全文获取类型
收费全文 | 4626篇 |
免费 | 584篇 |
国内免费 | 420篇 |
专业分类
化学 | 2579篇 |
晶体学 | 50篇 |
力学 | 591篇 |
综合类 | 42篇 |
数学 | 558篇 |
物理学 | 1810篇 |
出版年
2024年 | 10篇 |
2023年 | 39篇 |
2022年 | 104篇 |
2021年 | 93篇 |
2020年 | 134篇 |
2019年 | 118篇 |
2018年 | 134篇 |
2017年 | 159篇 |
2016年 | 217篇 |
2015年 | 178篇 |
2014年 | 301篇 |
2013年 | 376篇 |
2012年 | 240篇 |
2011年 | 308篇 |
2010年 | 237篇 |
2009年 | 278篇 |
2008年 | 274篇 |
2007年 | 269篇 |
2006年 | 304篇 |
2005年 | 230篇 |
2004年 | 239篇 |
2003年 | 209篇 |
2002年 | 165篇 |
2001年 | 128篇 |
2000年 | 102篇 |
1999年 | 137篇 |
1998年 | 91篇 |
1997年 | 96篇 |
1996年 | 65篇 |
1995年 | 61篇 |
1994年 | 58篇 |
1993年 | 36篇 |
1992年 | 32篇 |
1991年 | 37篇 |
1990年 | 21篇 |
1989年 | 18篇 |
1988年 | 25篇 |
1987年 | 20篇 |
1986年 | 12篇 |
1985年 | 14篇 |
1984年 | 25篇 |
1983年 | 5篇 |
1982年 | 8篇 |
1981年 | 7篇 |
1980年 | 8篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 1篇 |
1975年 | 1篇 |
排序方式: 共有5630条查询结果,搜索用时 31 毫秒
141.
The role of achiral sorbent matrix in chiral recognition of amino acid enantiomers in ligand-exchange chromatography 总被引:2,自引:0,他引:2
Summary As an alternative to the known three-point interaction model describing recognition of optical isomers by a chiral resolving agent, a new concept has been developed stating that two interaction points between the resolving agent and the enantiomers are also sufficient for achieving chiral recognition of the latter, provided that the diastereomeric adducts formed by the resolving agent with the enantiomers additionally interact with a non-chiral chromatographic sorbent. This concept is based on the results of ligand-exchange chromatography of -amino acid enantiomers with copper(II) complexes of chiral bifunctional ligands as the resolving agents in chromatographic systems. 相似文献
142.
The analysis of complex mixtures of chlorinated paraffins (CPs) with short (SCCPs, C10–C13) and medium (MCCPs, C14–C17) chain lengths can be disturbed by mass overlap, if low resolution mass spectrometry (LRMS) in the electron capture negative ionization mode is employed. This is caused by CP congeners with the same nominal mass, but with five carbon atoms more and two chlorine atoms less; for example C11H1737Cl35Cl6
(m/z 395.9) and C16H2935Cl5
(m/z 396.1). This can lead to an overestimation of congener group quantity and/or of total CP concentration. The magnitude of this interference was studied by evaluating the change after mixing a SCCP standard and a MCCP standard 1+1 (S+MCCP mixture) and comparing it to the single standards. A quantification of the less abundant C16 and C17 congeners present in the MCCP standard was not possible due to interference from the major C11 and C12 congeners in the SCCPs. Also, signals for SCCPs (C10–C12) with nine and ten chlorine atoms were mimicked by MCCPs (C15–C17) with seven and eight chlorine atoms (for instance C10H12Cl10
by C15H24Cl8). A similar observation was made for signals from C15–C17 CPs with four and five chlorine atoms resulting from SCCPs (C10–C12) with six and seven chlorine atoms (such as C15H28Cl4
by C10H16Cl6) in the S+MCCP mixture. It could be shown that the quantification of the most abundant congeners (C11–C14) is not affected by any interference. The determination of C10 and C15 congeners is partly disturbed, but this can be detected by investigating isotope ratios, retention time ranges and the shapes of the CP signals. Also, lower chlorinated compounds forming [M+Cl]– as the most abundant ion instead of [M-Cl]– are especially sensitive to systematic errors caused by superposition of ions of different composition and the same nominal mass. 相似文献
143.
Luo J Zheng QY Chen CF Huang ZT 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(20):5917-5928
A series of inherently chiral calix[4]arenes with cone and partial cone conformations and with crown ether moieties of variable size have been readily synthesized. By taking advantage of the carboxy appendage on the lower rim, these were condensed with the chiral auxiliary (S)-BINOL to form diastereomers which, in most cases, could be separated by preparative TLC, or more desirably, by column chromatography on silica gel (diastereomeric excess >99 % based on HPLC analysis). Seven enantiopure antipodes of inherently chiral calix[4]crowns were obtained after hydrolysis. It has been found that both the size of the crown moiety and alkylation of the last phenolic hydroxy group (accompanied with or without a change in the conformation) affect the separation of the diastereomers. 相似文献
144.
