全文获取类型
收费全文 | 9633篇 |
免费 | 144篇 |
国内免费 | 24篇 |
专业分类
化学 | 6483篇 |
晶体学 | 66篇 |
力学 | 142篇 |
数学 | 1830篇 |
物理学 | 1280篇 |
出版年
2019年 | 74篇 |
2016年 | 152篇 |
2015年 | 112篇 |
2014年 | 114篇 |
2013年 | 272篇 |
2012年 | 248篇 |
2011年 | 326篇 |
2010年 | 214篇 |
2009年 | 218篇 |
2008年 | 318篇 |
2007年 | 311篇 |
2006年 | 323篇 |
2005年 | 293篇 |
2004年 | 270篇 |
2003年 | 245篇 |
2002年 | 237篇 |
2001年 | 146篇 |
2000年 | 181篇 |
1999年 | 142篇 |
1998年 | 138篇 |
1997年 | 130篇 |
1996年 | 163篇 |
1995年 | 196篇 |
1994年 | 160篇 |
1993年 | 138篇 |
1992年 | 176篇 |
1991年 | 171篇 |
1990年 | 147篇 |
1989年 | 157篇 |
1988年 | 147篇 |
1987年 | 134篇 |
1986年 | 137篇 |
1985年 | 184篇 |
1984年 | 161篇 |
1983年 | 163篇 |
1982年 | 178篇 |
1981年 | 197篇 |
1980年 | 144篇 |
1979年 | 167篇 |
1978年 | 145篇 |
1977年 | 156篇 |
1976年 | 129篇 |
1975年 | 134篇 |
1974年 | 150篇 |
1973年 | 128篇 |
1972年 | 94篇 |
1971年 | 101篇 |
1970年 | 78篇 |
1968年 | 62篇 |
1966年 | 66篇 |
排序方式: 共有9801条查询结果,搜索用时 0 毫秒
81.
Various phenylsulfonyl allene derivatives were prepared with double bonds tethered either to the alpha-position or the gamma-position of the allene. These substrates underwent a highly regio- and stereospecific thermal [2 + 2]-cycloaddition across the nonactivated cumulene double bond, forming distal cycloadducts (i.e., 57) in the case of alpha-tethered allenes and proximal adducts (i.e., 25) in the case of gamma-tethered allenes. The mechanistic rationale for the observed stereospecificity involves initial diradical formation, followed by a rapid ring closure to the more stable cis-fused ring system. The tether may be equipped with heteroatoms, allowing for the formation of fused heterocycles (e.g., 61), and the cycloaddition can be facilitated by the introduction of sterically bulky groups and/or by conformational rigidity to the tether. Other modes of cyclization were observed in the presence of sodium benzenesulfinate or Lewis acids, in which cases polar mechanisms prevail. The chemoselectivity is reversed for [4 + 2]-cycloadditions, which prefer instead to engage the vinyl sulfone moiety, independent of whether the tether is attached to the alpha- or gamma-position of the allene. 相似文献
82.
Zur Synthese von fungizid wirksamen Isocamphanderivaten: Synthesen in der Isocamphanreihe, 34. Mitt.
Gerhard Buchbauer Helmut Spreitzer Margarete Kneidinger 《Monatshefte für Chemie / Chemical Monthly》1990,121(6-7):549-554
Summary Some esters of isocamphenilanic acid (4) have been prepared and tested for their fungicidal and insecticidal activity. Esters of various acids with isocamphanyl ethylalcohol (5) have also been synthesized and included in the fungicidal/acaricidal/insecticidal screening programme. All esters bear a geminal dimethyl group at C-3 of the bicyclus which is important because of its shielding effect. 相似文献
83.
