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   318篇
  免费   9篇
化学   248篇
数学   33篇
物理学   46篇
  2022年   3篇
  2021年   6篇
  2020年   7篇
  2019年   3篇
  2018年   7篇
  2017年   3篇
  2016年   10篇
  2015年   6篇
  2014年   11篇
  2013年   14篇
  2012年   18篇
  2011年   21篇
  2010年   16篇
  2009年   10篇
  2008年   15篇
  2007年   27篇
  2006年   14篇
  2005年   21篇
  2004年   17篇
  2003年   12篇
  2002年   17篇
  2001年   8篇
  2000年   2篇
  1999年   5篇
  1998年   2篇
  1997年   2篇
  1996年   3篇
  1995年   4篇
  1994年   6篇
  1993年   2篇
  1992年   2篇
  1991年   3篇
  1990年   4篇
  1989年   2篇
  1988年   1篇
  1987年   2篇
  1985年   1篇
  1984年   2篇
  1983年   1篇
  1982年   1篇
  1981年   2篇
  1980年   1篇
  1979年   3篇
  1978年   2篇
  1977年   2篇
  1976年   1篇
  1975年   1篇
  1974年   2篇
  1971年   2篇
排序方式: 共有327条查询结果,搜索用时 625 毫秒
101.
Summary: Polyaniline nanofibers (from 93 to 220 nm) doped with β‐naphtalenesulfonic acid (PANI‐NSA) have been characterized. Nonhomogeneity in the thickness of fibers, as can be seen with scanning electron microscopy, is the reason for their instability under the HCl doping process. Resonance Raman spectral profiles of PANI‐NSA fibers suggest a lower protonation state than the emeraldine salt forms. Observation of Raman bands, which are a result of cross‐linking, confirms that one of the properties of the polymeric structure formed in micellar polymerization is the type of connection between chains.

  相似文献   

102.
103.
The infections caused by the Herpes Simplex Virus are one of the most common sources of diseases in adults and several natural nucleoside analogues are currently used in the treatment of these infections. In vitro tests of a series of quinolonic ribonucleosides derivatives synthesized by part of our group indicated that some of them have antiviral activity against HSV-1.The conformational analysis of bioactive compounds is extremely important in order to better understand their chemical structures and biological activity. In this work, we have carried out a nuclear relaxation NMR study of 6-Me ribonucleoside derivative in order to determine if the syn or anti conformation is preferential. The NMR analysis permits the determination of inter-atomic distances by using techniques which are based on nuclear relaxation and related phenomena. Those techniques are non-selective longitudinal or spin-lattice relaxation rates and NULL pulse sequence, which allow the determination of distances between pairs of hydrogen atoms. The results of NMR studies were compared with those obtained by molecular modeling.  相似文献   
104.
A mechanism for chemical attachment and growth of a Cu-BTC Metal–Organic Framework, also known as MOF-199 or HKUST-1, onto cellulosic substrates is reported. Four different experimental procedures were attempted in order to elucidate the role of carboxylate groups on the anionic cellulose’s surface. The order of addition of Cu(OAc)2—copper acetate, BTH3, 1,3,5-benzenetricarboxylic acid and TEA—Triethylamine was found to be a critical factor for the attachment and growth of the MOF-199 crystals onto anionic cellulose. The presence of MOF-199 crystals was probed using XRD and XPS spectra and a strong chemical interaction to the carboxymethylated cellulose fibers was confirmed by intense and vigorous washing of the specimens with water, DMF and methanol. Based on the recognized ability of MOF-199 to capture gases and toxic chemicals, combined with the availability of cellulose-based fibrous materials, the described procedure provides the basis for future fabrication of functionalized fibers and active filtration media.  相似文献   
105.
Molecular dynamics simulations are used to examine the relationship between water-like anomalies and the liquid-liquid critical point in a family of model fluids with multi-Gaussian, core-softened pair interactions. The core-softened pair interactions have two length scales, such that the longer length scale associated with a shallow, attractive well is kept constant while the shorter length scale associated with the repulsive shoulder is varied from an inflection point to a minimum of progressively increasing depth. The maximum depth of the shoulder well is chosen so that the resulting potential reproduces the oxygen-oxygen radial distribution function of the ST4 model of water. As the shoulder well depth increases, the pressure required to form the high density liquid decreases and the temperature up to which the high-density liquid is stable increases, resulting in the shift of the liquid-liquid critical point to much lower pressures and higher temperatures. To understand the entropic effects associated with the changes in the interaction potential, the pair correlation entropy is computed to show that the excess entropy anomaly diminishes when the shoulder well depth increases. Excess entropy scaling of diffusivity in this class of fluids is demonstrated, showing that decreasing strength of the excess entropy anomaly with increasing shoulder depth results in the progressive loss of water-like thermodynamic, structural and transport anomalies. Instantaneous normal mode analysis was used to index the overall curvature distribution of the fluid and the fraction of imaginary frequency modes was shown to correlate well with the anomalous behavior of the diffusivity and the pair correlation entropy. The results suggest in the case of core-softened potentials, in addition to the presence of two length scales, energetic, and entropic effects associated with local minima and curvatures of the pair interaction play an important role in determining the presence of water-like anomalies and the liquid-liquid phase transition.  相似文献   
106.
Abstract