An easy to handle and stable racemization catalyst for secondary alcohols is obtained by an in situ mixture of readily available [Ru(cymene)Cl2]2 with chelating aliphatic diamines. Optimization of the reaction revealed that N,N,N′,N′-tetramethyl-1,3-propanediamine as ligand racemizes aromatic alcohols completely within 5 h. This easy to handle and stable catalytic system is combined with a lipase-catalyzed resolution to provide an efficient dynamic kinetic resolution of secondary alcohols. 相似文献
145.
We present the implementation and demonstration of the Chebyshev pseudo-spectral method coupled with an adaptive mesh method for performing fast and highly accurate electrophoresis simulations. The Chebyshev pseudo-spectral method offers higher numerical accuracy than all other finite difference methods and is applicable for simulating all electrophoresis techniques in channels with open or closed boundaries. To improve the computational efficiency, we use a novel moving mesh scheme that clusters the grid points in the regions with poor numerical resolution. We demonstrate the application of the Chebyshev pseudo-spectral method on a moving mesh for simulating nonlinear electrophoretic processes through examples of isotachophoresis (ITP), isoelectric focusing (IEF), and electromigration-dispersion in capillary zone electrophoresis (CZE) at current densities as high as 1000 A/m. We also show the efficacy of our moving mesh method over existing methods that cluster the grid points in the regions with large concentration gradients. We have integrated the adaptive Chebyshev pseudo-spectral method in the open-source SPYCE simulator and verified its implementation with other electrophoresis simulators. 相似文献
146.
T. Nagai K. Sakai T. Asaka A. Yamazaki Y. Matsui 《Journal of solid state chemistry》2006,179(6):1898-1903
The crystal structure of our newly discovered Sr-Co-O phase is investigated in detail through high-resolution electron microscopy (HREM) techniques. Electron diffraction (ED) measurement together with energy dispersive X-ray spectroscopy (EDS) analysis show that an ampoule-synthesized sample contains an unknown Sr-Co-O ternary phase with monoclinic symmetry and the cation ratio of Sr/Co=1. From HREM images a layered structure with a regular stacking of a CdI2-type CoO2 sheet and a rock-salt-type Sr2O2 double-layered block is observed, which confirms that the phase is the parent of the more complex “misfit-layered (ML)” cobalt oxides of [MmA2Om+2]qCoO2 with the formula of [Sr2O2]qCoO2, i.e. m=0. It is revealed that the misfit parameter q is 0.5, i.e. the two sublattices of the CoO2 sheet and the Sr2O2 block coexist to form a commensurate composite structure. We propose a structural model with monoclinic P21/m symmetry, which is supported by simulations of ED patterns and HREM images based on dynamical diffraction theory. 相似文献
147.
A variety of optically active carboxylic esters are produced by the kinetic resolution of racemic secondary benzylic alcohols using free carboxylic acids with benzoic anhydride and tetramisole derivatives. 4-Methoxybenzoic anhydride (PMBA) is the best reagent to use in producing the corresponding esters in high ee when the reaction is catalyzed by (+)-benzotetramisole (BTM); by contrast, when non-substituted benzoic anhydride is used as a coupling reagent, the resulting optically active alcohols are obtained with high selectivities. This protocol directly produces chiral carboxylic esters from free carboxylic acids and racemic secondary alcohols by utilizing the trans-acylation process to generate mixed anhydrides from acid components and benzoic anhydride derivatives under the influence of chiral catalysts. 相似文献
148.
Summary The partial resolution of some drug enantiomers by counter-current extraction in an aqueous two-phase system using bovine serum albumin (BSA) or ovomucoid (OVM) as chiral discriminator is described. The phase system was prepared with dextran 40, polyethylene glycol and a protein. Ofloxacin (OFLX) enantiomer was well recognized by the BSA system, whereas the enantiomers of carvedilol (-blocker) and DG-5128 (a new oral hypoglycaemic drug) were hardly recognized by the OVM system. The distribution coefficients of (S)-, (R)- and rac-OFLX varied with pH. Good enantioselectivity for OFLX was obtained at relatively high pH ( pH 9). The crude optically enriched OFLX enantiomers were extracted from the aqueous fractions and finally purified by thin-layer chromatography. 相似文献
149.
《Journal of separation science》2002,25(18):1317-1324
Multidimensional gas chromatography (2D GC) is demonstrated as a way to improve limits of detectability of spectrophotometric detectors. UV and IR detectors are generally less sensitive than mass spectrometers or other GC detectors. This has placed some limitations on the useful capabilities provided by spectrophotometric detectors, such as the ability to provide structure‐related information for a particular analyte. In this paper, we report results from interfacing a 2D GC instrument to a UV detector. Symmetry factor and the ratio of retention time divided by peak width did not show deterioration of the quality of chromatography when a megabore column was used with this detector. Furthermore, an increase in the limits of detectability over that attainable in a single‐column system was realized by using the 2D GC system. However, the low flow (1 mL/min) imposed by the use of a microbore column (250 μm ID) caused significant tailing when the UV detector was used. 相似文献
150.
The bis(dibutylstannylene) derivative of 1,2-cyclohexylidene-myo-inositol reacted with (S)-O-acetylmandeloyl chloride and diphosphate tetraesters to give 3,6-dimandelate and 3-phosphate, respectively. Using the stannylene methodology for the optical resolution and regioselective phosphorylation of the ketal, a concise synthesis of phosphatidylinositol with the natural configuration was accomplished. 相似文献