全文获取类型
收费全文 | 2282篇 |
免费 | 62篇 |
国内免费 | 14篇 |
专业分类
化学 | 1598篇 |
晶体学 | 22篇 |
力学 | 54篇 |
数学 | 241篇 |
物理学 | 443篇 |
出版年
2021年 | 22篇 |
2020年 | 38篇 |
2019年 | 22篇 |
2018年 | 21篇 |
2017年 | 17篇 |
2016年 | 40篇 |
2015年 | 48篇 |
2014年 | 39篇 |
2013年 | 105篇 |
2012年 | 109篇 |
2011年 | 168篇 |
2010年 | 83篇 |
2009年 | 77篇 |
2008年 | 167篇 |
2007年 | 126篇 |
2006年 | 154篇 |
2005年 | 143篇 |
2004年 | 89篇 |
2003年 | 94篇 |
2002年 | 67篇 |
2001年 | 29篇 |
2000年 | 37篇 |
1999年 | 28篇 |
1998年 | 25篇 |
1997年 | 17篇 |
1996年 | 32篇 |
1995年 | 17篇 |
1994年 | 19篇 |
1993年 | 36篇 |
1992年 | 18篇 |
1991年 | 13篇 |
1990年 | 17篇 |
1989年 | 17篇 |
1988年 | 20篇 |
1987年 | 20篇 |
1986年 | 14篇 |
1985年 | 21篇 |
1984年 | 17篇 |
1983年 | 17篇 |
1982年 | 18篇 |
1981年 | 16篇 |
1980年 | 15篇 |
1979年 | 28篇 |
1978年 | 16篇 |
1977年 | 14篇 |
1976年 | 13篇 |
1974年 | 12篇 |
1973年 | 17篇 |
1967年 | 9篇 |
1966年 | 7篇 |
排序方式: 共有2358条查询结果,搜索用时 31 毫秒
991.
Attempting to model superoxide dismutase (SOD) enzymes, we designed two new N3O-donor ligands to provide the same set of donor atoms observed in the active site of these enzymes: K(i)Pr2TCMA (potassium 1,4-diisopropyl-1,4,7-triazacyclononane-N-acetate) and KBPZG (potassium N,N-bis(3,5-dimethylpyrazolylmethyl) glycinate). Five new Co(II) complexes (1-5) were obtained and characterized by X-ray crystallography, mass spectrometry, electrochemistry, magnetochemistry, UV-vis, and electron paramagnetic resonance (EPR) spectroscopies. The crystal structures of 1 and 3-5 revealed five-coordinate complexes, whereas complex 2 is six-coordinate. The EPR data of complexes 3 and 4 agree with those of the Co(II)-substituted SOD, which strongly support the proposition that the active site of the enzyme structurally resembles these models. The redox behavior of complexes 1-5 clearly demonstrates the stabilization of the Co(II) state in the ligand field provided by these ligands. The irreversibility displayed by all of the complexes is probably related to an electron-transfer process followed by a rearrangement of the geometry around the metal center for complexes 1 and 3-5 that probably changes from a trigonal bipyramidal (high spin, d7) to octahedral (low spin, d6) as Co(II) is oxidized to Co(III), which is also expected to be accompanied by a spin-state conversion. As the redox potentials to convert the Co(II) to Co(III) are high, it can be inferred that the redox potential of the Co(II)-substituted SOD may be outside the range required to convert the superoxide radical (O2*-) to hydrogen peroxide, and this is sufficient to explain the inactivity of the enzyme. Finally, the complexes reported here are the first corroborative structural models of the Co(II)-substituted SOD. 相似文献
992.
