全文获取类型
收费全文 | 2575篇 |
免费 | 56篇 |
国内免费 | 8篇 |
专业分类
化学 | 1732篇 |
晶体学 | 23篇 |
力学 | 32篇 |
数学 | 564篇 |
物理学 | 288篇 |
出版年
2022年 | 17篇 |
2020年 | 45篇 |
2019年 | 29篇 |
2018年 | 17篇 |
2017年 | 25篇 |
2016年 | 42篇 |
2015年 | 68篇 |
2014年 | 52篇 |
2013年 | 92篇 |
2012年 | 92篇 |
2011年 | 100篇 |
2010年 | 76篇 |
2009年 | 58篇 |
2008年 | 119篇 |
2007年 | 112篇 |
2006年 | 96篇 |
2005年 | 103篇 |
2004年 | 79篇 |
2003年 | 63篇 |
2002年 | 50篇 |
2001年 | 23篇 |
2000年 | 33篇 |
1999年 | 25篇 |
1998年 | 30篇 |
1997年 | 22篇 |
1996年 | 23篇 |
1995年 | 27篇 |
1994年 | 28篇 |
1993年 | 36篇 |
1992年 | 29篇 |
1991年 | 18篇 |
1989年 | 25篇 |
1988年 | 30篇 |
1987年 | 33篇 |
1986年 | 17篇 |
1985年 | 35篇 |
1984年 | 47篇 |
1983年 | 26篇 |
1982年 | 20篇 |
1981年 | 32篇 |
1980年 | 38篇 |
1979年 | 23篇 |
1978年 | 23篇 |
1977年 | 26篇 |
1976年 | 22篇 |
1975年 | 26篇 |
1974年 | 26篇 |
1965年 | 16篇 |
1964年 | 16篇 |
1956年 | 24篇 |
排序方式: 共有2639条查询结果,搜索用时 15 毫秒
101.
Detailed analysis of Raman and Raman optical activity (ROA) of L-alanine zwitterion (ALAZW) revealed that shapes of the spectral bands are to a large extent determined by the rotation of the NH(3)(+), CO(2)(-), and CH(3) groups. Aqueous solution ALAZW spectra were measured down to 100 cm(-1) and compared to complex simulations based on ab initio (B3LYP/CPCM/6-31++G**) computations of molecular energies and spectral parameters. The bands exhibit different sensitivities to the motion of the rotating group; typically, for more susceptible bands the Raman signal becomes broader and the ROA intensity decreases. When these dynamical factors are taken into account in Boltzmann averaging of conformer contributions, simulated spectra not only better agree with the experiment, but shapes of the rotational potentials can be estimated. Effects of the molecular flexibility could be also demonstrated on differences in Raman spectra of the solution, crystalline, and glass (gellike) solid states of ALAZW. Experimental Raman and ROA spectra of four model dipeptides of different rigidities (Ala-Pro, Pro-Ala, Pro-Gly, and Gly-Pro) indicate that the broadening of spectral lines can be used as a general site-specific indicator of molecular rigidity or flexibility. 相似文献
102.
Bekesová S Kovácik V Chmelík J Kovác P 《European journal of mass spectrometry (Chichester, England)》2006,12(1):43-50
Saccharides (mono through hexasaccharides) that mimic the terminal epitopes of O-antigens of Vibrio cholerae O:1, serotypes Ogawa and Inaba, were studied by electrospray ion trap (ESI IT) mass spectrometry (MS) in the negative mode. Anionized adducts are the characteristic ions formed by the capture of H(3)O(2)(-) under the condition of ESI MS analysis. The reactive species are produced by reaction of hydroxyl anions with the molecule of water. Thus the [M + H(3)O(2)](-) have the highest m/z value in the ESI IT negative mass spectra. After dissociation of adducts by loss of 2H(2)O the [M-H](-) ions are produced. The fragmentation pathways were confirmed by multistage measurements (MS(n)). The predominant pathway of fragmentation of the mono- and oligomers is the elimination of a molecule of alpha- hydroxy--gammabutyrolactone from the 4-(3-deoxy-L-glycero-tetronamido) group. The other characteristic pathway occurs by shortening the length of oligosaccharides. In this way, conversion of the Ogawa to Inaba fragments takes place under the conditions of measurement. Negative ESI MS/MS provided sufficient information about molecular mass, the number of saccharide residues, basic structure of saccharides, about the tetronamide part of the compounds investigated and allowed Ogawa and Inaba serotypes to be distinguished. 相似文献
103.
