全文获取类型
收费全文 | 532篇 |
免费 | 19篇 |
国内免费 | 4篇 |
专业分类
化学 | 442篇 |
力学 | 7篇 |
数学 | 58篇 |
物理学 | 48篇 |
出版年
2023年 | 2篇 |
2022年 | 3篇 |
2021年 | 13篇 |
2020年 | 13篇 |
2019年 | 10篇 |
2018年 | 6篇 |
2017年 | 7篇 |
2016年 | 12篇 |
2015年 | 6篇 |
2014年 | 13篇 |
2013年 | 38篇 |
2012年 | 35篇 |
2011年 | 46篇 |
2010年 | 22篇 |
2009年 | 26篇 |
2008年 | 53篇 |
2007年 | 43篇 |
2006年 | 34篇 |
2005年 | 38篇 |
2004年 | 25篇 |
2003年 | 16篇 |
2002年 | 11篇 |
2001年 | 10篇 |
2000年 | 15篇 |
1999年 | 5篇 |
1998年 | 6篇 |
1997年 | 4篇 |
1996年 | 3篇 |
1995年 | 5篇 |
1994年 | 3篇 |
1993年 | 2篇 |
1992年 | 3篇 |
1991年 | 3篇 |
1990年 | 1篇 |
1989年 | 3篇 |
1988年 | 1篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1981年 | 3篇 |
排序方式: 共有555条查询结果,搜索用时 31 毫秒
11.
Six novel metabolites, acremines A-F have been isolated from agar cultures of a strain of Acremonium sp. Their structures and stereochemistry were elucidated using a combination of 13C and 1H homo and heteronuclear 2D NMR experiments and X-ray analysis. Acremines A-D inhibited the germination of sporangia of Plasmopara viticola. 相似文献
12.
Charles W. Jefford Manuel Jimenez Estrada Giacomo Barchietto Tho Berclaz Michel Geoffroy 《Helvetica chimica acta》1990,73(6):1653-1658
The photo-oxygenation of adamantylideneadamantane ( 1 ) on siliceous supports using admixed granules of ion-exchange resin fixed to methylene blue (MB) and rose bengal (RB) gave exclusively the corresponding dioxetane derivative 2 for the former sensitizer, while the latter gave 2 and traces of the epoxide 3. RB and the charge-transfer complex produced from N-ethylcarbazole and 2,4,5,6-tetranitrofluoren-9-one both reacted with chemically generated singlet oxygen to give superoxide radical anion. Trapping of the latter with 5,5-dimethyl-1-pyrroline 1-oxide gave an adduct exhibiting a characteristic ESR spectrum. The treatment of 1 in MeOH with 30% aqueous H2O2 for 22 h at 60° gave 3 in 100% yield. Repetition of this experiment in the presence of 2,6-di(tert-butyl)-p-cresol caused no significant change. These results indicate that singlet oxygen reacts with 1 , in the presence of RB, by two different processes. The first leads to dioxetane formation. The second process involves conversion of singlet oxygen by RB to superoxide radical anion which subsequently gives H2O2 so producing epoxide 3 from 1 . 相似文献
13.
Diacetylene-containing polyesters and polyurethanes were prepared by the reaction of m,m-butadiynylenedibenzyl alcohol with isophthaloyl chloride, sebacoyl chloride, hexamethylene diisocyanate and 2,4-tolylene diisocyanate. Their thermal behavior was studied by optical microscope and differential scanning calorimetry (DSC). The polyesters were crystalline. The polyurethane prepared from hexamethylene diisocyanate and polyphthalates crystallized on cooling from their melts. All turned a reddish brown color on heating at temperatures above 200°C, and prolonged heating led to a black resin, but thermal decomposition could not be avoided. Irradiation by γ-ray and UV light at room temperature deepened the color of the films, but the polymerization of the diacetylene groups was not appreciable. Simultaneous heating and irradiation was necessary to obtain transparent, amorphous, wine red colored films. 相似文献
14.
The role of hydrogen bonding on the microscopic properties of supercooled methanol has been analyzed by means of molecular dynamics simulations. Thermodynamic, structural, and dynamical properties have been investigated in supercooled methanol. The results have been compared with those of an ideal methanol-like system whose molecules have the same dipole moment as the methanol but lack sites for hydrogen bonding. Upon cooling the methanol samples, translational relaxation times increase more rapidly than reorientational ones. This effect is much more important when hydrogen bonds are suppressed. Suppression of hydrogen bonds also results in lower critical temperatures for diffusion and for several characteristic relaxation time constants. The anisotropy of individual dynamics and the existence of dynamical heterogeneities have also been investigated. 相似文献
15.
A. Segura Carretero C. Cruces Blanco R. Estrada Fernández A. Fernández Gutiérrez 《Analytical and bioanalytical chemistry》1998,360(5):605-608
A method for the determination of the fungicide thiabendazole (TBZ) by micellar-stabilized room-temperature phosphorescence is described. It does not require any separation step and allows the direct determination of the fungicide in canned pineapple samples. The effect of various experimental conditions is discussed in detail. The analytical curve of thiabendazole gives a linear dynamic range of 23.8–500.0 ng mL–1 and a detection limit of 23.8 ng mL–1. Recoveries of 103.9 and 89.2% for syrup and canned pineapple pulp, respectively, were obtained for 250 ng mL–1 thiabendazole. 相似文献
16.
