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   426篇
  免费   6篇
化学   254篇
力学   16篇
数学   78篇
物理学   84篇
  2022年   3篇
  2021年   4篇
  2018年   5篇
  2017年   3篇
  2016年   13篇
  2015年   10篇
  2014年   14篇
  2013年   18篇
  2012年   16篇
  2011年   22篇
  2010年   17篇
  2009年   13篇
  2008年   18篇
  2007年   16篇
  2006年   24篇
  2005年   11篇
  2004年   13篇
  2003年   6篇
  2002年   11篇
  2001年   10篇
  2000年   3篇
  1999年   3篇
  1998年   6篇
  1997年   5篇
  1996年   3篇
  1995年   8篇
  1994年   5篇
  1993年   3篇
  1992年   2篇
  1989年   6篇
  1988年   2篇
  1986年   2篇
  1985年   2篇
  1984年   4篇
  1983年   6篇
  1982年   5篇
  1981年   4篇
  1980年   2篇
  1979年   2篇
  1978年   5篇
  1977年   3篇
  1976年   4篇
  1974年   2篇
  1972年   3篇
  1970年   2篇
  1968年   2篇
  1967年   2篇
  1966年   3篇
  1961年   2篇
  1960年   70篇
排序方式: 共有432条查询结果,搜索用时 15 毫秒
141.
142.
Abstract

The leaving groups of organophosphorus compounds and carbo-xylic esters are differently located in the active centers of cholinesterases that is connected with the stereochemi-cal regularities of the nucleophilic displacement reactions occurring at the carboxyl and phosphoryl groups of these reagents. This different spatial location of quasisubstrates in the enzyme active center also reveals in the mechanism of the bond-breaking steps leading to the formation of the covalent enzymereagent intermediate. In the present coinmunication the mechanism of these reaction steps is discussed with the implication of the putative structures of the transition states. The analysis is based on the results of QSAR for the leaving group effects of ester substrates and organophosphorus quasisubstrates in their reactions with acetylcholinesterase and butyrylcholinesterase. It has been found that the sensitivity of the enzyme phosphorylat-ion rate against the inductive effect of the quasisubstrate leaving group was remarkably nigh pointing to the complete separation of charges in the transition state. On the other hand, the transition state of the butyrylcholinesterase acylation reaction has been found to be similar to the structure of the appropriate acyl-enzyme. aesides these two enzymes the reactions of ester substrates and organophosphorus quasisubstrates have been studied with a covalently modified derivative of acetylcholinesterase, synthesized by affinity labelling of the native enzyme with N,N-dimethyl-2-phenylaziridinium ions, The data obtained allow to discuss the role of the putative “anionic point” of acetylcholinesterase in the enzyme reactions with organophosphorus quasisubstrates and ester substrates.  相似文献   
143.
A class of reversible Markov jump processes on a periodic lattice is described and a result about their scaling behavior stated: Under diffusion scaling, the empirical measure converges to a solution of the porous medium equation on thed-dimensional torus. The process can be viewed as a randomly interacting configuration of sticks that evolves through exchanges of stick pieces between nearest neighbors through a zero-range pressure mechanism, with conservation of total stick length.  相似文献   
144.
The atom transfer radical polymerization of methyl methacrylate (MMA) and n‐butyl methacrylate (n‐BMA) was initiated by a poly(ethylene oxide) chloro telechelic macroinitiator synthesized by esterification of poly(ethylene oxide) (PEO) with 2‐chloro propionyl chloride. The polymerization, carried out in bulk at 90 °C and catalyzed by iron(II) chloride tetrahydrate in the presence of triphenylphosphine ligand (FeCl2 · 4H2O/PPh3), led to A–B–A amphiphilic triblock copolymers with MMA or n‐BMA as the A block and PEO as the B block. A kinetic study showed that the polymerization was first‐order with respect to the monomer concentration. Moreover, the experimental molecular weights of the block copolymers increased linearly with the monomer conversion, and the molecular weight distribution was acceptably narrow at the end of the reaction. These block copolymers turned out to be water‐soluble through the adjustment of the content of PEO blocks (PEO content >90% by mass). When the PEO content was small [monomer/macroinitiator molar ratio (M/I) = 300], the block copolymers were water‐insoluble and showed only one glass‐transition temperature. With an increase in the concentration of PEO (M/I = 100 or 50) in the copolymer, two glass transitions were detected, indicating phase separation. The macroinitiator and the corresponding triblock copolymers were characterized with Fourier transform infrared, proton nuclear magnetic resonance, size exclusion chromatography analysis, dynamic mechanical analysis, and differential scanning calorimetry. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5049–5061, 2005  相似文献   
145.
Cationic polymers bind DNA and form compacted nanoparticulates (i.e., polyplexes). Polyplexes augment DNA delivery into the cells as a nonviral method of gene therapy. DNA packing and release are the key factors in polyplex-mediated gene delivery, but they are poorly understood due to the lack of physical methods of investigation. We used time-resolved fluorescence spectroscopy to study poly(ethylenimine) (PEI) and poly(L-lysine) (PLL) polyplexes. Analysis of fluorescence lifetimes and time-resolved spectra revealed that DNA exists in several different states in PEI polyplexes and only in one tightly bound state in PLL polyplexes. The observed difference in the nature of the polyplexes may explain why PEI releases DNA more easily than PLL even though both polycations condense DNA effectively. The present method utilizing time-resolved fluorescence spectroscopy gives information on the specific interactions between DNA and the cationic polymers in the polyplexes. This kind of information is very important in the development of biologically effective nonviral systems for DNA delivery.  相似文献   
146.
