全文获取类型
收费全文 | 1102篇 |
免费 | 22篇 |
国内免费 | 14篇 |
专业分类
化学 | 721篇 |
晶体学 | 15篇 |
力学 | 12篇 |
数学 | 228篇 |
物理学 | 162篇 |
出版年
2022年 | 18篇 |
2021年 | 21篇 |
2020年 | 7篇 |
2019年 | 12篇 |
2018年 | 24篇 |
2017年 | 10篇 |
2016年 | 21篇 |
2015年 | 31篇 |
2014年 | 42篇 |
2013年 | 78篇 |
2012年 | 60篇 |
2011年 | 69篇 |
2010年 | 40篇 |
2009年 | 41篇 |
2008年 | 61篇 |
2007年 | 44篇 |
2006年 | 37篇 |
2005年 | 58篇 |
2004年 | 38篇 |
2003年 | 40篇 |
2002年 | 41篇 |
2001年 | 17篇 |
2000年 | 11篇 |
1999年 | 12篇 |
1997年 | 19篇 |
1996年 | 14篇 |
1995年 | 18篇 |
1994年 | 16篇 |
1993年 | 12篇 |
1992年 | 11篇 |
1991年 | 9篇 |
1990年 | 13篇 |
1989年 | 12篇 |
1988年 | 8篇 |
1987年 | 9篇 |
1986年 | 8篇 |
1985年 | 14篇 |
1984年 | 12篇 |
1983年 | 8篇 |
1982年 | 12篇 |
1981年 | 12篇 |
1980年 | 7篇 |
1979年 | 8篇 |
1978年 | 7篇 |
1977年 | 8篇 |
1975年 | 9篇 |
1974年 | 5篇 |
1968年 | 6篇 |
1965年 | 6篇 |
1962年 | 6篇 |
排序方式: 共有1138条查询结果,搜索用时 0 毫秒
971.
Aleksandra Konopacka Joanna Pająk Guido Maes Zbigniew Pawełka 《Monatshefte für Chemie / Chemical Monthly》2007,138(12):1233-1238
Summary. The optimal geometries, energies, polarities, infrared frequencies, and intensities of the non-polar and polar conformers
of 1,3,5-triacetylbenzene were calculated using DFT/B3LYP/6-311G** and semi-empirical (AM1 and PM3) methods. All the methods revealed that the non-polar conformer prevails in vacuum.
The infrared spectra in the solid state, in Ar matrix, and in liquid solutions as well as the dipole moments of 1,3,5-triacetylbenzene
in many solvents show the distinct influence of the environment on the non-polar conformer ⇌ polar conformer equilibrium.
The effect of solvent polarity on the standard Gibbs energy of this conformational equilibrium is quantitatively discussed in terms of the continuum-homogenous dielectric model. 相似文献
972.
Nowicka AM Zabost E Donten M Mazerska Z Stojek Z 《Bioelectrochemistry (Amsterdam, Netherlands)》2007,70(2):440-445
The electrooxidation of double-stranded DNA (dsDNA) from calf thymus was carried by using cyclic voltammetry. A glassy carbon disk-, a platinum disk-, a platinum mesh- and a carbon vapor-deposited platinum mesh electrodes were used. It is shown that the appropriate chemical and biological (steam treatment) purification of the complete cell allows, for the graphite electrode, formation of a wide anodic dsDNA signal with two visible anodic peaks. There was no necessity of preaccumulation of dsDNA on the electrode surface and of use of mediators to get well defined voltammetric signals. These peaks apparently reflect electrooxidation of the DNA's guanine and adenine. The spectrophotometric data obtained during the electrooxidation indicate that the absorbance increases with an increase in potential and electrooxidation current of dsDNA. However, the absorption band maximum either does or does not change its position depending on the mesh material. This different spectroscopic behavior may mean that the changes in the dsDNA structure upon electrooxidation are different in the case of Pt and C electrodes. 相似文献
973.
Postawa Z Czerwinski B Winograd N Garrison BJ 《The journal of physical chemistry. B》2005,109(24):11973-11979
Molecular dynamics computer simulations have been employed to model the bombardment of Ag{111} covered with three layers of C6H6 by 15 keV Ga and C60 projectiles. The study is aimed toward examining the mechanism by which molecules are desorbed from surfaces by energetic cluster ion beams and toward elucidating the differences between cluster bombardment and atom bombardment. The results show that the impact of the cluster on the benzene-covered surface leads to molecular desorption during the formation of a mesoscopic scale impact crater via a catapulting mechanism. Because of the high yield of C6H6 with both Ga and C60, the yield enhancement is observed to be consistent with related experimental observations. Specific energy and angle distributions are shown to be associated with the catapult mechanism. 相似文献
974.
The ability to focus the laser accurately onto the sample with a small beam diameter (2.0–3.0 μm) enables laser mass spectrometry to be used as a microprobe. Results from a fully automated ion-mapping system for laser mass spectrometry are described. These results show that the spatial resolution of the laser microprobe is primarily limited by the diameter of the laser beam. Factors such as laser power density, laser focus, sample preparation, and chemical environment influence the reproducibility of laser mass spectra significantly. Calibration curves obtained in the analysis of mixtures of phenanthrolines demonstrate that laser mass spectrometry can be used to quantify organic components. Preliminary results on the detection of neutral molecules resulting from metastable decomposition in the flight tube are also presented. 相似文献
975.