A. L. O. Damasceno A. Iwahara M. A. L. Silva J. J. S. Estrada 《Journal of Radioanalytical and Nuclear Chemistry》2006,269(2):317-323
Summary The radionuclide 192Ir has been increasingly used as a brachytherapy source and manufactured in different geometry forms (thin wires, hairpins,
needles or point sources). A procedure for the characterization of the activity content of 192Ir wire sources was developed in order to establish the secondary standard activity measurement system based on the ionization
chamber set up at Brazilian National Laboratory for Ionizing Radiation Metrology (LNMRI). Firstly, the ion current of the
ionization chamber is measured and, subsequently, the activity is determined in a destructive analysis. This procedure enables
obtaining the calibration factor for wire sources that can be used for further activity determination of similar sources in
a non-destructive analysis. Accurate activity determination is needed for dosimetric measurements and for helping the manufacturers
to improve its quality control programs. 相似文献
17.
Luis Adrio Gemma Alberdi Adriana Amoedo Darío Lata Alberto Fernndez Javier Martínez M. Teresa Pereira Jos M. Vila 《无机化学与普通化学杂志》2005,631(11):2197-2203
Reaction of the thiosemicarbazone ligands C4H4NC(H)=NN(H)C(S)NHR (R = Me, a ; Et, b ) with Li2[PdCl4] gave the dinuclear complexes [Pd{C4H4NC(H)=NNC(S)NHR}(μ‐Cl)]2 (R = Me, 1a ; Et, 1b ) with a central Pd2Cl2 core and with deprotonation of the thiosemicarbazones at the hydrazinic nitrogen atom. Treatment of 1a and 1b with triphenylphosphine gave the mononuclear compounds [Pd{C4H4C(H)=NNC(S)NHR}(Cl)(PPh3)] (R = Me, 2a ; Et, 2b ), whereas reaction of 1a and 1b with tertiary diphosphines gave mono‐ and dinuclear compounds, as appropriate, with the corresponding diphosphine acting as a monodentate ( 6b ), chelating ( 3a ) and bridging ligand ( 4a, 5a , 4b, 5b ). Treatment of 1a and 1b with (Ph2PCH2CH2PPh2)W(CO)5 gave the new heterobimetallic complexes 7a and 7b . The crystal structures of complexes 3a and 4a are described. 相似文献
18.
Transition metal (TM) hydrides display a remarkable range of bonding types, encompassing classical M-H moieties, dihydrogen complexes containing the eta 2-H2 ligand, and trihydrides which display quantum mechanical site exchange. Furthermore, C-H, Si-H and B-H moieties can bind to TM centres in an eta 2-manner, to give sigma-bond complexes with a spectrum of M...H contributions. In addition to these primary bonding modes, TM complexes also indulge in a wide spectrum of hydrogen-bonding interactions, including both M...H-X and the unique type M-H...H-X. This review begins with a historical perspective of the development of TM hydride chemistry, and proceeds to focus on three significant developments of the past two decades: the discovery of sigma-bond and dihydrogen complexes, the involvement of TM hydrides in hydrogen bonding, and the role played by quantum mechanical phenomena in the chemistry and dynamics of TM hydrides. The account concludes with an overview of the inter-relationship between these apparently disparate novel aspects of TM hydride chemistry. 相似文献
19.
Valkovska DS Shearman GC Bain CD Darton RC Eastoe J 《Langmuir : the ACS journal of surfaces and colloids》2004,20(11):4436-4445
A quantitative model for the kinetics of adsorption of ionic surfactants to an expanding liquid surface is presented for surfactant concentrations below and above the critical micelle concentration (cmc). For surfactant concentrations below the cmc, the electrostatic double layer is accounted for explicitly in the adsorption isotherm. An overflowing cylinder (OFC) was used to create nonequilibrium liquid surfaces under steady-state conditions. Experimental measurements of the surface excess for solutions of cationic surfactants CH3(CH2)n-1N+(CH3)3 Br- (CnTAB, n = 12, 14, 16) and the anionic fluorocarbon surfactant sodium bis(1H,1H-nonafluoropentyl)-2-sulfosuccinate (di-CF4) in the OFC are in excellent agreement with the theoretical predictions for diffusion-controlled adsorption for all concentrations studied below the cmc. For surfactant concentrations above cmc, the diffusion ofmicelles and monomers are handled separately under the assumption of fast micellar breakdown. This simplified model gives excellent agreement for the system C14TAB + 0.1 M NaBr above the cmc. Agreement between theory and experiment for C16TAB + 0.1 M NaBr is less good. A plausible explanation for the discrepancy is that micellar breakdown is no longer fast on the time scale of the OFC (ca. 0.1 s). 相似文献
20.
Philippe Quevauviller Gemma Rauret Allan Ure Roser Rubio José-Fírmin López-Sánchez Haidé Fiedler Herbert Muntau 《Mikrochimica acta》1995,120(1-4):289-300
The determination of extractable trace metal contents in soil using single extraction procedures is currently performed in many laboratories to assess the bioavailable metal fraction (and related phytotoxic effects) and the accessability to the environment (e.g. contamination of ground waters). Owing to the need for validation of the extraction schemes used and of the analytical techniques, the EC Measurements and Testing Programme (formerly BCR) has organized a project for improving the quality of determination of extractable trace metal contents in soil, the first step being an interlaboratory study to adopt common extraction procedures and the second being a certification campaign to certify two soils for their extractable trace element contents following these procedures. This paper gives a brief overview of the project organisation and describes the preparation, homogeneity and stability studies of two soil candidate reference materials (sewage sludge-amended and terra rossa soils). 相似文献