An alternative NMR method for determining nuclear shielding anisotropies in molecules is proposed. The method is quite simple, linear and particularly applicable for heteronuclear spin systems. In the technique, molecules of interest are dissolved in a thermotropic liquid crystal (LC) which is confined in a mesoporous material, such as controlled pore glass (CPG) used in this study. CPG materials consist of roughly spherical particles with a randomly oriented and connected pore network inside. LC Merck Phase 4 was confined in the pores of average diameter from 81 to 375 A and LC Merck ZLI 1115 in the pores of average diameter 81 A. In order to demonstrate the functionality of the method, the (13)C shielding anisotropy of (13)C-enriched methyl iodide, (13)CH(3)I, was determined as a function of temperature using one dimensional (13)C NMR spectroscopy. Methane gas, (13)CH(4), was used as an internal chemical shift reference. It appeared that methyl iodide molecules experience on average an isotropic environment in LCs inside the smallest pores within the whole temperature range studied, ranging from bulk solid to isotropic phase. In contrast, in the spaces in between the particles, whose diameter is approximately 150 microm, LCs behave as in the bulk. Consequently, isotropic values of the shielding tensor can be determined from spectra arising from molecules inside the pores at exactly the same temperature as the anisotropic ones from molecules outside the pores. Thus, for the first time in the solution state, shielding anisotropies can easily be determined as a function of temperature. The effects of pore size as well as of different LC media on the shielding anisotropy are examined and discussed.  相似文献   
147.
The isotropic average, JisoHgC, and the anisotropy, DeltaJHgC, of the 199Hg-13C spin-spin coupling tensor in methylmercury halides, CH3HgX (X=Cl, Br, I), were determined for the first time by utilizing the NMR spectra of these molecules dissolved in liquid crystals. Furthermore, density functional calculations were performed using the zeroth-order regular approximation, including also dimethylmercury. The temperature-dependence of the JisoHgC couplings in the isotropic phase was studied in each case in order to extrapolate their values into the liquid crystal state. Good agreement is found between the experimental and the calculated DeltaJHgC values as long as solvent effects are considered in the computations. Most of the magnitude of DeltaJ can be attributed to the spin mechanism of J-coupling, with additional sizable spin-orbital cross terms due to electronic spin-orbit coupling.  相似文献   
148.
Reliable and effective virtual high-throughput screening (vHTS) methods are desperately needed to minimize the expenses involved in drug discovery projects. Here, we present an improvement to the negative image-based (NIB) screening: the shape, the electrostatics, and the solvation state of the target protein's ligand-binding site are included into the vHTS. Additionally, the initial vHTS results are postprocessed with molecular mechanics/generalized Born surface area (MMGBSA) calculations to estimate the favorability of ligand-protein interactions. The results show that docking produces very good early enrichment for phosphodiesterase-5 (PDE-5); however, in general, the NIB and the ligand-based screening performed better with or without the added electrostatics. Furthermore, the postprocessing of the NIB screening results using MMGBSA calculations improved the early enrichment for the PDE-5 considerably, thus, making hit discovery affordable.  相似文献   
149.
Emerging contaminants from wastewater effluent samples were analysed, using posttarget and nontarget analysis techniques. The samples were analysed with an ultra performance liquid chromatograph‐time‐of‐flight mass spectrometer (UPLC‐TOF‐MS), and the resulting data were processed with commercial deconvolution software. The method works well for posttarget analysis with prior information about the retention times of the compounds of interest. With positive polarity, 63 of 66 compounds and with negative polarity, 18 of 20 compounds were correctly identified in a spiked sample, while two compounds of a total of 88 fell out of the mass range. Furthermore, a four‐stage process for identification was developed for the posttarget analysis lacking the retention time data. In the process, the number of candidate compounds was reduced by using the accurate mass of selected compounds in two steps (stages 1 and 2), structure–property relationships (stage 3) and isotope patterns of the analytes (stage 4). The process developed was validated by analysing wastewater samples spiked with 88 compounds. This procedure can be used to gain a preliminary indication of the presence of certain analytes in the samples. Nontarget analysis was tested by applying a theoretical mass spectra library for a wastewater sample spiked with six pharmaceuticals. The results showed a high number of false identifications. In addition, manual processing of the data was considered laborious and ineffective. Finally, the posttarget analysis was applied to a real wastewater sample. The analysis revealed the presence of six compounds that were afterwards confirmed with standard compounds as being correct. Three psycholeptics (nordiazepam, oxazepam and temazepam) could be tentatively identified, using the identification process developed. Posttarget analysis with UPLC‐TOF‐MS proved to be a promising method for analysing wastewater samples, while we concluded that the software for nontarget analysis will need improvement before it can be used in environmental analytical work with LC‐TOF‐MS systems. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   
150.
The structural characteristics of fully‐hydrogenated carbon and boron nitride mono‐ and multilayer slabs, together with nanotubes derived from the slabs, are investigated mainly by means of periodic local second‐order Møller–Plesset perturbation (LMP2) calculations and the results are compared with Hartree–Fock (HF), density functional theory (DFT), and dispersion function‐augmented DFT (DFT‐D) obtained ones. The investigated systems are structurally analogous to (111) and (110) slabs of diamond, where the hydrogenated (111) slab of diamond corresponds to the experimentally known graphane. Multilayering of monolayers and nanotubes is energetically favorable at the LMP2 level for both C and BN, while HF and DFT are not able to reproduce this behavior for CH systems. The work highlights the importance of utilizing methods capable of properly describing weak interactions in the investigation of dispersively‐bound systems such as the multilayered graphanes and the corresponding nanotubes.  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