全文获取类型
收费全文 | 2928篇 |
免费 | 86篇 |
国内免费 | 5篇 |
专业分类
化学 | 1993篇 |
晶体学 | 13篇 |
力学 | 47篇 |
数学 | 373篇 |
物理学 | 593篇 |
出版年
2020年 | 46篇 |
2019年 | 52篇 |
2018年 | 34篇 |
2016年 | 68篇 |
2015年 | 51篇 |
2014年 | 51篇 |
2013年 | 107篇 |
2012年 | 99篇 |
2011年 | 127篇 |
2010年 | 65篇 |
2009年 | 65篇 |
2008年 | 99篇 |
2007年 | 77篇 |
2006年 | 85篇 |
2005年 | 71篇 |
2004年 | 56篇 |
2003年 | 65篇 |
2002年 | 53篇 |
2001年 | 59篇 |
2000年 | 65篇 |
1999年 | 34篇 |
1998年 | 35篇 |
1996年 | 48篇 |
1995年 | 63篇 |
1994年 | 35篇 |
1993年 | 52篇 |
1992年 | 54篇 |
1991年 | 35篇 |
1990年 | 43篇 |
1989年 | 40篇 |
1988年 | 29篇 |
1987年 | 31篇 |
1986年 | 32篇 |
1984年 | 42篇 |
1983年 | 34篇 |
1982年 | 42篇 |
1981年 | 45篇 |
1980年 | 38篇 |
1979年 | 43篇 |
1978年 | 37篇 |
1977年 | 39篇 |
1976年 | 36篇 |
1975年 | 33篇 |
1974年 | 48篇 |
1973年 | 38篇 |
1968年 | 30篇 |
1967年 | 36篇 |
1966年 | 42篇 |
1964年 | 41篇 |
1963年 | 36篇 |
排序方式: 共有3019条查询结果,搜索用时 15 毫秒
41.
The rearrangement of aminoethanol catalyzed by ethanolamine ammonia lyase is investigated by computational means employing DFT (B3LYP/6-31G) and ab initio molecular orbital theory (QCISD/cc-pVDZ). The study aims at providing a detailed account on various crucial aspects, in particular a distinction between a direct intramolecular migration of the partially protonated NH(2) group vs elimination of NH(4)(+). Three mechanistic scenarios were explored: (i) According to the calculations, irrespective of the nature of the protonating species, intramolecular migration of the NH(3) group is energetically less demanding than elimination of NH(4)(+). However, all computed activation enthalpies exceed the experimentally derived activation enthalpy (15 kcal/mol) associated with the rate-determining step, i.e., the hydrogen abstraction from the 5'-deoxyadenosine by the product radical. For example, when imidazole is used as a model system for His interacting with the NH(3) group of the substrate, the activation enthalpy for the migration process amounts to 27.4 kcal/mol. If acetic acid is employed to mimic Asp or Glu, the activation enthalpy is somewhat lower, being equal to 24.2 kcal/mol. (ii) For a partial deprotonation of the substrate 2 at the OH group, the rearrangement mechanism consists of the dissociation of an NH(2) radical from C(2) and its association at C(1) atom. For all investigated proton acceptors (i.e., OH(-), HCOO(-), CH(3)COO(-), CH(2)NH, imidazole), the activation enthalpy for the dissociation step also exceeds 15 kcal/mol. Typical data are 20.2 kcal/mol for Ac(-) and 23.8 kcal/mol for imidazole. (iii) However, in a synergistic action of partial protonation of the NH(2) group and partial deprotonation of the OH group by the two conceivable catalytic auxiliaries Asp/Glu and His, the activation enthalpy computed is compatible with the experimental data. For imidazole and acetate as model systems, the activation enthalpy is equal to 13.7 kcal/mol. This synergistic action of the two catalytic groups is expected to take place in a physiologically realistic pH range of 6-9.5, and the present computational findings may help to further characterize the yet unknown structural details of the ethanolamine ammonia lyase's active site. 相似文献
42.
