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   1149篇
  免费   44篇
  国内免费   14篇
化学   803篇
晶体学   14篇
力学   10篇
数学   144篇
物理学   236篇
  2022年   11篇
  2021年   15篇
  2020年   14篇
  2019年   14篇
  2018年   17篇
  2017年   17篇
  2016年   31篇
  2015年   35篇
  2014年   37篇
  2013年   57篇
  2012年   63篇
  2011年   63篇
  2010年   54篇
  2009年   39篇
  2008年   75篇
  2007年   55篇
  2006年   47篇
  2005年   56篇
  2004年   39篇
  2003年   41篇
  2002年   44篇
  2001年   10篇
  2000年   13篇
  1998年   12篇
  1997年   9篇
  1996年   13篇
  1995年   7篇
  1994年   13篇
  1993年   8篇
  1992年   9篇
  1991年   8篇
  1990年   9篇
  1989年   8篇
  1985年   13篇
  1984年   9篇
  1982年   10篇
  1981年   8篇
  1980年   8篇
  1978年   6篇
  1977年   6篇
  1976年   7篇
  1975年   12篇
  1974年   7篇
  1973年   7篇
  1959年   13篇
  1958年   23篇
  1957年   16篇
  1956年   14篇
  1955年   11篇
  1954年   18篇
排序方式: 共有1207条查询结果,搜索用时 15 毫秒
21.
The configuration and dynamic behavior of O-allyl-S-methyl-N-(acridin-9-yl)iminothiocarbonate (1) and its S-allyl-O-methyl regioisomer (2) were studied using quantum chemical calculations and by applying a novel graphical method to scatter maps obtained from MD simulations for evaluation of an NOE-weighted internuclear distance (r(NOE)). Energy calculations indicated that the Z configuration was predominant for each compound and, further, this was supported both by the calculated chemical shifts and the r(NOE). Both N-inversion- and rotation-type transition-state structures were also calculated for the E/Z isomerization process, the results indicating that the preferred interconversion mechanism for 1 is N-inversion, but contrastingly, interconversion via rotation is equally as probable as N-inversion for 2. This supports the notion that one or the other or both pathways can be active and each system needs to be assessed on a case-by-case basis.  相似文献   
22.
Herein, we report the first use of chiral alkaloid brucine to synthesize novel porphyrin-brucine conjugate capable of acting as a gelator of methanol and acetonitrile at extremely low level of concentration. The synthesis, characterization and spectral properties of gelator based on a novel structural motif, quaternized alkaloid conjugates, are described. Different spectroscopic methods (1H NMR spectroscopy, Raman and infrared spectroscopy, and spectroscopy of electronic and vibrational circular dichroism) were used for characterization of the prepared organogel. The aggregation of the gelator studied by UV-vis spectroscopy and electronic circular dichroism showed the formation of chiral J-aggregates in water and water-methanol (1:1) mixture. A new methodology for the determination of functional groups involved in gel formation based on vibrational circular dichroism is presented.  相似文献   
23.
Fluorescence-detected magnetic resonance of triplets in zero magnetic field (FDMR), fluorescence fading (FF) due to triplet-formation, both at 4.2 K, and prompt fluorescence decay kinetics (FDK) at room temperature have been measured for free pheophorbide- a (f-Pheo) and bound (b-Pheo) to a synthetic polypeptide (L-L ys -L-A la -L-A la )n, dissolved in dimethylformamide (DMF). Fluorescence decay kinetics measurements of f-Pheo in DMF yielded 1-5 ns lifetimes, for b-Pheo in DMF a ~ 50 ps decay-component was found emitting at 730–750 nm. Zero-field splitting parameters |D| and |E| of the lowest triplet state T1 were determined from FDMR spectra as (337 and 24) 10-4 cm-1 for f-Pheo and (359 and 25) 10-4 cm-1 for b-Pheo, both in DMF. Decay rate constants of the three spin levels of T1 of b-Pheo ( K x= 1200 50 s-1, k y= 440 25 s-1, k z= 80 5 s-1) and relative steady-state populations (Nx= 28 2%, Ny= 47 2%, Nz= 26 2%) determined from FF curves predict a fluorescence decrease at the D–E and D + E FDMR transitions, whereas experimentally a fluorescence increase is observed. The FDMR sign-inversion results from singlet-singlet energy transfer from b-Pheo monomers to their aggregates, followed by fast intersystem crossing to T1. These results indicate that aggregates are formed by two or more b-Pheo molecules at different positions on the folded polypeptide chain. This situation resembles that in chlorophyll-proteins, containing low-lying traps, resulting from interaction of chromophores with other chromophores and with the protein environment.  相似文献   
24.
The locality of underground water, contaminated with cyanides, has been successfully cleaned by using the hydraulic barrier method (assembly of pumped wells) since 1986. The average cyanide concentrations in the outflow exceeded 35 m per litre. Contamination had to be eliminated before the discharge into the sewer system. The radiation approach “in situ” i.e. decomposition of cyanides by barrier, was applied and is still being used today. The cyanide concentration was lowered more than one order of magnitude. This process was approved by the Czechoslovak radiation security authorities and further applications of “in Situ” regeneration of underground water contamination is anticipated.  相似文献   
25.
26.
