全文获取类型
收费全文 | 1074篇 |
免费 | 48篇 |
国内免费 | 11篇 |
专业分类
化学 | 733篇 |
晶体学 | 11篇 |
力学 | 10篇 |
数学 | 135篇 |
物理学 | 244篇 |
出版年
2024年 | 3篇 |
2023年 | 6篇 |
2022年 | 36篇 |
2021年 | 28篇 |
2020年 | 29篇 |
2019年 | 37篇 |
2018年 | 35篇 |
2017年 | 30篇 |
2016年 | 56篇 |
2015年 | 25篇 |
2014年 | 38篇 |
2013年 | 123篇 |
2012年 | 59篇 |
2011年 | 60篇 |
2010年 | 47篇 |
2009年 | 50篇 |
2008年 | 44篇 |
2007年 | 33篇 |
2006年 | 31篇 |
2005年 | 32篇 |
2004年 | 47篇 |
2003年 | 22篇 |
2002年 | 28篇 |
2001年 | 10篇 |
2000年 | 12篇 |
1999年 | 4篇 |
1998年 | 15篇 |
1997年 | 4篇 |
1996年 | 13篇 |
1995年 | 6篇 |
1994年 | 19篇 |
1993年 | 12篇 |
1992年 | 15篇 |
1991年 | 10篇 |
1990年 | 7篇 |
1989年 | 14篇 |
1988年 | 9篇 |
1987年 | 3篇 |
1985年 | 8篇 |
1984年 | 9篇 |
1983年 | 8篇 |
1982年 | 7篇 |
1981年 | 10篇 |
1980年 | 5篇 |
1979年 | 6篇 |
1977年 | 6篇 |
1975年 | 4篇 |
1972年 | 3篇 |
1971年 | 4篇 |
1969年 | 2篇 |
排序方式: 共有1133条查询结果,搜索用时 15 毫秒
991.
992.
Aïcha Yacouta-Nour Abobaker K. T. Maki Monsen M. Mostafa Kamal M. Ibrahim Ashraf A. El-Bindary 《Transition Metal Chemistry》1991,16(1):23-27
Summary Uranyl acetate dihydrate reacts with several hydrazone-oximes, derived from aliphatic (mon-annd di-) hydrazides and 2,3-butanedione monoxime in the absence of NaOAc, to form complexes of general formulae [UO2(HL)2] and [UO2(HL)2SZ] (where H2L=aliphatic acid monohydrazone-oximes; S=EtOH and Z=H2O). With aliphatic acid dihydrazone and monomaleoyl-hydrazone-oximes, different complexes have been synthesized in 50% EtOH and in the absence of NaOAc. The products have been characterized by elemental analyses, molar conductivities, spectral (u.v., i.r., n.m.r.), pH, conductometric titrations, molecular weights and magnetic measurements. I.r. spectral data indicate that the aliphatic hydrazone-oximes, except monomaleoyl-hydrazone-oxime, behave in a similar way to tridentate ligandsvia the azomethine nitrogen and the NOH groups, with replacement of a hydrogen atom from the latter group. On the other hand, malonyl dihydrazone- and adipoyldihydrazone-oximes coordinate in a bi-, and tridentate manner, respectively, towards one uranyl ion, while monomaleoyl- and oxaloyldihydrazone-oximes behave in a penta-and hexadentate fashion, respectively, towards two uranyl ions. Elemental analyses, molecular weight measurements as well as spectral data confirm the existence of a binuclear complex for monomaleoyl- and oxaloyldihydrazone-oximes, whereas a monomer for the rest. Finally, the role of solvent, absence of NaOAc and the type of ligand on the structure of the complexes are discussed. 相似文献
993.
Three new 9-(N-alkynyl)acridinamines 5, 6 and 7 have been synthesized from 9-chloroacridine using a simple aromatic nucleophilic substitution reaction. 相似文献
994.
OH radical reactions with ethanolamine, diethanolamine and triethanolamine were studied at pH values below and above the pKa values of these compounds. The rate constants were found to be lower for the protonated amines than those for their neutral forms. The OH radical reaction led to the formation of both oxidizing, as well as reducing species, as observed by their reactions with methyl viologen and ascorbic acid. The oxidizing species formed by OH radical reaction at the amine site was not found to react with the parent molecules and thereby no secondary yield of reducing species was obtained, as in the case of glycine (except in the case of triethanolamine at pH 9.2). 相似文献
995.