Winfried Wagner-Redeker Karsten Levsen Helmut Schwarz Waltraud Zummack 《Journal of mass spectrometry : JMS》1981,16(8):361-371
Using the relative abundance of metastable ions, collisional activation spectra, field ionization kinetic measurements, isotopic labelling, appearance energy and kinetic energy release data, it is shown that linear alkyne radical cations with more than six carbon atoms do not isomerize to equilibrating structures prior to decomposition. At the shortest ion lifetimes the molecular ions of linear alkynes decompose mainly by simple β-bond fission which allows an unequivocal localization of the triple bond. At medium ion lifetimes fragmentation occurs predominantly via a McLafferty rearrangement, while at long ion lifetimes competing alkyl losses prevail. These alkyl losses occur via cyclic intermediates leading to thermochemically stable cycloalkenyl ions. All these reactions occur with a high specificity with respect to the carbon and hydrogen atoms involved and are preceded by little or no hydrogen exchange reactions. 相似文献
84.
Basic metals. XXIV. Mono- and dinuclear cobaltthiolato complexes obtained from disulfides. Splitting of a S? S bond by a metal base The dinuclear complex C5H5(PMe3)Co(μ-CO)2Mn(CO)C5H4Me ( 3 ) reacts with the disulfides S2R2 (R ? Ph, CH2Ph) by splitting of the sulfur-sulfur bond to form C5H5(PMe3)Co(SR)2 ( 4, 5 ). From 3 and S2Me2 a mixture of C5H5(PMe3)Co(SMe)2 ( 6 ) and [C5H5Co(μ-SMe)]2 ( 7 ) is obtained. The synthesis of C5H5(PMe3)Co(SCF3)2 ( 8 ) succeeds by treating 3 with N(SCF3)3. Whereas the reactions of 4 and 5 with MeI lead to the complex C5H5(PMe3)CoI2 ( 9 ), the dinuclear complex [C5H5(PMe3)Co(μ-SPh)]2(BF4)2 ( 11 ) is formed from 4 and [OMe3]BF4. The reactions of 11 with L = PMe3 and P(OMe)3 produce the compounds [C5H5Co(PMe3)(L)SPh]BF4 ( 12, 13 ), which react with [OMe3]BF4 to yield [C5H5Co(PMe3)(L)(MeSPh)](BF4)2 ( 14, 15 ). 相似文献
85.
Basic Metals. LXIV. Lewis-basic Bis(trimethylphosphine)cobalt Complexes with Indenyl and Trifluormethylcyclopentadienyl as Ligands The half-sandwich type compounds C9H7Co(PMe3)2 ( 1 ) and (C5H4CF3)Co(PMe3)2 ( 6 ) are prepared from CoCl(PMe3)3 and C9H7Li or TlC5H4CF3, respectively. They behave like metal bases and react with HBF4, CH3I (or CF3SO3CH3), I2, and CH3COCl by oxidative addition to give the cationic complexes [C9H7CoX(PMe3)2]+ and [(C5H4CF3)CoX(PMe3)2]+ (X ? H, CH3, I, COCH3) which are isolated as the PF6 salts ( 2–5 and 7–10 ). The 1HNMR and the IR spectra of the compounds 1–10 are discussed, also in comparison to those of the corresponding cyclopentadienylcobalt complexes. 相似文献
86.
Helmut Quast Josef Christ Yvonne Görlach Wolfgang von der Saal 《Tetrahedron letters》1982,23(36):3653-3656
The 2,6-dicyano-1,5-dimethylsemibullvalene (2d) synthesized via the bicyclo[3.3.0]octanedione 7 exists as a classical ground state which is lower in energy by less than 5 kcal/mole than the homoaromatic transition of the Cope rearrangement of 2d. 相似文献
87.
