全文获取类型
收费全文 | 4011篇 |
免费 | 90篇 |
国内免费 | 21篇 |
专业分类
化学 | 2708篇 |
晶体学 | 16篇 |
力学 | 105篇 |
数学 | 770篇 |
物理学 | 523篇 |
出版年
2020年 | 29篇 |
2019年 | 44篇 |
2016年 | 73篇 |
2015年 | 62篇 |
2014年 | 69篇 |
2013年 | 184篇 |
2012年 | 171篇 |
2011年 | 196篇 |
2010年 | 114篇 |
2009年 | 97篇 |
2008年 | 215篇 |
2007年 | 182篇 |
2006年 | 173篇 |
2005年 | 174篇 |
2004年 | 137篇 |
2003年 | 122篇 |
2002年 | 127篇 |
2001年 | 77篇 |
2000年 | 69篇 |
1999年 | 54篇 |
1998年 | 50篇 |
1997年 | 57篇 |
1996年 | 63篇 |
1995年 | 52篇 |
1994年 | 44篇 |
1993年 | 57篇 |
1992年 | 49篇 |
1991年 | 42篇 |
1990年 | 41篇 |
1989年 | 53篇 |
1988年 | 55篇 |
1987年 | 46篇 |
1986年 | 26篇 |
1985年 | 69篇 |
1984年 | 63篇 |
1983年 | 53篇 |
1982年 | 69篇 |
1981年 | 57篇 |
1980年 | 75篇 |
1979年 | 70篇 |
1978年 | 62篇 |
1977年 | 50篇 |
1976年 | 38篇 |
1975年 | 51篇 |
1974年 | 46篇 |
1973年 | 49篇 |
1972年 | 24篇 |
1971年 | 32篇 |
1968年 | 22篇 |
1966年 | 20篇 |
排序方式: 共有4122条查询结果,搜索用时 15 毫秒
101.
Beech CL Coope JF Fairley G Gilbert PS Main BG Plé K 《The Journal of organic chemistry》2001,66(7):2240-2245
A new "safety catch" linker for esters has been synthesized on polystyrene resin. This 2-tert-butoxyphenol resin 10 may be acylated to give a relatively stable ester that will allow nucleophilic chemistry without reaction at the linking ester group. Removal of the tert-butyl group with acid unmasks a highly reactive 2-hydroxyphenyl ester that reacts readily with nucleophiles to cause release of the product from the resin. This sequence has been exemplified by acylating the resin with various bromo acids, carrying out nucleophilic displacements with thiols, phenols, or amines, activating the ester with trifluoroacetic acid and cleaving from the resin with amines to give the (nucleophile) substituted carboxamides in high yield and purity. Kinetic studies with a model ester revealed half-lives for reaction with morpholine of 119 h for the tert-butoxyphenyl ester and 1 min for the corresponding phenol. 相似文献
102.
Gerald Kainz und Franz Scheidl 《Fresenius' Journal of Analytical Chemistry》1964,202(4):251-258
Zusammenfassung In der vorliegenden Arbeit wurde untersucht, wie man die vergaste Probe am besten durch die Verbrennungszone führt, damit man eine vollständige Vermischung mit Sauerstoff erhält; denn bei hohen Temperaturen ist nicht so sehr die Umsatzgeschwindigkeit der Probe mit Sauerstoff bestimmend, sondern die Mischungsgeschwindigkeit mit Sauerstoff. Es wurden zwei Möglichkeiten untersucht. Bei der schnellen Vergasung im geraden Rohr ergab sich, daß sich dabei ein Substanzpfropf bildet, der durch Sauerstoff (im Wege der Diffusion) durchdrungen werden muß. Eine quantitative Oxydation ist nur dann möglich, wenn die Einwaage auf den Durchmesser des Verbrennungsrohres abgestimmt ist. Bei Mikroeinwaagen (3 mg) muß der Durchmesser des Verbrennungsrohres mindestens 20 mm betragen.Eine wesentlich intensivere Vermischung der Substanzdämpfe mit Sauerstoff wird bei der Verbrennung in der Flamme erreicht, die auf Wickbold bzw. Radmacher zurückgeht. Hierbei wird die Probe in einem engen Rohr unter Durchleiten von Sauerstoff oder Stickstoff vergast und durch eine enge Düse in die heiße Verbrennungszone eingeblasen. An der Düse bildet sich eine Flamme, in die Sauerstoff intensiv eindiffundiert. Eine einfache Apparatur, die eine schnelle, bei Mikroeinwaagen von der Geschicklichkeit des Analytikers unabhängige Vergasung und Verbrennung zuläßt, wird beschrieben.
