid handlers and the 96-well plate format, more samples can be prepared for subsequent liquid chromatography/tandem mass spectrometry (LC/MS/MS) analysis. Protein precipitation is still widely used despite potential loss of sensitivity or variable results due to ion suppression. The use of solid-phase extraction (SPE) clearly gives superior results but may not be as cost effective as protein precipitation due to the labor and material costs associated with the process. Here, a novel 96-well SPE plate is described that was designed to minimize the elution volume required for quantitative elution of analytes. The plate is packed with 2 mg of a high-capacity SPE sorbent that allows loading of up to 750 microL of plasma, while the novel design permits elution with as little as 25 microL. Therefore, the plate offers up to a 15-fold increase in sample concentration. The evaporation and reconstitution step that is typically required in SPE is avoided due to the concentrating ability of the plate. Examples of applications in drug discovery/development are shown and results are compared to protein precipitation. Excellent sensitivity and linearity are demonstrated.  相似文献   
107.
The formation of 3-chloro-2-fluoroacrylates 2 and 2-fluoroacrylates 3 by hydrogenolysis of 3,3-dichloro-2-fluoroacrylates 1 was studied by using Bu3SnH, zinc, the sodium sulphite/sodium formate mixture or iron pentacarbonyl in the presence of a hydrogen donor (Et3SiH or CH3OH). The two last couples can be used to prepare the 3-chloro derivatives 2, whereas for the preparation of the 3,3-dihydro derivatives 3, zinc is the most appropriate reducing agent. Keywords: 2-Fluoroacrylate; 3-Chloro-2-fluoroacrylate; 3,3-Dichloro-2-fluoroacrylate; Tributyltin hydride; Zinc; Sodium sulphite; Sodium formate; Iron pentacarbonyl; NMR spectroscopy; IR spectroscopy  相似文献   
108.
This paper describes original approaches aimed at the preparation of electron-rich 1-aryl-3-carboxylisoquinolines. Our first attempt led to an efficient preparation of 1-hydroxyisoquinoline-3-carboxylic acid methyl ester starting from bromophthalide via a rearrangement of 2-acetylamino-2-(3-oxo-1,3-dihydroisobenzofuran-1-yl)-malonic acid dimethyl ester. However, as its eventual application to the synthesis of methylenedioxy-bearing substrates seemed rather long, a second approach involving an extension of the Ritter reaction to safrole was devised. We thus report that, under proper experimental settings, the use of 54% tetrafluoroboric acid in ether enables a Ritter reaction between safrole and 3,4,5-trimethoxybenzonitrile yielding 17% of 7-methyl-5-(3,4,5-trimethoxyphenyl)-7,8-dihydro-[1,3]dioxolo[4,5-g]isoquinoline. This acidic reagent avoids the extensive decomposition seen when using the classical Ritter reaction conditions (i.e.: concentrated sulfuric acid). Further chemical transformations of this methyl-bearing dihydroisoquinoline led to the methylenedioxy-bearing 1-aryl-3-carboxylisoquinoline. These derivatives are related to the peripheral benzodiazepine receptor ligand PK 11195 as well as falcipain-2 inhibitors and other potential antitumor agents.  相似文献   
109.
Time-resolved adsorption behavior of a human immunoglobin G (hIgG) protein on a hydrophobized gold surface is investigated using multitechniques: quartz crystal microbalance/dissipation (QCM-D) technique; combined surface plasmon resonance (SPR) and Love mode surface acoustic wave (SAW) technique; combined QCM-D and atomic force microscopy (AFM) technique. The adsorbed hIgG forms interfacial structures varying in organization from a submonolayer to a multilayer. An "end-on" IgG orientation in the monolayer film, associated with the surface coverage results, does not corroborate with the effective protein thickness determined from SPR/SAW measurements. This inconsistence is interpreted by a deformation effect induced by conformation change. This conformation change is confirmed by QCM-D measurement. Combined SPR/SAW measurements suggest that the adsorbed protein barely contains water after extended contact with the hydrophobic surface. This limited interfacial hydration also contributed to a continuous conformation change in the adsorbed protein layer. The viscoelastic variation associated with interfacial conformation changes induces about 1.5 times overestimation of the mass uptake in the QCM-D measurements. The merit of combined multitechnique measurements is demonstrated.  相似文献   
110.
A theoretical study including full geometry optimizations is carried out at the IMOMM(MP2:MM3) (IMOMM = integrated molecular orbital molecular mechanics) computational level on the [ReH(5)(PPh(i)()Pr(2))(2)(SiHPh(2))(2)] and [ReH(5)(PCyp(3))(2)(SiH(2)Ph)(2)] systems, the results being compared with available experimental diffraction data, as well as with MP2 results on the model system [ReH(5)(PH(3))(2)(SiH(3))(2)]. A simple scheme for the analysis of the relative weight of different contributions to the "steric" distortion is also proposed and applied to the same [ReH(5)(PPh(i)()Pr(2))(2)(SiHPh(2))(2)] and [ReH(5)(PCyp(3))(2)(SiH(2)Ph)(2)] species.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号