Gryko DT Zhao F Yasseri AA Roth KM Bocian DF Kuhr WG Lindsey JS 《The Journal of organic chemistry》2000,65(22):7356-7362
One approach toward storage of multiple bits of information at the molecular level requires the construction of molecular architectures comprised of multiple redox-active units. Four new ferrocene-porphyrins have been synthesized to investigate questions concerning (1) the scope of redox-active molecules that can be employed in molecular information-storage schemes and (2) writing/reading rates as well as retention of charge in redox-active units located at different sites in a molecular architecture. Three of the ferrocene-porphyrins have linkers of different lengths between the ferrocene and porphyrin. The fourth ferrocene-porphyrin has two ferrocenes positioned at the lateral sites on the porphyrin. The latter architecture is designed to provide a shorter distance between the electroactive surface and the ferrocene while maintaining an upright orientation of the porphyrin. Each ferrocene-porphyrin affords three cationic oxidation states (ferrocene monocation, porphyrin monocation, porphyrin dication) in addition to the neutral state, thereby affording the capability of storing two bits of information. Each ferrocene-porphyrin bears an S-acetyl or S-(N-ethyl)carbamoyl-protected thiol moiety, thereby avoiding handling of free thiols. Each ferrocene-porphyrin forms a self-assembled monolayer (SAM) on gold via in situ cleavage of the thiol protecting group. The SAM of each array is electrochemically robust and exhibits three well-resolved, reversible oxidation waves. 相似文献
993.
A minicomputer system is described for the off-line reduction of gamma spectral data from neutron activation analysis. The
hardware and software systems are described. Both data printouts and spectral plots are produced. The versatility of system
permits the availability of computer capability for both on- and off-line functions in a variety of analytical techniques
at a moderate cost and under the control of the experimenter. 相似文献
994.
The preparation, single crystal growth, and crystallographic properties of a close-packed, eight-layer, hexagonal (a = 5.803 Å, c = 19.076 Å) modification having the stoichiometry Ba8Nb6Li2O24 and of a close-packed, ten-layer, hexagonal (a = 5.760 Å, c = 23.742 Å) phase with Ba10W6Li4O30 stoichiometry are discussed. The isostructural Ba8Ta6Li4O24 form of the eight-layer phase was also prepared (a = 5.802 Å, c = 19.085 Å). Proposed crystal structures involve the pairing of lithium and metal (Nb, Ta, or W) octahedra to yield face-sharing units. The relationship of this phenomenon to other known close-packed phases containing Li is demonstrated. An investigation of the Ba8Nb6Li2O24Ba10W6Li4O30 system is reported.A tetragonal bronze phase homogeneity region was delimited at 1200°C in the BaONb2O5Li2O system. A new orthorhombic phase (a = 10.197 Å, b = 14.882 Å, c = 7.942 Å) was prepared with the stoichiometry Ba4Li2Nb10O30. 相似文献
995.
Smith BB Hill DE Cropp TA Walsh RD Cartrette D Hipps S Shachter AM Pennington WT Kwochka WR 《The Journal of organic chemistry》2002,67(15):5333-5337
Reaction of the bis-9-BBN adduct of several dienes with 1,3-dibromobenzene via Suzuki coupling leads to a series of [n]metacyclophanes ranging in size from 10 to 17 atom members. In each case, two carbon-carbon bonds are formed in one reaction vessel. However, when the bis-9-BBN adduct of 1,5-hexadiene is coupled with a variety of aryl dihalides, larger [n.n]cyclophanes were formed in preference to the [n]cyclophanes. Four carbon-carbon bonds are formed in this instance. Single-crystal X-ray analyses of these [n.n]cyclophanes reveal interestingly shaped molecules with large cavities. 相似文献
996.
Wangler M Roth DA Krivtsun VM Pak I Winnewisser G Geleijns M Wormer PE van der Avoird A 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2002,58(11):2499-2504
The spectrum of the weakly bound complex Ar-CH4 in the 7 microm region was discovered, analysed, and compared with a spectrum, predicted from ab initio calculations. The measurements were made by probing a supersonic gas expansion with a tunable diode laser (TDL). Several bands of Ar-CH4 associated with different ro-vibrational transitions of the v4 vibration of CH4 were recorded and analysed in a spectral region from 1295 to 1330 cm(-1). In particular the following transitions were studied: j = 1 <-- 0 (at 1311 cm(-1)) reported in Pak et al. [Z. Naturforsch. 53 (1998) 725], j = 0 <-- 1 (at 1301 cm(-1)), j = 2 <-- 1 (at 1316 cm(-1)), and j = 3 <-- 2 transitions (at 1322 cm(-1)). Here, j denotes the angular momentum of the methane unit inside the complex. Analysis of the recently recorded j = 1 <-- 1 transitions at about 1306 cm(-1) in the region of methane Q(1) is in progress. The experimental results are compared with ab initio calculations. The close agreement between observed and ab initio spectra is convincingly demonstrated with respect to the gross spectral features, including many details of the spectra. 相似文献
997.
