全文获取类型
收费全文 | 651篇 |
免费 | 4篇 |
国内免费 | 1篇 |
专业分类
化学 | 443篇 |
晶体学 | 13篇 |
力学 | 11篇 |
数学 | 113篇 |
物理学 | 76篇 |
出版年
2023年 | 7篇 |
2022年 | 4篇 |
2021年 | 8篇 |
2020年 | 11篇 |
2019年 | 4篇 |
2017年 | 4篇 |
2014年 | 17篇 |
2013年 | 29篇 |
2012年 | 16篇 |
2011年 | 26篇 |
2010年 | 6篇 |
2009年 | 19篇 |
2008年 | 25篇 |
2007年 | 28篇 |
2006年 | 29篇 |
2005年 | 17篇 |
2004年 | 22篇 |
2003年 | 22篇 |
2002年 | 12篇 |
2001年 | 6篇 |
2000年 | 15篇 |
1999年 | 16篇 |
1998年 | 19篇 |
1997年 | 16篇 |
1996年 | 15篇 |
1995年 | 13篇 |
1994年 | 13篇 |
1993年 | 8篇 |
1992年 | 14篇 |
1991年 | 14篇 |
1990年 | 13篇 |
1989年 | 13篇 |
1988年 | 8篇 |
1987年 | 18篇 |
1986年 | 10篇 |
1985年 | 11篇 |
1984年 | 10篇 |
1983年 | 16篇 |
1982年 | 8篇 |
1981年 | 5篇 |
1980年 | 10篇 |
1979年 | 5篇 |
1978年 | 13篇 |
1977年 | 6篇 |
1976年 | 8篇 |
1975年 | 5篇 |
1974年 | 10篇 |
1973年 | 4篇 |
1970年 | 4篇 |
1966年 | 3篇 |
排序方式: 共有656条查询结果,搜索用时 15 毫秒
91.
A method is described for the determination of polycyclic aromatic hydrocarbons (PAH) in the diluted exhaust gas of diesel vehicles. Sampling is done by drawing off proportional streams from the dilution tunnel. The particulates deposited on filters are sublimed, and the sublimate is purified and prefractionated on silica gel. Further separation and quantitative determination of the PAH is performed by two dimensional TLC in conjunction with In situ fluorescence spectrometry. Results of theoretical considerations and experimental data on the distribution of the emitted PAH between particulate matter and the corresponding gas phase in diluted exhaust are presented. A modified Langmuir adsorption model is used to explain the effects of dilution ratio and sample temperature in the dilution tunnel. Comparison of the emission values for PAH obtained from diluted and undiluted exhaust shows good agreement. Furthermore, the PAH contents of the exhaust gas of several diesel-engined cars are also compared for various driving cycles. 相似文献
92.
Hans-Peter Isenring Engelbert Zass Keith Smith Heinz Falk Jean-Luc Luisier Albert Eschenmoser 《Helvetica chimica acta》1975,58(8):2357-2367
Enol derivatives in the chlorophyll series. 132-Desmethoxycarbonyl-173-desoxy-132,173-cyclochlorophyllide a-enol and a method for the introduction of magnesium into porphinoid ligands under mild conditions Magnesium transfer from the iodo-magnesium salt of 3,5-di-t-butyl-4-hydroxy-toluene (BHT) into methyl pheophorbide a is a fast process in methylenechloride/ether solution at ambient temperature. This procedure for the introduction of magnesium into labile chlorin ligands has made possible the preparation of a crystalline 132, 173-cyclochlorophyllide a-enol. Concomitant use of the lithium salt of BHT facilitates the more critical insertion of magnesium into methyl bacteriopheophorbide a. The preparative success of these magnesium transfers depends crucially upon the solvent system used. Under conditions where the complexation of methyl pheophorbide a with iodo-magnesium-BHT is essentially complete within 2 minutes at 12° in methylenechloride/ether solution, strong inhibition of the magnesium transfer is observed by cosolvents such as pyridine, dimethylacetamide, dioxan or tetrahydrofuran. 相似文献
93.
Structures of Cesium-containing Fluorides. V. Pseudohexagonal Tungsten Bronze Structure of the Monoclinic Phase Cs0.4Zn0.4Fe1.6F6 and Related Cesium Transition Metal Fluorides The crystal structure of the pseudohexagonale phase Cs0.4Zn0.4Fe1.6F6 was refined in the monoclinic space group P21 (a = 747.4, b = 763.6, c = 746.1 pm, β = 120.0°, z = 3). Using 1067 single crystal reflections R = 6.3% was reached. Compared to the hexagonal tungsten bronze structure the corner-sharing of octahedra shows pronounced angles also along the unique axis (M? F? M = 159.0°). The average bridge angle in the framework of tilted octahedra is 149.6°. Structural relations are discussed. 相似文献
94.
