全文获取类型
收费全文 | 371篇 |
免费 | 9篇 |
专业分类
化学 | 307篇 |
晶体学 | 14篇 |
力学 | 17篇 |
数学 | 15篇 |
物理学 | 27篇 |
出版年
2021年 | 4篇 |
2020年 | 4篇 |
2017年 | 3篇 |
2016年 | 6篇 |
2015年 | 3篇 |
2014年 | 3篇 |
2013年 | 11篇 |
2012年 | 20篇 |
2011年 | 13篇 |
2010年 | 3篇 |
2009年 | 3篇 |
2008年 | 10篇 |
2007年 | 19篇 |
2006年 | 14篇 |
2005年 | 12篇 |
2004年 | 10篇 |
2003年 | 18篇 |
2002年 | 17篇 |
2001年 | 10篇 |
2000年 | 13篇 |
1999年 | 4篇 |
1998年 | 3篇 |
1997年 | 7篇 |
1996年 | 10篇 |
1995年 | 7篇 |
1994年 | 7篇 |
1993年 | 8篇 |
1992年 | 5篇 |
1990年 | 9篇 |
1989年 | 6篇 |
1988年 | 7篇 |
1987年 | 5篇 |
1986年 | 5篇 |
1985年 | 5篇 |
1983年 | 4篇 |
1981年 | 3篇 |
1980年 | 4篇 |
1979年 | 3篇 |
1976年 | 3篇 |
1969年 | 7篇 |
1967年 | 5篇 |
1966年 | 8篇 |
1955年 | 4篇 |
1954年 | 3篇 |
1936年 | 2篇 |
1935年 | 2篇 |
1934年 | 2篇 |
1928年 | 4篇 |
1915年 | 2篇 |
1873年 | 3篇 |
排序方式: 共有380条查询结果,搜索用时 15 毫秒
371.
Christian Schenk Florian Henke Marco Neumaier Matthias Olzmann Hansgeorg Schnöckel Prof. Dr. Andreas Schnepf PD Dr. 《无机化学与普通化学杂志》2010,636(7):1173-1182
The cluster anion {Ge9[Si(SiMe3)3]3}– ( 1 ) is transferred intact into the gas phase via the electro spray method. Subsequently the fragmentation of 1 after resonant excitation as well as the oxidation reaction with O2 and Cl2 are investigated in an FT‐ICR mass spectrometer (Fourier Transform Ion Cyclotron Resonance). Unlike former results with off‐resonant excitation the fragmentation leads mainly to the end‐product Ge9–. Moreover, applying an on‐resonant excitation the dissociation experiment can be quantified; 2.0 ± 0.15 eV (193 ± 15kJ · mol–1) for the elimination of the first two ligands and 2.7 ± 0.15 eV (261 ± 15 kJ · mol–1) for all ligands, respectively. Particular attention is turned on the first step, where sterically encumbered Si2(SiMe3)6 molecules are formed in a concerted reaction. This result, which is also important for elemental reactions on metal surfaces in catalyses, is based on experimentally determined threshold energies, DFT calculations and calculations on the lifetime of the involved species., In contrast to the high reactivity of crystalline 1 ·Li(THF)4, gaseous 1 is inert against oxygen. The analogy to recently published spin forbidden reactions of Al13– with O2 hints to a general importance of spin conversion during gas phase reactions of larger cluster molecules. The oxidation of 1 with Cl2 proceeds through different reaction channels. DFT calculations give a first insight on the complex primary oxidation steps. These calculations also reveal that the delocalized bonding situation in the Ge9 core is distorted upon oxidation. This result together with the dissociation experiments shed more light on differences and similarities between metalloid clusters and Zintl ions. 相似文献
372.
R H Blaauw J F Brière R de Jong J C Benningshof A E van Ginkel J Fraanje K Goubitz H Schenk F P Rutjes H Hiemstra 《The Journal of organic chemistry》2001,66(1):233-242
A synthesis of the bicyclo[2.1.1]hexane substructure of solanoeclepin A (1), the most active natural hatching agent of potato cyst nematodes, was approached via an intramolecular [2 + 2] photocycloaddition. Aldehyde 12 containing the dioxenone chromophore served as a useful starting material, allowing the synthesis of a variety of photocycloaddition substrates via Grignard addition or via a Nozaki-Hiyama-Kishi reaction. Photolysis of the unsubstituted alkene 14 led to the expected crossed cycloadduct bicyclo[2.1.1]hexane 15 according to the so-called rule of five. However, several functionalized alkenes 18, 20, and 31 exhibited a complete reversal of cycloaddition regioselectivity, providing straight cycloadducts bicyclo[2.2.0]hexanes 21-26 and 4, respectively. Their structures were proved by a combination of extensive NMR measurements, X-ray analyses, and subsequent retro-aldol reactions. The latter de Mayo process allowed the formation of spiro-[3.5]nonane 35 and spiro[3.4]octane 36 as well as the cyclobutanes 37 and 38. Finally, the cyclization of the more rigid lactone precursor 28 occurred in high yield in the desired fashion with complete regio- and stereoselectivity to give 3 containing the core bicyclo[2.1.1]hexane skeleton of the natural product. 相似文献
373.
