首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   751篇
  免费   28篇
  国内免费   3篇
化学   485篇
晶体学   9篇
力学   12篇
数学   85篇
物理学   191篇
  2023年   5篇
  2022年   7篇
  2021年   6篇
  2020年   7篇
  2019年   19篇
  2018年   7篇
  2017年   9篇
  2016年   25篇
  2015年   15篇
  2014年   8篇
  2013年   27篇
  2012年   35篇
  2011年   41篇
  2010年   24篇
  2009年   24篇
  2008年   62篇
  2007年   48篇
  2006年   57篇
  2005年   37篇
  2004年   21篇
  2003年   23篇
  2002年   32篇
  2001年   28篇
  2000年   12篇
  1999年   6篇
  1998年   5篇
  1997年   9篇
  1996年   8篇
  1995年   9篇
  1994年   5篇
  1993年   7篇
  1992年   19篇
  1991年   11篇
  1990年   11篇
  1989年   12篇
  1988年   9篇
  1987年   6篇
  1986年   8篇
  1985年   13篇
  1984年   8篇
  1983年   7篇
  1982年   7篇
  1981年   5篇
  1980年   10篇
  1978年   8篇
  1977年   3篇
  1975年   2篇
  1974年   3篇
  1971年   2篇
  1930年   2篇
排序方式: 共有782条查询结果,搜索用时 109 毫秒
11.
The orientation of hydrogenase bound covalently to a pyrolytic graphite edge electrode modified with a 4-aminophenyl monolayer can be modulated via electrostatic interactions during the immobilization step. At low ionic strength and when the amino groups of the electrode surface are mostly protonated, the hydrogenase is immobilized with the negatively charged region that surrounds its 4Fe4S cluster nearer to the protein surface facing the electrode. This allows direct electron transfer between the immobilized hydrogenase and the electrode, which is observed by the strong catalytic currents measured in the presence of the H2 substrate. Therefore, a very stable enzymatic electrode is produced that catalyzes nonmediated H2 oxidation.  相似文献   
12.
Abstract— Absorption and fluorescence spectra of 5′-deoxypyridoxal (DPL) in various pure solvents and mixtures were recorded both at room temperature and over the range10–65°C. The areas under the absorption bands were analyzed to obtain the mole fraction (fN, fz) of two tautomers (the zwitterionic, Z, and neutral, N, forms) in the ground state. The following spectral parameters were determined from the fluorescence spectra: Stokes shift (Δv), fluorescence quantum yield of the neutral form (QN), fluorescence ratio of the neutral to the zwitterionic form (øNZ) and the rate constant of tautomerization (k1) from Z to N in the excited state. Some of these parameters (fN, Δv, QN, k1) were found to depend on the proton donor character of the solvent, whereas others (øNZ) depended on its dipole moment. Thus, the absorption and fluorescence spectra of DPL allow one to obtain information on the polarity and the concentration of –OH groups on its environment.  相似文献   
13.
Calculations involving molecular orbitals have appeared in the chemical literature over the past few years, but all have used smalltrans-polyenes. We now report extended Huckel molecular orbital calculations ontrans-polyenes of up to 18 double bonds (36 carbons and 38 hydrogen atoms). Information obtained from these calculations include total energies, bond gaps, and net charges on each atom. Also found is that the band gap approaches 1.41 eV as the number of double bonds approaches infinity. This value is quite close to reported experimental values. Data are also presented for polyenes assuming equal C-C bond lengths.  相似文献   
14.
1-Azatricyclo[3.3.1.13-7]decan-4-one (4-oxo-1-azaadamantane) and 1-azatricyclo[3.3.1.13-7]decane-4-α(β)-ol (4-α-(β)-hydroxy-1-azaadamantane) have been studied by 1H and 13C nmr methods. From this study several stereo and stereoelectronic effects have been deduced. The complete proton and carbon chemical shift assignments for the title compounds have been made, with the aid of two-dimensional nmr techniques.  相似文献   
15.