95.
96.
Complex Formation of N-(Thiocarbamoyl) benzamidines N-(Thiocarbamoyl)benzamidines 1 and N-(Thiocarbamoyl)-N′-phenyl-benzamidines 2 form in alcoholic solutions with Ni2+, Cu2+, Pd2+, Co3+, and Ag+ ions well crystallized chelates 1a ? h and 2a? h , respectively, with N/S coordination. The X-ray photoelectron spectra evidence the NH group as ligator in the case of chelates of 1 and the increased donor capacity compared with the isosteric oxygen ligator atom bound to nickel. This is confirmed by d-1H-n.m.r. spectroscopy analyzing the value of ΔGc≠ for the rotational barrier for the Et2N group of 1a 相似文献
97.
Hans-Peter Thier Tjakko Stijve Wolfgang Specht 《Fresenius' Journal of Analytical Chemistry》1989,334(6):546-549
Summary During the last few years, the Working Group Pesticide Residues of the Food Chemistry and Forensic Chemistry Division of the Gesellschaft Deutscher Chemiker (German Chemical Society) has organised six laboratory performance tests on the analysis of pesticide residues in which numerous laboratories were involved. In these tests the choice of the analytical methods for the examination of fats or vegetable substrates was free. Organochlorine pesticides in concentrations of over 0.01 mg/kg were mostly readily identified, whereas in the analysis of organophosphorus residues often only the classic compounds (parathion, diazinon, etc.) were reported. Many a false positive result could have been avoided by using more adequate methods for confirmatory analysis. The quantitative results, however, were generally quite reliable and were mostly within the official acceptable dispersion range. It can be concluded that the performance of a residue laboratory is by no means constant, and that it is necessary to assess regularly the quality of the analytical results by participating in such interlaboratory tests.
Qualitätssicherung bei Rückstandsanalysen von PflanzenschutzmittelnErfahrungen aus Ringversuchen相似文献
98.
Hans-Peter?MeyerEmail author Karen T.?Robins 《Monatshefte für Chemie / Chemical Monthly》2005,136(8):1269-1277
Summary. A competitive production method using the biotransformation of 4-butyrobetaine to enantiomerically pure L-carnitine was developed and scaled-up by Lonza. The process produces L-carnitine in 99.5% yield, and >99.9% enantiomeric excess (ee). Continuous and discontinuous processes were developed but the fed-batch process was found to be economically the most favourable process mode. 相似文献
99.
Hitzeroth G Vater J Franke P Gebhardt K Fiedler HP 《Rapid communications in mass spectrometry : RCM》2005,19(20):2935-2942
Streptocidins, a family of tyrocidine-like cyclic decapeptides, are an ideal demonstration object for the detection and in situ structure analysis of natural compounds directly in microbial cells using whole cell matrix-assisted laser desorption/ionization time-of-flight-mass spectrometry (MALDI-TOFMS), an emerging technique that can be used for rapid sensitive metabolic profiling of microorganisms. Five main members of the streptocidin family (A-E) were detected in Brevibacillus cells picked from agar plates and identified by in situ structure analysis with post-source decay MALDI-TOFMS. This efficient modern method allows the precise detection of metabolites within minutes without the need to isolate and purify the target compounds. The generated mass spectra are of similar quality to those obtained for the purified peptides. In addition, surface extracts were prepared by treating Brevibacillus cells with 70% acetonitrile in the presence of 0.1% trifluoroacetic acid and fractionated by high-resolution reversed-phase high-performance liquid chromatography (HPLC). In this way ten minor streptocidins were detected demonstrating the full biosynthetic variety of streptocidin production on the cellular level. The streptocidins differ from the well-known tyrocidines essentially in position 3 of the decapeptide chain by replacement of the aromatic amino acid (F/W) found in tyrocidines by L-leucine or L-valine. 相似文献
100.
The levorotatory diol 7 has been sequentially monosilylated, dehydrated, and oxidized at the allylic methylene group to provide (+)-12. The enantiomeric dextrorotatory diol 7 has been directed down a different sequence of steps involving monosilylation, dehydration, hydroboration, Swern oxidation, and regioselective introduction of a conjugated double bond to generate (-)-33. The novel feature of these transformations is that two key deoxycarbospironucleoside intermediates of the proper absolute configuration have been made available from enantiomerically related precursors. Also reported is a highly practical and reliable means for the formation of novel 2'-deoxyribonucleosides of novel structural type from these spirocyclic cyclopentenones. 相似文献