S. Roth F. Grass F. De Corte L. Moens K. Buchtela 《Journal of Radioanalytical and Nuclear Chemistry》1993,169(1):159-175
k0- and Q0-factors of 18 short-lived nuclides used in reactor neutron activation analysis were determined using the Fast Irradiation and Measurement System (FIMS) of the TRIGA MARK II reactor at the Atominstitut der Österreichischen Universitäten. The Q0-factors were either critically selected from literature, or experimentally determined according to theCd-ratio method, while the k0-factors were additionally determined by theBare monitor method. A comparison is made with the values calculated by introduction of literature data for the input parameters. Moreover, a user oriented tabulation is presented of k0-, Q0-factors and related nuclear data. All results were critically tested with respect to their accuracy and precision. 相似文献
998.
L. Roth 《Mikrochimica acta》1959,47(4):582-585
Zusammenfassung Durch Säulen aus Filtrierkarton kann in aufsteigender Chromatographie eine Substanz von störenden Begleitstoffen befreit werden. Es ist möglich, während des Betriebes durch Sprühreagenzien Feststellungen über die Lage des Stoffes in der Säule zu machen oder — bei kurzer Unterbrechung — mit einzelnen Scheiben Reinheitsprüfungen durchzuführen. Die Säule kann während des Betriebes verlängert werden. Eine quantitative chromatographische Bestimmung ist möglich.
Karlsruhe, Richard-Wagner-Straße 8. 相似文献
Summary A substance can be freed of interfering accompanying materials by ascending chromatographing in columns made of filtering paperboard. It is possible, during the operation, to locate the materials in the column by sprayed-on reagents or—with brief interruption—to conduct purity tests with individual disks. The column can be lengthened during the operation. It is possible to make a quantitative chromatographic determination.
Résumé Il est possible de purifier une substance de produits gênants l'accompagnant par chromatographic ascendante sur des colonnes en carton filtre. Il est possible de se rendre compte de la position de la substance pendant l'opération à l'aide de réactifs pulvérisés et également après une courte interruption d'effectuer des essais de pureté sur des disques isolés. Pendant l'opération, il est possible d'allonger la colonne. Il est également possible d'effectuer une détermination chromatographique quantitative.
Karlsruhe, Richard-Wagner-Straße 8. 相似文献
999.
Brock A Chang E Ho CC LeDuc P Jiang X Whitesides GM Ingber DE 《Langmuir : the ACS journal of surfaces and colloids》2003,19(5):1611-1617
Mammalian cells redirect their movement in response to changes in the physical properties of their extracellular matrix (ECM) adhesive scaffolds, including changes in available substrate area, shape, or flexibility. Yet, little is known about the cell's ability to discriminate between different types of spatial signals. Here we utilize a soft-lithography-based, microcontact printing technology in combination with automated computerized image analysis to explore the relationship between ECM geometry and directional motility. When fibroblast cells were cultured on fibronectin-coated adhesive islands with the same area (900 micrometers2) but different geometric forms (square, triangle, pentagon, hexagon, trapezoid, various parallelograms) and aspect ratios, cells preferentially extended new lamellipodia from their corners. In addition, by imposing these simple geometric constraints through ECM, cells were directed to deposit new fibronectin fibrils in these same corner regions. These data indicate that mammalian cells can sense edges within ECM patterns that exhibit a wide range of angularity and that they use these spatial cues to guide where they will deposit ECM and extend new motile processes during the process of directional migration. 相似文献
1000.
Moore AA Jacobson ML Belabas N Rowlen KL Jonas DM 《Journal of the American Chemical Society》2005,127(20):7292-7293
The role and the nature of the continuum in Surface Enhanced Raman Spectroscopy (SERS) are unclear. Here, two-dimensional (2D) covariance and correlation analysis is applied to single molecule SERS spectra on silver colloids with and without rhodamine 6G (native colloid). The resulting 2D covariance and correlation maps show that the sharp molecular Raman peaks from rhodamine 6G and the molecule responsible for the SERS peaks from the native colloid are correlated to different continua even though both continua are present in each data set. This suggests that two distinct active sites on the silver colloids produce the two different continua, and that each site has some molecular specificity. 相似文献