El-Sayed Mohamed A. El-Wakil Hoda Ismail Kamal Z. El-Zayat Tahane A. Davies Geoffrey 《Transition Metal Chemistry》1998,23(6):795-800
Copper(I) dimer [(DEED)CuBr]2 (4, DEED=N,N-diethylethylenediamine) is rapidly oxidized by O2 to mixed valence peroxocopper complex [(DEED)CuBr]4O2 (1) in CH2Cl2 at –50 to 30°C. The long half- life for conversion of (1) into oxocopper(II) complex [(DEED)]CuBr]2O (3) allows (1), (3) and their carbonato derivative of [(DEED)CuBr]2CO3 (5) to be compared as oxidants of 2,6–dimethylphenol (DMPOH) to the corresponding diphenoquinone (DPQ) over a range of concentrations and temperatures. DPQ production is: 1)less than stoichiometric with deficits or slight excesses of DMPOH, but 2) mildly catalytic at moderate [DMPOH], as found with tetranuclear oxo-halo(pyridine)copper(II) oxidants. This behaviour is attributed to 1) co-product water destruction of initiators, and 2) inhibition by water of copper(I) reoxidation to complete the catalytic cycle. These inhibiting factors apparently are ameliorated by water incorporation in hydrogen-bonded phenol clusters in aprotic solvents. Initial rate measurements show that (1), (3) and (5) form monophenolate complexes with DMPOH in methylene chloride. The rate-determining step for conversion of these complexes to DPQ is fastest for oxocopper(II) complex (3) which is expected to be the strongest proticbase. Highest rates with (3) and activation parameter comparisons suggest that the ability of phenolatocoppercomplexes to accept protons from coordinated phenolate is an important factor in determining overall copper- catalyzed phenolic oxidative coupling rates. 相似文献
996.
The determination of the interaction between lumichrome (LC), one of the products of decomposition of the biologically important flavins, and calf thymus double-stranded DNA was performed by using cyclic voltammetry (CV) and differential pulse stripping voltammetry (DPSV) in connection with a hanging mercury drop electrode (HMDE) or glassy carbon electrode (GCE). The nature of the process taking place at both electrode surfaces was clarified. It was found that the addition of DNA to a buffered LC solution results in the decrease of redox peak currents with changes in the peak potentials at both electrodes. We assume that LC interacting with DNA produces an electrochemically inactive supramolecular complex via intercalation. There was a difference between the electrochemical parameters determined at the HMDE and those at the GCE. The binding constants ( K) of the LC-DNA complex at HMDE and GCE were determined through the changes of peak currents and their values at the 10(5) level and 10(4) level with each nucleotide residue of DNA binding one LC molecule, respectively. Furthermore, the calibration graph for the determination of DNA was obtained by the decrease in the DPSV peak current of LC in the presence of DNA. Different variables, such as the concentration of LC, the accumulation time and solution conditions, were studied and optimised to maximize the sensitivity; in addition, the detection limit and the reproducibility were determined. 相似文献
997.
Six new 4-alkynyl-2-bromoquinolines have been synthesised via palladium catalysed C? C bond formation between 2,4-dibromoquinoline and mono-substituted alkynes. Position of the alkyne substituent was confirmed to be at C-4 by an nOe experiment on some methoxy derivatives prepared by nucleophilic displacement of bromo substituent. 相似文献
998.
Summary As the transition state for the solvolysis of [Co(NH3)5Cl]2+ ions is known to have Cl- ions in a situation closely similar to that in the bulk solvent, the kinetics of this solvolysis have been investigated for comparison in H2O with added cosolvents of low and high hydrophobicities. A linear variation of log(rate constant) with the reciprocal of the dielectric constant is found with the former, but not with the latter cosolvent. Maxima in the enthalpies and entropies of activation found using the more hydrophobic cosolvent appear at solvent compositions where extrema occur in the physical properties influenced by structural changes in the solvent. The application of a free energy cycle to the solvolysis in H2O and in the mixtures shows that the emergent solvated cobalt(III) ion in the transition state is more stabilised in the latter than [Co(NH3)5Cl]2+ with both cosolvents. The application of such a cycle to cases where the initial state is destabilised in the mixture is discussed. 相似文献
999.
Hanessian S Papeo G Fettis K Therrien E Viet MT 《The Journal of organic chemistry》2004,69(15):4891-4899
A bicyclic indolizidinone carboxylic acid and a tricyclic constrained analogue of l-proline were synthesized and evaluated for their ability to induce helix formation as l-Ala tetrapeptides. Variable-temperature NMR, DMSO titration, CD spectra, and X-ray structure analyses, in conjunction with molecular modeling, confirmed the existence of 3(10)-helical motifs with di- and tetrapeptides of l-Ala. 相似文献
1000.
A simple kinetic spectrophotometric method was developed for the determination of josamycin in its dosage forms. The method is based on oxidation of the drug with alkaline potassium permanganate at room temperature for a fixed time of 20 min and measuring the produced green color at 611 nm. The absorbance-concentration plot is rectilinear over the range of 2-10 microg/mL (2.4 x 10(6)-1.2 x 10(-5)M) with minimum detectability of 1.0 microg/mL (1.2 x 10(-6)M). The determination of josamycin by fixed concentration and the rate-constant methods is also feasible with the calibration equations obtained, but the fixed-time method proved to be more applicable. The procedure was applied successfully to commercial tablets, and statistical analysis showed that the results compared favorably with those obtained by reference methods. The effect of sensitizers and surfactants on the performance of the proposed method was also studied. A proposal of the reaction pathway was presented. 相似文献