In a cellular environment, the presence of macromolecular cosolutes and membrane interfaces can influence the folding-unfolding behavior of proteins. Here we report on the pressure stability of alpha-chymotrypsin in the ternary system bis(2-ethylhexyl)sodium sulfosuccinate-octane-water using FTIR spectroscopy. The ternary system forms anionic reverse micelles which mimic cellular conditions. We find that inclusion of a single protein molecule in a reverse micelle does not alter its conformation. When pressurized in bulk water, alpha-chymotrypsin unfolds at 750 MPa into a partially unfolded structure. In contrast, in the ternary system, the same pressure increase induces a random coil-like unfolded state, which collapses into an amorphous aggregate during the decompression phase. It is suggested that the unfolding pathway is different in a cell-mimicking environment due to the combined effect of multiple factors, including confinement. A phase transition of the reverse micellar to the lamellar phase is thought to be essential to provide the conditions required for unfolding and aggregation, though the unfolding is not a direct result of the phase transition. Our observations therefore suggest that membranes may cause the formation of alternative conformations that are more susceptible to aggregation.  相似文献   
27.
The kinetic method is applied to differentiate and quantify mixtures of isomeric tripeptides based on the competitive dissociations of divalent metal ion-bound clusters in an ion trap mass spectrometer. This methodology is extended further to determine compositions of ternary mixtures of the isomers Gly-Gly-Ala (GGA), Ala-Gly-Gly (AGG), and Gly-Ala-Gly (GAG). This procedure also allows to perform chiral quantification of a ternary mixture of optical isomers. The divalent metal ion Ca(II) is particularly appropriate for isomeric distinction and quantification of the isobaric tripeptides Gly-Gly-Leu/Gly-Gly-Ile (GGL/GGI). Among the first-row transition metal ions, Cu(II) yields remarkably effective isomeric differentiation for both the isobaric tripeptides, GGI/GGL using GAG as the reference ligand, and the positional isomers GAG/GGA using GGI as the reference ligand. This is probably due to agostic bonding: alpha-agostic bonding occurs between Cu(II) and GAG and beta-agostic bonding between Cu(II) and GGI, each produces large but different steric effects on the stability of the Cu(II)-bound dimeric clusters. These data form the basis for possible future quantitative analyses of mixtures of larger peptides such as are generated, for example, in combinatorial synthesis of peptides and peptide mimics.  相似文献   
28.
Zusammenfassung Es wurde eine Mikromethode für die Bestimmung von Oxyäthylengruppen ausgearbeitet. Das Prinzip der Methode beruht darauf, daß Äthylenoxydverbindungen beim Sieden mit Jodwasserstoffsäure neben anderen Produkten eine äquivalente Menge Jod liefern, die durch Titration mit Natriumthiosulfat bestimmt wird.Es wurde eine einfache Apparatur erprobt, die selbst geringe Jodverluste verhindert.
Summary A micromethod has been worked out for the determination of oxyethylene groups. Its principle is based on the fact that on boiling with hydriodic acid, the ethylene oxide compounds yield, along with other products, an equivalent quantity of iodine, which can be measured by titration with sodium thiosulfate.A simple apparatus has been tested, which prevents the loss of even slight amounts of iodine.

Résumé On a mis au point une microméthode pour doser les groupes oxéthyles. Le principe de la méthode repose sur le fait que les composés de l'oxyde d'éthylène, portés à l'ébullition avec de l'acide iodhydrique, libèrent, à côté d'autres produits, une quantité équivalente d'iode que l'on titre par le thiosulfate de sodium.On a expérimenté un appareil simple qui empêche les pertes en iode, même faibles.


Herrn Dipl.-Ing.J. Simandl danken wir für die anregende Diskussion und die Überlassung von oxyäthylierten Aminen. Herrn Dipl.-Ing.J. Kami danken wir für die Überlassung von Proben reiner Polyglykoläther. Die Wasserbestimmung nachK. Fischer führte HerrZ. Netuil aus.  相似文献   
29.
Summary.  In this investigation the crystallization of PbTiO3 upon annealing of pure nanopowders and PbTiO3–SiO2 (1:1 v/v) nanocomposite powders prepared by the sol-gel technique was studied. Using X-ray diffraction phase analysis, the start of PbTiO3 crystallization in pure PbTiO3 powders was detected at 400°C. Distinct crystallization of PbTiO3 in PbTiO3–SiO2 nanocomposites starts at 700°C, whereas SiO2 remains amorphous. There are indications that an interface interaction between the PbTiO3 and the SiO2 phase plays an important role in hindering the crystallization of PbTiO3. The particle size (size of coherently scattering regions) was estimated from the broadening of the X-ray diffraction line profiles. The average size of PbTiO3 nanocrystallites increases with temperature and time of annealing, the influence of temperature being more significant than that of the annealing time. Differential scanning calorimetry confirmed the results of the X-ray diffraction with respect to the start of the crystallization. Laser beam scattering and scanning electron microscopy provided the statistical distribution of the grain size and the morphology of the powder grains, showing that each grain of the powders contains several nanocrystallites (coherently scattering regions). Received October 4, 2001. Accepted (revised) December 14, 2001  相似文献   
30.
A method has been developed for calculating more accurate and reproducible equivalent chain length (ECL) values of 33 straight-chain fatty acids (FA), with various positions and numbers of cis double bonds, chromatographed on DB-Wax and DB-1 columns. The dependence of ECL values on the position and the number of double bonds is described, as is a method of utilizing these relationships for the characterization of FA by gas chromatography.  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