Summary In this paper the problem is investigated how to pass the vaporised sample through the combustion zone in order to attain a complete mixture with oxygen; at high temperatures the completeness of combustion depends not so much on the rate of reaction with oxygen than on the rate of mixing with oxygen. Two experimental procedures have been investigated. High speed vaporisation in straight tubes leads to a plugshaped concentration of the sample, which must be penetrated by oxygen (diffusion). Using this procedure, a quantitative oxidation is only possible, if the sample weight is in a proper relation to the diameter of the tube. In microanalysis (3 mg of sample) the diameter of the combustion tube must be 20 mm or more.A much more intensive mixing of the vaporised sample with oxygen is attained by the combustion in the flame proposed by Wickbold and Radmacher. The vaporised sample is blown out of a small tube through a narrow nozzle into the heated combustion zone. On the nozzle a flame is formed, in which oxygen is diffusing intensively. A simple apparatus is described, which permits a rapid vaporisation and quantitative oxidation. Analysing micro samples the vaporisation was found to be independent of the skill of the analyst.相似文献
103.
Density functional theories of solvation forces in charged fluids are extended to treat electrolytes consisting of finitesized ions and neutral solvent particles. The resulting forces display pronounced oscillations whose magnitude is a strong function of the bulk density of the neutral species. 相似文献
104.
Amperometric method for determining the degree of complexation of polyelectrolytes with cationic surfactants 总被引:1,自引:0,他引:1
Hakkarainen S Gilbert SL Kontturi AK Kontturi K 《Journal of colloid and interface science》2004,272(2):404-410
The complexation of sodium polystyrene sulfonate with monovalent cationic surfactants at a microsized liquid/liquid interface has been studied using electrochemistry. The method is based on measurement of surfactant ion transfer across the interface between two immiscible electrolyte solutions (ITIES). The complexation of various cationic surfactants (alkylpyridinium- and trimethylammonium-) with oligosized polystyrene sulfonate was measured. Binding isotherms were used to determine the degree of binding as a function of the surfactant chain length and type of head group. It was found that the hydrophobicity of the surfactant was the predominant factor. The effect of the polyelectrolyte chain length on the binding mechanism was studied using cetylpyridinium chloride as a complexing agent. It was found that binding affinity, as well as cooperativity of the binding process, decreases with decreasing polyelectrolyte chain length. Thermodynamics of surfactant binding was measured using titration microcalorimetry. The thermodynamic data obtained show that the enthalpy of surfactant binding is not dependent on polymer chain length, but an increase in chain length makes the binding process entropically more favorable. 相似文献
105.
Zhang B Harb JN Davis RC Kim JW Chu SH Choi S Miller T Watt GD 《Inorganic chemistry》2005,44(10):3738-3745
Horse spleen ferritin (HoSF) containing 800-1500 cobalt or 250-1200 manganese atoms as Co(O)OH and Mn(O)OH mineral cores within the HoSF interior (Co-HoSF and Mn-HoSF) was synthesized, and the chemical reactivity, kinetics of reduction, and the reduction potentials were measured. Microcoulometric and chemical reduction of HoSF containing the M(O)OH mineral core (M = Co or Mn) was rapid and quantitative with a reduction stoichiometry of 1.05 +/- 0.10 e/M forming a stable M(OH)(2) mineral core. At pH 9.0, ascorbic acid (AH(2)), a two-electron reductant, effectively reduced the mineral cores; however, the reaction was incomplete and rapidly reached equilibrium. The addition of excess AH(2) shifted the reaction to completion with a M(3+)/AH(2) stoichiometry of 1.9-2.1, consistent with a single electron per metal atom reduction. The rate of reaction between M(O)OH and excess AH(2) was measured by monitoring the decrease in mineral core absorbance with time. The reaction was first order in each reactant with second-order rate constants of 0.53 and 4.74 M(-1) min(-1), respectively, for Co- and Mn-HoSF at pH 9.0. From the variation of absorbance with increasing AH(2) concentration, equilibrium constants at pH 9.0 of 5.0 +/- 1.9 for Co-HoSF and 2.9 +/- 0.9 for Mn-HoSF were calculated for 2M(O)OH + AH(2) = 2M(OH)(2) + D, where AH(2) and D are ascorbic acid and dehydroascorbic acid, respectively. Consistent with these equilibrium constants, the standard potential for the reduction of Co(III)-HoSF is 42 mV more positive than that of the ascorbic acid reaction, while the standard potential of Mn(III)-HoSF is 27 mV positive relative to AH(2). Fe(2+) in solution with Co- and Mn-HoSF under anaerobic conditions was oxidized to form Fe(O)OH within the HoSF interior, resulting in partial displacement of the Co or Mn by iron. 相似文献
106.
