全文获取类型
收费全文 | 437篇 |
免费 | 18篇 |
专业分类
化学 | 278篇 |
晶体学 | 16篇 |
力学 | 11篇 |
数学 | 26篇 |
物理学 | 124篇 |
出版年
2023年 | 5篇 |
2022年 | 7篇 |
2021年 | 11篇 |
2020年 | 12篇 |
2019年 | 11篇 |
2018年 | 10篇 |
2017年 | 7篇 |
2016年 | 18篇 |
2015年 | 9篇 |
2014年 | 17篇 |
2013年 | 27篇 |
2012年 | 33篇 |
2011年 | 37篇 |
2010年 | 20篇 |
2009年 | 19篇 |
2008年 | 20篇 |
2007年 | 25篇 |
2006年 | 14篇 |
2005年 | 19篇 |
2004年 | 27篇 |
2003年 | 12篇 |
2002年 | 22篇 |
2001年 | 3篇 |
2000年 | 4篇 |
1999年 | 9篇 |
1997年 | 4篇 |
1996年 | 2篇 |
1995年 | 2篇 |
1994年 | 2篇 |
1993年 | 2篇 |
1991年 | 4篇 |
1990年 | 1篇 |
1989年 | 4篇 |
1988年 | 2篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1985年 | 4篇 |
1984年 | 4篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1972年 | 2篇 |
1971年 | 1篇 |
排序方式: 共有455条查询结果,搜索用时 453 毫秒
411.
Homocysteine is an independent risk factor for cardio- and/or cerebrovascular diseases. Many methods are used to measure plasma homocysteine levels in physiological fluids. Current gas chromatographic/mass spectrometric (GC/MS) methods allow determination not only of plasma homocysteine concentration, but also of its turnover. However, they have some methodological limitations due to the reduction of disulfide bonds between homocysteine and other thiols or proteins often requiring the use of several very toxic compounds or multi-step procedures that are particularly time-consuming, and/or utilize expensive instruments. Herein is described a rapid and precise GS/MS method to determine homocysteine turnover from a relatively low volume of plasma (200 microL). First disulfide bonds were reduced by 2-mercaptoethanol, which allows the maintenance of the reduced status preventing the rebuilding of the disulfide bond. Then the sample was derivatized to form the bis-tert-butyldimethylsilyl derivative. A deuterated internal standard, DL-[3,3,3',3',4,4,4',4'-2H8]-homocystine, was employed to account for losses associated with each analytical step. To evaluate the 'in vivo' homocysteine metabolic turnover, [1-13C]-methionine was infused and the derived [1-13C]-homocysteine quantitated. So a standard curve of [1-13C]-homocysteine was prepared by the decomposition of the [1-13C] methionine. The ions at m/z 325 and 326 were monitored, corresponding to the unlabeled [12C]-homocysteine and to labeled [13C]-homocysteine, respectively. The ion at m/z 325 ([M-114)]+) probably resulted from the loss of one derivatizing group to regenerate a free amino group. The intra-assay coefficient of variation (CV-intra%) was consistently less than 1.06%, the inter-assay (CV-inter%) less than 1.05%. The method described here seems to be simpler, more rapid, and less toxic than those published so far. In particular, its main strength appears to be the degree of precision obtained. We suggest applying this method to the measurement of the 'in vivo' rate of production of homocysteine (by the plasma 13C-homocysteine enrichment) from its precursor (13C-methionine). 相似文献
412.
Second-order MS-MA calculations by the variation-perturbation method for the triplet state of H2 show that in the van der Waals region induction including exchange is practically negligible and the depth and position of the potential minimum are determined by the competition between repulsive first-order exchange and attractive Coulomb dispersion. Accurate interaction energies are obtained in second order only if all non-expanded multipoles up to the dipole-octupole term are properly accounted for. 相似文献
413.
Bragieri M Liverani A Zanotti MC Borzatta V Fiori J Cavrini V Andrisano V 《Journal of separation science》2004,27(1-2):89-95
A GC/FID/MS method was developed for the identification and quantification of d-allethrin (DA) and its major impurities in commercial samples. Optimisation of the experimental conditions was carried out considering such important requirements as resolution, reproducibility, detection limits of 0.1% (m/m) for the impurities, and short analysis time. Under the optimised final conditions the method was validated for specificity, precision (CV% = 0.133 at 2.10 mg/mL and CV% = 0.035 at 3.00 mg/mL), linearity (0-3.00 microg injected), limits of detection (0.09 ng injected) and quantitation (0.28 ng injected), and robustness. The DA related impurities were identified by using a GC/MS method with ion trap mass detection and also by comparison with synthesised standards. The most abundant impurities were crysolactone, allethrolone, chrysanthemic acid, and chloro-derivatives of DA. 相似文献
414.
Pavel Hradil Petr Krej
í Jan Hlav
Iveta Wiedermannov Antonín Ly
ka Valerio Bertolasi 《Journal of heterocyclic chemistry》2004,41(3):375-379
As known, some derivatives of quinolin‐4(1H)‐one possess interesting biological properties. The biological and cytostactic activity of 2‐substituted 3‐hydroxyquinolin‐4(1H)‐ones has not been reported yet. In this paper the synthesis of a series of chloro and dichloro 2‐phenyl‐3‐hydroxyquinolin‐4(1H)‐ones and their characterization by NMR spectra and X‐ray data is described. Their cytostatic properties have been evaluated and the results are reported. 相似文献
415.