Amarie S Standfuss J Barros T Kühlbrandt W Dreuw A Wachtveitl J 《The journal of physical chemistry. B》2007,111(13):3481-3487
Nonphotochemical quenching (NPQ) is a fundamental mechanism in photosynthesis which protects plants against excess excitation energy and is of crucial importance for their survival and fitness. Recently, carotenoid radical cation (Car*+) formation has been discovered to be a key step for the feedback deexcitation quenching mechanism (qE), a component of NPQ, of which the molecular mechanism and location is still unknown. We have generated and characterized carotenoid radical cations by means of resonant two color, two photon ionization (R2C2PI) spectroscopy. The Car*+ bands have maxima located at 830 nm (violaxanthin), 880 nm (lutein), 900 nm (zeaxanthin), and 920 nm (beta-carotene). The positions of these maxima depend strongly on solution conditions, the number of conjugated C=C bonds, and molecular structure. Furthermore, R2C2PI measurements on the light-harvesting complex of photosystem II (LHC II) samples with or without zeaxanthin (Zea) reveal the violaxanthin (Vio) radical cation (Vio*+) band at 909 nm and the Zea*+ band at 983 nm. The replacement of Vio by Zea in the light-harvesting complex II (LHC II) has no influence on the Chl excitation lifetime, and by exciting the Chls lowest excited state, no additional rise and decay corresponding to the Car*+ signal observed previously during qE was detected in the spectral range investigated (800-1050 nm). On the basis of our findings, the mechanism of qE involving the simple replacement of Vio with Zea in LHC II needs to be reconsidered. 相似文献
104.
Richard Barner Josef Hübscher John J. Daly Peter Schnholzer 《Helvetica chimica acta》1981,64(3):915-938
Configuration of the Vitamin-D3-Metabolite 25,26-Dihydroxycholecalciferol: Synthesis of (25S,26)- and (25R,26)-Dihydroxycholecalciferol For selective synthesis of the title compounds, (25S)- 1b and (25R)- 1b (Scheme 1), the protected cholesterol precursors (25S)- 6 and (25R)- 6 were prepared from stigmasterol-derived steroid-units 4a-d and C5-side chain building blocks 5a–d by Grignard- or Wittig-coupling (Scheme 2), the configuration at C(25) of the target compounds being already present in the C5-units. Conversion of the cholesterol intermediates to the corresponding vitamin-D3 derivatives was carried out via the 7,8-didehydrocholesterol compounds (25S)- 2b and (25R)- 2b (Scheme 1), using the established photochemical-thermal transformation of the 5,7-diene system to the seco-triene system of cholecalciferol. The configuration at C(25) of the cholesterol precursors as assigned on basis of the known configuration of the C5-units used, was found to be in agreement with the result of a single crystal X-ray analysis on compound 11 . The configuration at C(25) remained untouched on conversion of the cholesterol ring system to the seco-triene system of vitamin D3 as evident from comparison of the lanthanide-induced CD. Cotton effects observed for (25S)- 3b and (25S) 1b . 25,26-Dihydroxycholecalciferol observed as a natural vitamin-D3 metabolite has (25S)-configuration. 相似文献
105.
Previously unknown polyfluorocyclohexenyl, and acyclic perfluoroalkenyliodine tetrafluorides were prepared in high yields. Perfluorocyclohex-1-enyliodine tetrafluoride was obtained from pentafluoroiodobenzene using XeF2-NbF5 in aHF. The reaction of C6F5I with the weaker fluorooxidant XeF2-BF3 in 1,1,1,3,3-pentafluorobutane (PFB) yielded C6F5IF2, perfluorocyclohexa-1,4-dienyliodine difluoride, C6F5IF4, perfluorocyclohexa-1,4, and 1,3-dienyliodine tetrafluoride as intermediate products on parallel reaction routes. Both perfluoroalkenyl iodides, cis- and trans-(CF3)2CFCFCFI, reacted with XeF2-BF3 in PFB to give the corresponding perfluoroalkenyliodine tetrafluorides, cis- and trans-(CF3)2CFCFCFIF4. Even perfluoroalkyl iodides can be fluorinated by this reagent as was demonstrated by the preparation of C6F13IF4 from C6F13I. Generally, the CFCIFn fragment (n = 0, 2, or 4) in cyclic or acyclic perfluoroalkenyliodine compounds RFIFn did not undergo a transformation to the corresponding perfluoroalkyliodine compound. Furthermore, no perfluoroorganoiodine hexafluorides were detected in reactions with the fluorooxidant XeF2-aHF or BF3 or NbF5. 相似文献
106.
Stará IG Alexandrová Z Teplý F Sehnal P Starý I Saman D Budesínský M Cvacka J 《Organic letters》2005,7(13):2547-2550
[reaction: see text] A new approach to nonracemic [7]helicene-like molecules has been developed. Stereoselective Co(I)-mediated [2 + 2 + 2] cycloisomerization of aromatic triynes containing an asymmetric carbon atom produces [7]helicene-like scaffolds in diastereomeric ratios up to 100:0. This central-to-helical chirality transfer can be controlled by the absolute configuration at the asymmetric center and by the presence of carbon substituents. 相似文献
107.