A new procedure for simultaneous preconcentration, separation, and determination of permanganate, chromate and vanadate was developed for aqueous solutions containing p.p.b. levels of these anions (and molybdate). The presence of other (non-complexing) ions in p.p.m. concentrations did not interfere. The procedure consisted of reactive adsorption of all three anions as their reduced cations, on iron(II)-treated resin, followed by elution of vanadium(IV) with 0.01 M HNO3/H2O2, manganese(II) with 0.35 M HCl, and chromium(III) with 4M HCl. Concentration factors of 40 were obtained with 1-l “samples” 10 p.p.b. concentrations were determined with standard deviations of 4–5 % by a.a.s. and conventional spectrophotometry. 相似文献
107.
Prof. Gerald B. Porter 《Theoretical chemistry accounts》1972,24(2-3):265-270
Analysis of the transient and steady-state kinetics of reversible energy transfer shows that while the interpretation of lifetime measurements is difficult unless the donor and acceptor lifetimes are appreciably different, quantum yield measurements are relatively easy to interpret.
Dedicated to the memory of Professor K. H. Hansen. 相似文献
Zusammenfassung Die Analyse der Kinetik der Übergangszustände und der stationären Zustände der reversiblen Energieübertragung zeigt, daß im Gegensatz zu einer schwierigen Interpretation der Messungen der Lebensdauer — es sei denn die Lebensdauer von Donor und Acceptor sind wesentlich voneinander verschieden — die Messungen der Quantumausbeute verhältnismäßig einfach zu interpretieren sind.
Résumé L'analyse de la cinétique de l'état transitoire et de l'état stationnaire du transfert réversible d'énergie montre que, si l'interprétation des mesures de durée de vie est difficile, à moins queles durées de vie du donneur et de l'accepteur soient très différentes, il est par contre relativement facile d'interpréter les mesures de rendement quantique.
Dedicated to the memory of Professor K. H. Hansen. 相似文献
108.
Godwin O. Mbagwu R. Gerald Bass Richard A. Glennon 《Journal of heterocyclic chemistry》1985,22(2):465-474
The ring-opening reactions of seven mesoionic thiazolo[3,2-a]pyrimidine-5,7-diones by a series of primary and secondary amines have been investigated. The rates of the ring fission of five N(8)-substituted mesoionic xanthines with benzylamine were measured and found to follow second order kinetics. The Hammett relationship is followed with ? value of + 0.48 in p-dioxane as solvent. The dependence of rates on temperatures has been studied for the N(8)-ethyl derivative; the energy of activation (Δ E* ) is 25.3 kcals mol?1, the enthalpy of activation (Δ H* ) is 24.7 kcals mol?1 and the entropy of activation (Δ S* ) is — 4.9 e.u. A slight increase in rate of reaction was observed when the solvent was changed from p-dioxane to dimethyl sulfoxide. In p-dioxane at constant mesoionic xanthine concentration, the rate constant for ring opening decreased with increasing benzylamine concentration. These results are consistent with a bimolecular nucleophilic mechanism proceeding by the rate-determining formation of a charged tetrahedral transition state. 相似文献
109.
Fifteen of twenty-four silver(I) carboxylates examined give useful electron impact mass spectra. The compounds vaporize at moderate temperatures, apparently mainly as dimer with traces of higher oligomer in only a few cases. The molecular ion for the dimer is generally weak or absent, with the most abundant silver containing ion being [Ag2(O2CR)]+ in most cases. Metastable defocusing and deuterium labeling experiments on silver acetate have established some of the fragmentation pathways. The reported loss of carbon dioxide from perfluorocarboxylates to give intense peaks for organosilver ions was not observed in this study. Attempts to obtain spectra on the silver salts of organic materials other than carboxylic acids were successful in several cases. Silver trifluoromethanesulfonate, although much less volatile, gives a spectrum and fragmentation very much like the carboxylates, whereas silver trifluoromethanethiolate gives a complex spectrum which suggests tetramer as a major gas phase species. Of three compounds examined which have silver to nitrogen bonding only silver(II) phthalocyanine is sufficiently volatile to give a spectrum without decomposition. The field desorption spectra of the four compounds examined all show the ions AgnXn ? 1 for X=acetate (n=1 ? 6), X=p-chlorobenzoate (n=1 ? 4), X=methanesulfonate (n=1 ? 7) and X=p-toluenesulfonate (n=1 ? 5). 相似文献
110.
In a combination of cobalt-mediated cationic and free radical chemistry, treatment of the allyl silane substituted β-hydroxycobaloxime 13 with pTSA led predominantly to the trans-disubstituted cyclopentane 14, which on irradiation with TEMPO produced 15. In a similar manner, the cobaloxime 18 was cyclised to 19 in the presence of pTSA which, on irradiation with a sunlamp then underwent radical mediated cyclisation to the substituted indane 20. 相似文献