The problem of calculating the second moment characteristics of the response of a general class of nonconservative linear distributed parameter systems with stochastically varying surface roughness, excited by a moving concentrated load, is investigated. In particular the case of an accelerated load is discussed. The surface roughness is modeled as a Gaussian stationary second order process. For the stochastic representation of the surface roughness a orthogonal series expansion of the covariance kernel, the so called Karhunen‐Loéve expansion, is applied. The resulting initial/boundary value problem is transformed by eigenfunction expansion into the modal state space. Second moment characteristics of the response are determined numerically by direct integration using a Runge‐Kutta method. 相似文献
374.
Gilles Muller Boris Schmidt Jan Jii
ek Jean‐Claude G. Bünzli Kurt J. Schenk 《Acta Crystallographica. Section C, Structural Chemistry》2003,59(6):o353-o356
L4, or 3‐[2,6‐bis(diethylcarbamoyl)pyridin‐4‐yl]‐N‐(tert‐butoxycarbonyl)alanine methyl ester, C24H38N4O6, crystallizes in neat [010] laths stabilized by abundant intra‐ and intermolecular hydrogen bonds. The strongest of these form [010] chains of molecules, thus rationalizing the fastest growth direction, while the slowest direction coincides with the normal to the (110) layers, which are linked by very weak hydrogen bonds. There exist two independent molecules, the distances and bond angles of which differ in a random manner only. The torsion and dihedral angles, however, differ so as to achieve optimal packing. The influence of the chiral group in the 4‐position of the pyridine ring on the helical wrapping and on the ensuing diastereomeric induction is briefly discussed. 相似文献
375.
David J. Brauer Konstantin W. Kottsieper Stefan Schenk Othmar Stelzer 《无机化学与普通化学杂志》2001,627(6):1151-1156
Phosphino derivatives of serine R2P–CH2–CH(NHBOC)(COOMe) ( 2 a – 2 d ) have been obtained in high yield by nucleophilic phosphination of N‐(tert.butoxycarbonyl)‐3‐iodo‐L‐alanine methylester with secondary phosphines R2PH (R = Ph, 2‐tolyl, 3,5‐xylyl, cyclohexyl) in DMF using potassium carbonate as the base. Deprotection of 2 b with HCl affords the amino acid ester hydrochloride [2‐Tol2P–CH2–CH(NH3)(COOMe)]+Cl– ( 3 a ). The X‐ray structures of 2 a (space group P21/n) and 2 c (space group P 1) have been determined. The two enantiomers of 2 a or 2 c are interconnected by N–H…O hydrogen bridges forming dimers in the solid state. 相似文献
376.
377.
Raymond Behrendt Christian Renner Michaela Schenk Fengqi Wang Josef Wachtveitl Dieter Oesterhelt Luis Moroder 《Angewandte Chemie (International ed. in English)》1999,38(18):2771-2774
The cis ⇌ trans photoisomerization of the azobenzene building block 4-(4-aminophenylazo)benzoic acid incorporated in a cyclic peptide (see scheme) facilitated a two-state transition of the peptide chain from a rigid constrained conformation in the trans isomer into the largely free conformational space of the cis isomer. 相似文献
378.
379.
M. Schenk I. Hhnert L. T. H. Duong H.-H. Niebsch 《Crystal Research and Technology》1996,31(5):665-672
Cd1-xZnxTc crystals of different composition (0 ⩽ x ⩽ 1) were grown by the vertical Bridgman method and by synthesis in Te excess. After careful surface preparation of crystal slices, their Zn:Cd ratios were ascertained by wavelength-dispersive X-ray spectroscopy (WDXS), and the lattice parameters of the same slice regions were measured by X-ray diffractometry. The Vegard rule concerning the linear lattice-parameter dependence on composition is – in contrast with earlier literature data – exactly valid within the limits of error (Δx ⩽ ±0.01 and Δa/a ⩽ 3.7. 10−4) and follows the equation a(x) = (0.64822 - 0.03792x) nm. After annealing some slices at different temperatures and controlled partial pressure conditions in ordcr to find possible phase separations or cation ordering effects, neither the lattice parameters were changed nor additonal interferences were found. A published occurrence of a rhomhohedral phase could not be confirmed by means of powder diffraction analysis. The results are discussed in relation to own electron diffraction investigations and to EXAFS literature data. 相似文献
380.