Depecker G Patino N Di Giorgio C Terreux R Cabrol-Bass D Bailly C Aubertin AM Condom R 《Organic & biomolecular chemistry》2004,2(1):74-79
A cyclic molecule including a hexameric PNA sequence has been designed and synthesized in order to target the TAR RNA loop of HIV-1 through the formation of a "kissing complex". For comparison, its linear analogue has also been investigated. The synthesis of the cyclic and linear PNA has been accomplished following a liquid-phase strategy using mixed PNA and fully N-protected (aminoethylglycinamide) fragments. The interactions of this cyclic PNA and its linear analogue with TAR RNA have been studied and the results indicate clearly that no interaction occurs between the cyclic antisense PNA and TAR RNA, whereas a tenuous interaction has been detected with its linear PNA analogue. 相似文献
43.
Attempts to prepare phosphate granates were without success. However, three arsenate granates of the type {NaCa2}[M](As3)O12 with MII ? Mg, Co, Ni have been prepared by solid state reaction and their lattice constants determined. All these arsenates are characterised by a thermal transformation into a high-temperature form, proceeding monotropically for the Mg compound, and reversibly for the Ni and Co compound. 相似文献
44.
45.
The enantiomers of adrenaline, noradrenaline, ephedrine and pseudoephedrine were separated by capillary electrophoresis on a micromachined device. Detection was carried out with a new two-electrode amperometric detector, eliminating the need for individual counter and reference electrodes. Separation of the isomers was achieved by employing carboxymethyl-beta-cyclodextrin as chiral selector in the buffer, partly with the additional inclusion of the crown ether 18-crown-6. Plate numbers of up to 20,000, chiral resolutions of 2.5 and detection limits of the order of 10(-7) M were achieved. All separations were completed in less then 3 min. 相似文献
46.
Fourier-transform ion-cyclotron-resonance (FTICR) mass spectrometry has been used to uncover the mechanisms by which FeO+ dehydrates heptan-4-one ( 5a ) and nonan-5-one ( 6a ) in the gas phase. The study of isotopomeric ketones provides evidence that H2O loss is not due to a 1,1-elimination, thus ruling out the intermediacy of high-valent iron-carbene species. Rather, H2O is generated in a formal 1,2-elimination involving the ω/ω ? 1 positions of the alkyl chain (‘remote C? H bond activation’). In the consecutive alkene/H2O elimination, the olefins (ethylene from 5a and propene from 6a ) originate from the terminal part of one alkyl chain, and the H-atom is transferred to the FeO+ moiety in the course of this process, builds up together with an H-atom from the ω/ω-1 position of the other alkyl chain the H2O molecule. In either case, the O-atom of H2O is provided by the FeO+ species. 相似文献
47.
Detlef Schrder Jessica Loos Helmut Schwarz Roland Thissen Jana Roithova Zdenek Herman 《International journal of mass spectrometry》2003,230(2-3):113
Singly and doubly charged C4H3+/2+ ions generated upon electron ionization (EI) of the neutral precursors 1,3-butadiene, benzene, and exo-methylene cyclopropane, respectively, are examined by sector-field mass-spectrometry. Charge stripping of the mass-selected monocations affords the corresponding dications and charge exchange of the C4H32+ dications allows for the reverse redox process. Refined analysis and additional MS/MS studies suggest that the monocations are mixtures of isomeric ions formed upon ionization, whereas only a single type of dication seems to be formed. As an average of energy-resolved measurements, a vertical ionization energy of IEv(C4H3+)=16.5±0.4 eV is derived. In addition to the experimental work, density functional theory is used for a computational exploration of the mono- and dicationic species. The best theoretical estimates are IEa(C4H3+)=16.33 eV and IEv(C4H3+)=16.49 eV for the most stable isomer H2C=C---CCH+. Combination of the experimental and theoretical findings leads to the conclusion that the diacetylene cation C4H2+ has indeed a positive proton affinity of PA(C4H2+)=1.50±0.42 eV. 相似文献
48.