A compound formulated as (C4H12N2)[Ge2(pmida)2(OH)2] x 4 H2O (where pmida(4-) = N-(phosphonomethyl)iminodiacetate and C4H12N2(2+) = piperazinedium cation), containing the anionic [Ge2(pmida)2(OH)2]2- complex, has been synthesised by the hydrothermal approach and its structure determined by single-crystal X-ray diffraction analysis. Several high-resolution solid-state magic-angle spinning (MAS) NMR techniques, in particular two-dimensional 1H-X(13C,31P) heteronuclear correlation (HETCOR) and 1H-1H homonuclear correlation (HOMCOR) experiments incorporating a frequency-switched Lee-Goldburg (FS-LG) decoupling scheme, have been employed for the first time in such a material. Using these tools in tandem affords an excellent general approach to study the structure of other inorganic-organic hybrids. We assigned the NMR resonances with the help of C...H and P...H internuclear distances obtained through systematic statistical analyses of the crystallographic data. The compound was further characterised by powder X-ray diffraction techniques, IR and Raman spectroscopy, and by elemental and thermal analyses (thermogravimetric analysis and differential scanning calorimetry).  相似文献   
16.
The addition of the lithium anions derived from (R)- and (S)-methyl and -ethyl p-tolyl sulfoxides to (S)-N-benzylidene-p-toluenesulfinamide provides an easy access route to enantiomerically pure beta-(N-sulfinyl)amino sulfoxides. Stereoselectivity can be achieved when the configurations at the sulfur atoms of the two reagents are opposite (matched pair), thus resulting in only one diastereoisomer, even for the case in which two new chiral centers are created. The N-sulfinyl group primarily controls the configuration of the carbon bonded to the nitrogen, whereas the configuration of the alpha-sulfinyl carbanion seems to be responsible for the level of asymmetric induction, as well as for the configuration of the new stereogenic C-SO carbon in the reactions with ethyl p-tolyl sulfoxides. An efficient method for transforming the obtained beta-(N-sulfinyl)amino sulfoxides into optically pure beta-amino alcohols, based on the stereoselective non-oxidative Pummerer reaction, is also reported.  相似文献   
17.
Several 5,6-dialkyl-2,4-diarylpyrimidines were prepared and their electron ionization (EI) mass spectra reported. The benzylic cleavage takes place easily together with an important McLafferty rearrangement. The involvement of the nitrogen atom appears to be important in the fragmentation of 5-methyl-substituted pyrimidines. In contrast, the 6-methyl-substituted pyrimidines undergo benzylic cleavage without hydrogen transfer. Thus, the difference in the mass spectrometric behaviour allows the identification of these isomeric compounds which, in contrast, exhibit only small differences in their NMR spectra. Copyright 1999 John Wiley & Sons, Ltd.  相似文献   
18.
The structures of three brominated terpenoids which are natural products from the red alga Laurencia pinnatifida (Gmal. Lamour) are described. The structures of the sesquiterpenes 4 and 5 were determined by spectral comparison and chemical interconversion. The structure of the squalene-derived terpenoid 8 was secured by chemical transformation into thyrsiferol, a brominated triterpene previously isolated from the red alga Laurencia thyrsifera.  相似文献   
19.