The present study explores the application of the diffusion limited evaporation theory to the estimation of vapor pressure from TG experimental data. A simplified method was developed to calculate the apparent values of the vapor pressure of pure substances from TG data, based on isothermal TG runs with crucibles having different surface areas available for evaporation. Antoine parameters are estimated through a numerical procedure based on a non-linear least square algorithm. The procedure also evaluates the substance diffusivity in nitrogen. The methodology developed might be used for a preliminary screening of the vapor pressure of pure compounds, due to the limited amounts of sample that are necessary and to the limited time frame required for the experimental runs. However, the estimation of diffusivity and vapor pressures values by the TG technique is possible with limited accuracy. Possible sources of error were thoroughly investigated and discussed. 相似文献
416.
Triallyl- and diallyltin carboxylates [(CH2CHCH2)3SnOOCR′ with R′ = CH3 and CH2Cl, (CH2CHCH2)2Sn(OOCR′)2 with R′ = CH2Cl and CHCl2)] and tetraallyl-1,3-diacyloxydistannoxanes {[(CH2CHCH2)2SnOOCR′]2 with R′ = CH2Cl,CHCl2 and CCl3}, have been prepared from the reaction of tetraallytin with carboxylic acids in methanol. 相似文献
417.
Edoardo Guerrini Valerio Consonni Sergio Trasatti 《Journal of Solid State Electrochemistry》2005,9(5):320-329
The kinetics of Cl2 evolution from concentrated NaCl solutions on the (110) and (230) faces of RuO2 single crystals has been investigated by determining the Tafel slope, the stoichiometric number and the reaction orders with respect to Cl– and H+. The experimental parameters suggest that the mechanism is presumably similar to that put forward earlier by Krishtalik [51] for RuO2 layers, but a step common to oxygen evolution, like the case of polycrystalline samples, is present only with the (230) face. Reasons for this difference and for the apparent lower activity of the (110) face with respect to the (230) are discussed. A detailed analysis of the surface behavior of the two faces in Cl– free acid and alkaline solutions has also been carried out by cyclic voltammetry.Dedicated to Professor G. Horanyi on the occasion of his 70th birthday 相似文献
418.
Bernardi F Garavelli M Scatizzi M Tomasini C Trigari V Crisma M Formaggio F Peggion C Toniolo C 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(11):2516-2525
As a part of a program evaluating substituted gamma-lactams as conformationally constrained building blocks of pseudopeptide foldamers, we synthesized the homo-oligomers of L-pyroglutamic acid up to the tetramer level by solution methods. The preferred conformation of this pseudopeptide series in structure-supporting solvents was assessed by FT-IR absorption, 1H NMR and CD techniques. In addition, the crystal structure of the N alpha-protected dimer was established by X-ray diffraction. A high-level DFT computational modeling was performed based on the crystallographic parameters. In this analysis, we demonstrated that an alpha C-H...O=C intramolecular hydrogen bond is responsible for the stabilization of the s-trans L-pGlu-L-pGlu conformation by 1.4 kcal mol-1. This effect can be easily detected by 1H NMR spectroscopy, owing to the anomalous chemical shifts of the alpha CH protons present in all of the oligomers. In summary, we have developed a new polyimide-based, foldameric structure that, if appropriately functionalized, has promise as a rigid scaffold for novel functions and applications. 相似文献
419.
The chemical interactions of 1-methyl-3-hydroxy-4-pyridinecarboxylic acid (1M3H4P) and 1-methyl-4-hydroxy-3-pyridinecarboxylic acid (1M4H3P) with Fe(III) were investigated in aqueous 0.6 m (Na)Cl at 25 °C by means of potentiometric titrations and UV–Vis spectrophotometry. A large number of complexes were formed in solution with stoichiometry FeLxHy, x = 1, 2, 3 and y = 0, −1, −2. In view of a possible application to Fe(III) chelation therapy, the efficiencies of the ligands to chelate the metal ion were evaluated in vitro at physiological pH. 1M3H4P and 1M4H3P show a lower complexation efficiency with Fe(III) than that of other available chelators, but higher than that of their non-methylated analogs 3H4P and 4H3P. 相似文献
420.
Pinalli R Cristini V Sottili V Geremia S Campagnolo M Caneschi A Dalcanale E 《Journal of the American Chemical Society》2004,126(21):6516-6517
This communication reports design, self-assembly, solution, and solid-state characterization of nanoscale coordination cages formed by tetradentate cavitand ligands and appropriate metal precursors. The preorganization of the cavitand ligand in terms of structural rigidity and relative orientation of the pyridyl units leads to the exclusive formation of coordination cages in a wide temperature and concentration range. Desired features of the cage self-assembly process, such as reversibility in the presence of a competitive ligand and self-recognition of the cavitand components, have been assessed. 相似文献