Rigorous regio- and stereospecific labeling experiments are performed to demonstrate the operation of the previously suggested operation of ‘isotopically sensitive branching’ in FeI-mediated C? H bond activation. For the hexane-1,6-diol/Fe+-complex, it is shown that dehydrogenation involves specifically the central C(3)/C(4) position, and the study of the stereospecifically labeled D ,L - and meso-[3,4-D2]-isotompomers 1e and 1f demonstrates that dehydrogenation proceedes via two competing pathways (i.e. ‘anti’- vs. ‘syn’-route). The contribution of these routes to the product formation is – due to a kinetic isotope effect – controlled by the relative configuration at the labeled positions C(3)/C(4). For the D ,L -form 1e , we estimate a ratio of 49:1 in favor of the ‘anti’-route; due to an isotope effect, this ratio drops to 4.3:1 for the meso-form 1f . 相似文献
49.
Hans Weidmann Hanns Wolf Oberwalder Egon Wildschek Helmut Schwarz 《Monatshefte für Chemie / Chemical Monthly》1972,103(1):210-217
Zusammenfassung Durch acylierende Dehydratisierung von 1,2-Isopropyliden-d-xylo-und-d-ribo-pentodialdo-furanoseoxim bzw. aus Furanuronsäureestern und-amiden werden über eine Reihe von Zwischenstufen verschiedene 3-O-Acyl-pentofuranuronsäurenitrile synthetisiert und deren Reaktionen untersucht.
Mit 1 Abbildung
Aus den DissertationenH. W. Oberwalder (1970) undE. Wildschek (1967), Technische Hochschule in Graz. 相似文献
Synthesis of xyluronic and riburonic acid derivatives
Various 3-O-acyl-pentofuranurononitrils were synthesized by acylating dehydration of 1,2-isopropylidene-d-xylo- andd-ribo-pentodialdo-furanose oximes, or pentofuranuronic acids via intermediates. Their reactions were investigated.
Mit 1 Abbildung
Aus den DissertationenH. W. Oberwalder (1970) undE. Wildschek (1967), Technische Hochschule in Graz. 相似文献
50.
J. Schwarz 《Colloid and polymer science》1973,251(3-4):215-224
Summary The deviation from the statistical theory of rubberelasticity in the stress-strain-behavior of elastomers, which is described empirically by theRivlin-Mooney equation for the case of simple elongation, is explained by the cooperation of the short range order, which is to assume in amorphous polymers, and the chain length distribution and the occurrence of chain ends in crosslinked systems. This explanation is in qualitative agreement with all experimental results regarding the influence of swelling, preparation of the networks and type of deformation. The quantitative agreement is as good as one can expect from the underlying simple model. In particular the influence of the crosslinking density and the primary molecular weight is considered. In connection with this theory conclusions are drawn to the short range order and a remark is made concerning the problem of the crosslinking density.
Zusammenfassung Die Abweichung von der statistischen Theorie der Kautschukelastizität im Spannungs-Dehnungs-Verhalten von Elastomeren, die im Falle der uniaxialen Dehnung durch dieRivlin-Mooney-Gleichung beschrieben wird, wird erklärt durch das Zusammenwirken der Nahordnung, die in amorphen Polymeren anzunehmen ist, und der Kettenlängenverteilung sowie das Vorhandensein von freien Kettenenden in vernetzten Systemen. Die gegebene Erklärung stimmt qualitativ mit allen experimentellen Ergebnissen überein bezüglich des Einflusses der Quellung, der Herstellung der Netzwerke und der Art der Deformation. Die quantitative Übereinstimmung ist so gut, wie man es nach dem zugrunde liegenden einfachen Modell erwarten kann. Insbesondere wird der Einfluß der Vernetzungsdichte und des Molekulargewichts vor der Vernetzung betrachtet. Im Zusammenhang mit dieser Theorie werden Folgerungen bezüglich der Nahordnung gezogen und eine Betrachtung zum Problem der Vernetzungsdichte angestellt.相似文献