Rudowska M Wojewska D Kluczyk A Bąchor R Stefanowicz P Szewczuk Z 《Journal of the American Society for Mass Spectrometry》2012,23(6):1024-1028
Derivatization of peptides as quaternary ammonium salts (QAS) is a known method for sensitive detection by electrospray ionization
tandem mass spectrometry. Hydrogens at α-carbon atom in N,N,N-trialkylglycine residue can be easily exchanged by deuterons. The exchange reaction is base-catalyzed and is dramatically
slow at lower pH. Introduced deuterons are stable in acidic aqueous solution and are not back-exchanged during LC-MS analysis.
Increased ionization efficiency, provided by the fixed positive charge on QAS group, as well as the deuterium labeling, enables
the analysis of trace amounts of peptides. 相似文献
976.
Joanna Ruszkowska Robert Chrobak Jan K. Maurin Zbigniew Czarnocki 《Acta Crystallographica. Section C, Structural Chemistry》2004,60(5):o377-o380
Vindoline, C25H32N2O6, and 16‐demethoxyvindoline, C24H30N2O5, both of which are naturally occurring biologically active products derived from plants, are important as possible starting materials for the synthesis of valuable anticancer antibiotics, viz. vincristine and vinblastine, and other pharmaceuticals. The vindoline framework consists of two five‐ and three six‐membered condensed rings. One of the six‐membered rings adopts a boat conformation, one adopts a sofa conformation and the third is planar. Both five‐membered rings have envelope structures. The intramolecular hydrogen bonds present in the structures are characteristic of vinca alkaloids. 相似文献
977.
Zbigniew Kałuża Krzysztof Bielawski Rafał Ćwiek Piotr Niedziejko Przemysław Kaliski 《Tetrahedron: Asymmetry》2013,24(21-22):1435-1442
The synthesis of novel, enantiomerically pure C2-symmetrical hemiaminal ethers and diamines containing piperazine core is presented. The key steps of the synthesis involve the dimerization of an in situ generated α-amino aldehyde into the corresponding cyclic bis-hemiaminal, followed by dehydration in the presence of a base to give a 7-oxa-2,5-diaza-bicyclo[2.2.1]heptane derivative, which can be regarded as a bicyclic bis-hemiaminal inner ether. These compounds represent a new class of molecule, with a structure unambiguously established for the first time. Finally, sodium triacetoxy-borohydride reduction gave the corresponding diamines. Both classes of compounds, new diamines and hemiaminal ethers, were shown to be good ligands for the copper(II)-catalyzed desymmetrization of meso-diols (up to 87% ee) and Henry reactions (up to 84% ee). 相似文献
978.
It has been shown that ITO electrodes could be modified with a volume‐phase‐transition gel based on poly(N‐isopropylacrylamide) cross‐linked with N,N′‐methylenebisacrylamide. Prussian blue (PB) was deposited electrochemically inside the gel while glucose oxidase was added at the time of electropolymerization. The electrocatalytic activity of glucose oxidase towards the oxidation of glucose (measured as the reduction current of hydrogen peroxide) was strongly depressed by the shrinking process. Since the volume phase transition triggered by the temperature change is reversible the obtained change in the enzyme activity may be useful for getting a switchable system. 相似文献
979.
Piotr Stefanowicz Katarzyna Kapczynska Mariusz Jaremko Łukasz Jaremko Zbigniew Szewczuk 《Journal of mass spectrometry : JMS》2009,44(10):1500-1508
Gas phase fragmentation of peptide‐derived Amadori products was investigated using synthetic compounds regioselectively deuterated as well as labeled with 18O at aminofructose moiety. The eliminated molecule CH2O contains exclusively protons attached to carbon C6 of the aminofructose moiety. The hydrogen atoms connected with the carbon C1 of the aminofructose moiety are partially eliminated as a component of water molecules during the dehydration process and partially dislocated within the fragmented peptide molecule. The labeled oxygen atom attached to the carbon C2 is eliminated in 100% along with the first loss of water. The MS3 experiments revealed that the product ion formed by triple dehydration of the Amadori product does not eliminate the formaldehyde molecule. On the basis of these observations we proposed a hypothetical mechanism of Amadori products' fragmentation. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
980.
Poly(acrylic acid) gels are known to swell to different extent (measured by the swelling ratio) depending on pH and ionic strength of the solution. The experimental method employed by us to monitor the concentration and diffusion coefficient of the probes allows the simultaneous determination of their concentration and diffusion coefficient for any value of the ionic strength and pH of the solution trapped in the polymeric network. Two different electroactive probe systems, 1,1′-ferrocenedimethanol and 2,2,6,6-tetramethylpiperidine 1-oxyl, were selected for the examination. Small values of the swelling ratio, obtained either by application of a high ionic strength or a low pH, result in an unexpected high concentration of both probes in the gel samples. Correspondingly, the probes diffusion coefficients were much smaller compared to their values in the swollen gels. 相似文献