Structural characteristics of CeO(2)-ZrO(2)/TiO(2) (CZ/T) and V(2)O(5)/CeO(2)-ZrO(2)/TiO(2) (V/CZ/T) mixed oxide catalysts have been investigated using X-ray diffraction (XRD), BET surface area, Raman spectroscopy (RS), and high-resolution transmission electron microscopy (HREM) techniques. The CeO(2)-ZrO(2) (1:1 mole ratio) solid solution was deposited over a finely powdered TiO(2) support by a deposition precipitation method. A nominal 5 wt % V(2)O(5) was impregnated over the calcined (773 K) CZ/T mixed oxide carrier by a wet impregnation technique. The obtained CZ/T and V/CZ/T samples were further subjected to thermal treatments from 773 to 1073 K to understand the dispersion and temperature stability of these materials. In the case of CZ/T samples, the XRD results suggest the formation of different cubic and tetragonal Ce-Zr-oxide phases, Ce(0.75)Zr(0.25)O(2), Ce(0.6)Zr(0.4)O(2), Ce(0.5)Zr(0.5)O(2), and Ce(0.16)Zr(0.84)O(2) in varying proportions depending on the treatment temperature. With increasing calcination temperature from 773 to 1073 K, the intensity of the lines pertaining to cubic Ce(0.6)Zr(0.4)O(2) and Ce(0.5)Zr(0.5)O(2) phases increased at the expense of cubic Ce(0.75)Zr(0.25)O(2), indicating more incorporation of zirconia into the ceria lattice. The TiO(2) was mainly in the anatase form whose crystallite size also increased with increasing treatment temperature. A better crystallization and more incorporation of zirconia into the ceria lattice was noted when CZ/T was impregnated with V(2)O(5). However, no crystalline V(2)O(5) could be seen from both XRD and RS measurements. In particular, a preferential formation of CeVO(4) compound and an intense tetragonal Ce(0.16)Zr(0.84)O(2) phase were noted beyond 873 K. The HREM results indicate, in the case of CZ/T samples, a well-dispersed Ce-Zr-oxide of the size approximately 5 nm over the bigger crystals ( approximately 40 nm) of TiO(2) when treated at 873 K. The exact structural features of these crystals as determined by digital diffraction analysis of experimental images reveal that the Ce-Zr-oxides are mainly in the cubic fluorite geometry and the TiO(2) is in anatase form. A better crystallization of Ce-Zr-oxides ( approximately 8 nm) over the surface of bigger crystals of TiO(2) was noted at 1073 K. A further enhancement in the crystallite size and zirconia-rich tetragonal phase was noted in the case of V/CZ/T samples. Further, the structure of CeVO(4) formed was also clearly identified in conformity with XRD and RS results.  相似文献   
20.
Ce(x)Zr(1)(-)(x)O(2) solid solutions deposited over silica surface were investigated by X-ray diffraction (XRD), Raman spectroscopy (RS), and high-resolution transmission electron microscopy (HREM) techniques in order to understand the role of silica support and the temperature stability of these composite oxides. For the purpose of comparison, an unsupported Ce(x)Zr(1)(-)(x)O(2) was also synthesized and subjected to characterization by various techniques. The Ce(x)Zr(1)(-)(x)O(2)/SiO(2) (CZ/S) (1:1:2 mole ratio based on oxides) was synthesized by depositing Ce(x)Zr(1)(-)(x)O(2) solid solution over a colloidal SiO(2) support by a deposition precipitation method and unsupported Ce(x)Zr(1)(-)(x)O(2) (CZ) (1:1 mole ratio based on oxides) was prepared by a coprecipitation procedure, and the obtained catalysts were subjected to thermal treatments from 773 to 1073 K. The XRD measurements disclose the presence of cubic phases with the composition Ce(0.75)Zr(0.25)O(2) and Ce(0.6)Zr(0.4)O(2) in CZ samples, while CZ/S samples possess Ce(0.75)Zr(0.25)O(2), Ce(0.6)Zr(0.4)O(2), and Ce(0.5)Zr(0.5)O(2) in different proportions. The crystallinity of these phases increased with increasing calcination temperature. The cell a parameter estimations indicate contraction of ceria lattice due to the incorporation of zirconium cations into the CeO(2) unit cell. Raman measurements indicate the presence of oxygen vacancies, lattice defects, and displacement of oxygen ions from their normal lattice positions in both the series of samples. The HREM results reveal, in the case of CZ/S samples, a well-dispersed nanosized Ce-Zr-oxides over the surface of amorphous SiO(2). The structural features of these crystals as determined by digital diffraction analysis of experimental images reveal that the Ce-Zr-oxides are mainly in the cubic geometry and exhibit high thermal stability. Oxygen storage capacity measurements by a thermogravimetric method reveal a substantial enhancement in the oxygen vacancy concentration of CZ/S sample over the unsupported CZ sample.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号