Electrokinetic phenomena at grafted polyelectrolyte layers 总被引:1,自引:0,他引:1
During the last decades the electrokinetic theory of Smoluchowski (Z. Phys. Chem. 92 (1918) 129) was extended to be applicable for soft surfaces (grafted polyelectrolyte layers (PL), biological and artificial membranes, etc.) by either using the Debye approximation or numerical solutions. In the theory of Ohshima (Colloids Surf. A 103 (1995) 249) the nonlinearized Poisson-Boltzmann (PB) equation for thick and uniform PL is solved analytically and a general hydrodynamic equation is derived in an integral form. These advantages in the theory of Ohshima provided a base for the further development of a generalized electrokinetic theory for soft surfaces. In his theory the final equation for the electroosmotic (electrophoretic) velocity is specified for the case of the complete dissociation of ionic sites within PL. Accordingly, the equation may be used only if the difference between pK and pH is very large. However, it turned out that an analytical solution of the nonlinearized PB equation for thick PL is possible for any degree of dissociation. This was achieved using the approximation of excluded coions if the absolute value of the reduced Donnan potential is larger than 2 and due to the simplification in the case of weak dissociation, when the absolute value of the reduced Donnan potential is less than 2. Combining this generalized double layer (DL) theory for PL and the theory of Ohshima enables to obtain an analytical equation for electroosmosis for the general case of any degree of dissociation. This equation creates for the first time a theoretical base for the interpretation of electrokinetic fingerprinting (EF) for the characterization of soft surfaces. 相似文献
88.
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
89.
Werner Fuss Wolfram E Schmid Sergei A Trushin Paul S Billone William J Leigh 《Chemphyschem》2007,8(4):592-598
Photochemical pericyclic reactions are believed to proceed via a so-called pericyclic minimum on the lowest excited potential surface (S(1)), which is common to both the forward and backward reactions. Such a common intermediate has never been directly detected. The photointerconversion of 1,3-butadiene and cyclobutene is the prevailing prototype for such reactions, yet only diene ring closure proceeds with the stereospecificity that the Woodward-Hoffmann rules predict. This contrast seems to exclude a common intermediate. Using ultrafast spectroscopy, we show that the excited states of two cyclobutene/diene isomeric pairs are linked by not one, but by two common minima, p* and ct*. Starting from the diene side (cyclohepta-1,3-diene and cycloocta-1,3-diene), electrocyclic ring closure passes via the pericyclic minimum p*, whereas ct* is mainly responsible for cis-trans isomerization. Starting from the corresponding cyclobutenes (bicyclo[3.2.0]heptene-6 and bicyclo[4.2.0]octene-7), the forbidden isomer is formed from ct*. The path branches at the first (S(2)/S(1)) conical intersection towards p* and ct*. The fact that the energetically unfavorable ct* path can compete is ascribed to a dynamic effect: the momentum in C=C twist direction, acquired--such as in other olefins--in the Franck-Condon region of the cyclobutenes. 相似文献
90.
The factors affecting host-guest complexation between the molecular container compound cucurbit[6]uril (CB6) and various guests in aqueous solution are studied, and a detailed complexation mechanism in the presence of cations is derived. The formation of the supramolecular complex is studied in detail for cyclohexylmethylammonium ion as guest. The kinetics and thermodynamics of complexation is monitored by NMR as a function of temperature, salt concentration, and cation size. The binding constants and the ingression rate constants decrease with increasing salt concentration and cation-binding constant, in agreement with a competitive binding of the ammonium site of the guest and the metal cation with the ureido carbonyl portals of CB6. Studies as a function of guest size indicate that the effective container volume of the CB6 cavity is approximately 105 A(3). It is suggested that larger guests are excluded for two reasons: a high activation barrier for ingression imposed by the tight CB6 portals and a destabilization of the complex due to steric repulsion inside. For example, in the case of the nearly spherical azoalkane homologues 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH, volume ca. 96 A(3)) and 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO, volume ca. 110 A(3)), the former forms the CB6 complex promptly with a sizable binding constant (1300 M(-1)), while the latter does not form a complex even after several months at optimized complexation conditions. Molecular mechanics calculations are performed for several CB6/guest complexes. A qualitative agreement is found between experimental and calculated activation energies for ingression as a function of both guest size and state of protonation. The potential role of constrictive binding by CB6 is discussed. 相似文献