Zwickl H Traxler E Staettner S Parzefall W Grasl-Kraupp B Karner J Schulte-Hermann R Gerner C 《Electrophoresis》2005,26(14):2779-2785
The secretome of cells and tissues may reflect a broad variety of pathological conditions and thus represents a rich source of biomarkers. The identity of secreted proteins, usually isolated from cell supernatants or body fluids, is hardly accessible by direct proteome analysis, because these proteins are often masked by high amounts of proteins actually not secreted by the investigated cells. Here, we present a novel method for the specific detection of proteins secreted by human tissue specimen as well as cultured cells and chose liver as a model. The method is based on the metabolic labelling of proteins synthesized during a limited incubation period. Then, the cell supernatant is filtered, precipitated, and subjected to two-dimensional gel electrophoresis. Whereas fluorography detected a large number of proteins derived from residual plasma and dead cells, the autoradiographs selectively displayed genuinely secreted proteins. We demonstrate the feasibility of this approach by means of the secretomes of the hepatocellular carcinoma-derived cell line HepG2 and human liver slices. The selective identification of cell- and tissue-specific protein secretion profiles may help to identify novel sets of biomarkers for wide clinical applications. 相似文献
108.
On basis of earlier experimental experience, the transfer of protons in salts of the organic cation-inorganic anion type occurs primarily through directional arrangement of the anion-anion type short hydrogen bonds. The submitted work presents the preparation of quaternary pyridinium salts of inorganic hydrogen anions in the absence of solvent molecules in their crystal structure. These substances can form only the above-described anion-anion type hydrogen bonds; in addition, the absence of solvate anions increases the stability of the prepared compounds. A total of six substituted pyridinium salts were prepared, four of which have not been described yet: 1,2,4,6-tetraphenylpyridinium perchlorate, 1-benzyl-2,4,6-trimethylpyridinium perchlorate, 1,4-dimethylpyridinium hydrogen sulphate, 1,4-dimethylpyridinium dihydrogen phosphate, 1,4-dimethylpyridinium hydrogen sulphate, and 1,2-dimethyl-5-ethylpyridinium dihydrogen phosphate. Three of these substances were characterised by X-ray structural analysis: 1,2,4,6-tetraphenylpyridinium perchlorate crystallises in the orthorhombic system, space group Pbca; 1-benzyl-2,4,6-trimethylpyridinium perchlorate crystallises in the monoclinic system, space group P21/c; and 1,4-dimethylpyridinium dihydrogen phosphate crystallises in the monoclinic system, space group C2/c. This structure contains an oriented anion network bond by short anion-anion type hydrogen bonds with the donor acceptor lengths of 2.567(3) Å and 2.557(3) Å and thus fulfils the requirements of a good proton conductor. 相似文献
109.
Selective Template Removal by Thermal Depolymerization to Obtain Mesostructured Molybdenum Oxycarbide
下载免费PDF全文
Martin Schieder Thomas Lunkenbein Carina Bojer Martin Dulle Julia vom Stein Gudrun Auffermann Tina Löbling Judith Schöbel Holger Schmalz Josef Breu 《无机化学与普通化学杂志》2015,641(10):1829-1834
The carbon content of mesostructured organic‐inorganic hybrid material of a cylindrical block copolymer template of poly(2‐vinylpyridine)‐block‐poly(allyl methacrylate) (P2VP‐b‐PAMA) and ammonium paramolybdate (APM) could be reduced by thermal depolymerization. By calcination in vacuo at 320 °C the PAMA core can be completely removed while the remaining P2VP brush preserves the mesostructure. The P2VP‐APM composite can then be carburized in‐situ to MoOxCy in a second pyrolysis step without any additional carbon source but P2VP. The molybdenum oxycarbide nanotubes obtained, form hierarchically porous non‐woven structures, which were tested as catalyst in the decomposition of NH3. They proved to be catalytically active at temperatures above 450 °C. The activation energy was estimated from an Arrhenius Plot to be 127 kJ · mol–1. 相似文献
110.
Rearrangements of (2′-Propinyl)cyclohexadienols and -semibenzenes The acid-catalyzed dienol-benzene rearrangement of 3- and 5-methyl-substituted (2′-propinyl)cyclohexadienols has been investigated. Treatment of the dienols with CF3COOH in CCl4 yields allenyl- and (2′-propinyl)benzenes via [3,4]- and [1,2]-sigmatropic rearrangements, respectively. The reaction with H2SO4 in Et2O leeds to a mixture of allenyl-, 2′-propinyl-, 3′-butinyl- and (2′,3′-butadienyl)benzenes (Scheme 3). The latter are products of a thermal semibenzene-benzene rearrangement (cf. Scheme 9). The corresponding semibenzenes have been prepared by dehydration of the cyclohexadienols with H2SO4 or POCl3 (Schemes 6 and 7). Under acidic conditions, the p-(2′-propinyl)semibenzenes 33–35 (Scheme 8) undergo [3,4]- and [1,2]-sigmatropic rearrangements to give again allenyl- and (2′-propinyl)benzenes, whereas the thermal rearrangements to the 3′-butinyl- and (2′,3′-butadienyl)benzenes (Scheme 9) involves a radical mechanism. In contrast, the o-(2′-propinyl)semibenzene b (Scheme 7) leads to (2′,3′-butadienyl)benzene 32 via a thermal [3,3]-sigmatropic rearrangement. 相似文献