The inclusion properties of the first helical tubuland diol hosts 1–5 have been surveyed and are reported. Bicyclic diol 1 forms helical tubulate inclusion compounds with most small solvent molecules, but with certain phenols hydrogen bonded co-crystalline materials are produced instead. Diol 2 yields helical tubulates with larger guest molecules, but if smaller solvents are used then an alternative host system is obtained. This has the guests trapped in ellipsoidal cavities located along constricted canals and is termed the ellipsoidal clathrate type. In some cases both inclusion types can be produced from 2 by varying the experimental conditions. The free volume present in the helical tubuland lattices of 3 and 5 is insufficient for guest inclusion. Diol 4 has the largest void space and can form inclusion compounds with large molecules such as ferrocene and squalene. X-ray crystal structures of representative examples of these compounds are discussed.  相似文献   
107.
A number of inorganic anions (e.g., nitrate, fluoride, bromate, phosphate, and perchlorate) have been reported in alarming concentrations in numerous drinking water sources around the world. Their presence even in very low concentrations may cause serious environmental and health related problems. Due to the presence and significance of iron minerals in the natural aquatic environment and increasing application of iron in water treatment, the knowledge of the structure of iron and iron minerals and their interactions with aquatic pollutants, especially inorganic anions in water are of great importance. Iron minerals have been known since long as potential adsorbents for the removal of inorganic anions from aqueous phase. The chemistry of iron and iron minerals reactions in water is complex. The adsorption ability of iron and iron minerals towards inorganic anions is influenced by several factors such as, surface characteristics of the adsorbent (surface area, density, pore volume, porosity, pore size distribution, pHpzc, purity), pH of the solution, and ionic strength. Furthermore, the physico-chemical properties of inorganic anions (pore size, ionic radius, bulk diffusion coefficient) also significantly influence the adsorption process. The aim of this paper is to provide an overview of the properties of iron and iron minerals and their reactivity with some important inorganic anionic contaminants present in water. It also summarizes the usage of iron and iron minerals in water treatment technology.  相似文献   
108.
A mild, practical, and simple procedure for peptide‐bond formation is reported. Instead of activation of the carboxylic acid functionality, the reaction involves an unprecedented use of activated α‐aminoesters. The method provides a straightforward entry to dipeptides and was effective when a sensitive cysteine residue was used, as no epimerization was detected in this case. The applicability of this method to iterative peptide synthesis was illustrated by the synthesis of a model tetrapeptide in the challenging reverse N→C direction.  相似文献   
109.
A free‐base porphyrin carrying two hexabenzocoronene (HBC) substituents in a trans arrangement and its zinc complex have been prepared. The compounds were characterized extensively and found to form tricationic dimers in the gas phase. X‐ray crystallography confirms for the zinc complex a profound π‐stacking of the HBC moieties. In contrast, the free‐base porphyrin incarcerates n‐heptane which essentially prevents π‐stacking. Upon excitation of the HBC substituents, efficient energy transfer to the central porphyrin is observed.  相似文献   
110.
Summary We consider a spin system with competing interactions isotropic with respect to the axes of a cubic lattice in the presence of an external field. We show that for small values of the external fieldH, the paramagnetic to modulated phase transition is fluctuation-induced first order, while for larger fields, such transition changes to continuous at a tricritical point. Applications for fluids systems are proposed. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.  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