全文获取类型
收费全文 | 1024篇 |
免费 | 35篇 |
国内免费 | 1篇 |
专业分类
化学 | 794篇 |
晶体学 | 13篇 |
力学 | 26篇 |
数学 | 61篇 |
物理学 | 166篇 |
出版年
2022年 | 9篇 |
2021年 | 12篇 |
2020年 | 15篇 |
2019年 | 18篇 |
2017年 | 6篇 |
2016年 | 34篇 |
2015年 | 29篇 |
2014年 | 23篇 |
2013年 | 43篇 |
2012年 | 63篇 |
2011年 | 59篇 |
2010年 | 49篇 |
2009年 | 34篇 |
2008年 | 57篇 |
2007年 | 55篇 |
2006年 | 64篇 |
2005年 | 40篇 |
2004年 | 50篇 |
2003年 | 27篇 |
2002年 | 42篇 |
2001年 | 20篇 |
2000年 | 20篇 |
1999年 | 13篇 |
1998年 | 10篇 |
1997年 | 20篇 |
1996年 | 17篇 |
1995年 | 12篇 |
1994年 | 13篇 |
1993年 | 12篇 |
1992年 | 17篇 |
1991年 | 7篇 |
1990年 | 5篇 |
1989年 | 7篇 |
1988年 | 4篇 |
1987年 | 9篇 |
1986年 | 7篇 |
1984年 | 5篇 |
1983年 | 5篇 |
1982年 | 9篇 |
1981年 | 10篇 |
1980年 | 5篇 |
1979年 | 8篇 |
1978年 | 5篇 |
1977年 | 4篇 |
1976年 | 14篇 |
1975年 | 7篇 |
1972年 | 4篇 |
1958年 | 4篇 |
1955年 | 4篇 |
1914年 | 4篇 |
排序方式: 共有1060条查询结果,搜索用时 15 毫秒
1.
Dihydrothiinone 9a undergoes photocycloaddition regioselectively to all three C?C bonds of penta‐1,2,4‐triene ( 10 ), the relative stabilities of the biradical intermediates determining the product distribution. In contrast, cyclohexenone 9b and dihydropyranone 9c afford more complex mixtures of bicyclo[4.2.0]octanones, which also turn out to be less stable on chromatographic workup, reflecting the higher strain due to the shorter bond lengths (C? O and C? C vs. C? S) in the six‐membered rings, respectively. 相似文献
2.
Sodium phosphate tellurite glasses in the system (NaPO(3))(x)(TeO(2))(1-) (x) were prepared and structurally characterized by thermal analysis, vibrational spectroscopy, X-ray photoelectron spectroscopy (XPS) and a variety of complementary solid-state nuclear magnetic resonance (NMR) techniques. Unlike the situation in other mixed-network-former glasses, the interaction between the two network formers tellurium oxide and phosphorus oxide produces no new structural units, and no sharing of the network modifier Na(2)O takes place. The glass structure can be regarded as a network of interlinked metaphosphate-type P(2) tetrahedral and TeO(4/2) antiprismatic units. The combined interpretation of the O 1s XPS data and the (31)P solid-state NMR spectra presents clear quantitative evidence for a nonstatistical connectivity distribution. Rather, the formation of homoatomic P--O--P and Te--O--Te linkages is favored over mixed P--O--Te connectivities. As a consequence of this chemical segregation effect, the spatial sodium distribution is not random, as also indicated by a detailed analysis of (31)P/(23)Na rotational echo double-resonance (REDOR) experiments. 相似文献
3.
A method is described for the determination of particulate chromium and dissolved chromium(III) and (VI) in water at μg l-1 levels. Particulate material is collected by filtration of the water sample through a membrane filter (0.4-μm pore-size). Chromium(III) and chromium(VI) are then coprecipitated, separately and in that order, with iron(III) hydroxide (at pH 8.5) and a cobalt—pyrrolidinedithiocarbamate carrier complex (at pH 4.0). Both precipitates are collected as thin films on membrane filters and, with the particulate material, analysed directly for chromium by x-ray fluorescence spectrometry. Detection limits, for a 100-ml water sample and counting times of 100 s, are 0.1 μg Cr l-1. The method is unaffected by sea salt and is applicable, without modifications, to river and estuarine waters. 相似文献
4.
Nicola Armaroli Gianluca Accorsi JohnN. Clifford Jean‐Franois Eckert Jean‐Franois Nierengarten 《化学:亚洲杂志》2006,1(4):564-574
Oligophenylenevinylene (OPV)‐terminated phenylenevinylene dendrons G1 – G4 with one, two, four, and eight “side‐arms”, respectively, were prepared and attached to C60 by a 1,3‐dipolar cycloaddition of azomethine ylides generated in situ from dendritic aldehydes and N‐methylglycine. The relative electronic absorption of the OPV moiety increases progressively along the fullerodendrimer family C60G1 – C60G4 , reaching a 99:1 ratio for C60G4 (antenna effect). UV/Vis and near‐IR luminescence and transient absorption spectroscopy was used to elucidate photoinduced energy and electron transfer in C60G1 – C60G4 as a function of OPV moiety size and solvent polarity (toluene, dichloromethane, benzonitrile), taking into account the fact that the free‐energy change for electron transfer is the same along the series owing to the invariability of the donor–acceptor couple. Regardless of solvent, all the fullerodendrimers exhibit ultrafast OPV→C60 singlet energy transfer. In CH2Cl2, the OPV→C60 electron transfer from the lowest fullerene singlet level (1C60*) is slightly exergonic (ΔGCS≈0.07 eV), but is observed, to an increasing extent, only in the largest systems C60G2 – C60G4 with lower activation barriers for electron transfer. This effect has been related to a decrease of the reorganization energy upon enlargement of the molecular architecture. Structural factors are also at the origin of an unprecedented OPV→C60 electron transfer observed for C60G3 and C60G4 in apolar toluene, whereas in benzonitrile, electron transfer occurs in all cases. Monitoring of the lowest fullerene triplet state by sensitized singlet oxygen luminescence and transient absorption spectroscopy shows that this level is populated through intersystem crossing and is not involved in photoinduced electron transfer. 相似文献
5.
Contributions to the Chemistry of Transition Metal Alkyl Compounds. XLVI. Polarographic Properties of Dibenzyl and Diphenyl Titanium in Aprotic Solvents The polarographic properties of dibenzyl and diphenyl titanium were investigated using acetonitrile (AN), dimethylformamide (DMF), and dimethylsulfoxide (DMSO) as solvents. Both compounds are reduced irreversibly at very negative potentials in a two-electron step with formation of titanium. The half-wave potentials depend on the donor strength of the solvents. With increasing donor number the half-wave potentials are shifted to more negative values. The fact that the limiting currents turned out to be diffusion controlled suggests a monomeric structure of the titanium diorganyls in AN, DMF, and DMSO. 相似文献
6.
Herbert Schumann Kerstin Zietzke Frank Erbstein Roman Weimann 《Journal of organometallic chemistry》1996,520(1-2):265-268
LnCl3 reacts with Na(C5Me41Bu) in THF to form the organolanthanoid chlorides [(C5Me41Bu)2LnCl(THF)] (Ln = La (1a), Lu (1b)). Compounds 1a and 1b yield in reaction with NaO2CCH3 the monomeric organolanthanoid acetates [(C5Me41Bu)2LnO2CCH3] (Ln = La (2a), Lu (2b)). The single crystal X-ray structure analysis of 2b as well as the cryoscopic molecular weight investigation of 2a verify the monomeric structure of these complexes. 相似文献
7.
G. Eckert 《Fresenius' Journal of Analytical Chemistry》1956,153(4):261-267
Zusammenfassung Es wird ein Verfahren zur colorimetrischen Bestimmung kleiner Aluminiumgehalte (0,1–0,002% Al) in Nickellegierungen für Oxydkathoden beschrieben. Die Trennung des Aluminiums von dem großen Nickelüberschuß und anderen Schwermetallen wird durch negative Extraktion der Diäthyldithiocarbaminate in bestimmten Halogenkohlenwasserstoffen erreicht. Anschließend kann Aluminium mit dem Ammoniumsalz der Aurintricarbonsäure photometrisch bestimmt werden. Der Einfluß verschiedener Elemente auf die Farblackbildung wurde quantitativ untersucht.Das beschriebene Verfahren ist auch für die Aluminiumbestimmung in Eisen und Eisenlegierungen geeignet.Bei der Durchführung der beschriebenen Untersuchungen wurde ich von Frl. E. Bauersachs unterstützt. Herrn Dr. W. Schmidt, Röhrenfabrik der Siemens u. Halske AG, danke ich für wertvolle Diskussionen. 相似文献
8.
A miniaturised technique to analyse and detect heterocyclic aromatic amines (HAs) using micro solid-phase extraction (SPE) coupled on-line (in-capillary) to capillary electrophoresis (CE) separation with nanospray (nESI) mass spectrometry (MS) detection has been developed. HAs are mutagenic and carcinogenic compounds formed at low levels in protein-rich food during cooking. Due to the low concentrations of HAs and the high complexity of the matrix in which they exist, sensitive and selective analytical methods are required for quantification. SPE was performed on a packed bed of C18 particles inside the CE capillary, which minimised the dead volume. The on-line coupling of SPE, CE and nESI-MS reduced the time for extraction and identification to less than half an hour, which will allow for screening of several samples per day. The new technique provides short analysis time, low sample and solvent consumption, and HAs in standard solutions were easily detected at 12–17 fmol injections, and in spiked urine samples at 750–810 fmol injections. 相似文献
9.
A method is described for the determination of anionic detergents. The detergent anions are extracted into chloroform as an ion-association compound with the bis(ethylenediamine)copper(II) cation. Determinations are completed by colorimetry or atomic absorption spectrometry. With a 150-ml water sample, the limit or detection is 0.03 μg ml-1 (as LAS) for colorimetry or 0.06 μg ml-1 for a.a.s. The method requires only one phase separation step and is highly selective. It is directly applicable to brine and sea-water samples. 相似文献
10.
Dietmar Rahner Stefan Machill Heike Schlörb Kerstin Siury Matthias Kloss Waldfried Plieth 《Journal of Solid State Electrochemistry》1998,2(2):78-84
An overview is given of intercalation materials for both the negative and the positive electrodes of lithium batteries, including
the results of our own research. As well as lithium metal as a negative electrode, we consider insertion materials based on
aluminium alloys. In the case of the positive electrode metal-oxides based on manganese, nickel and cobalt are discussed.
Received: 27 May 1997 / Accepted: 30 July 1997 相似文献