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排序方式: 共有133条查询结果,搜索用时 673 毫秒
1.
The site of preferred nucleophilic attack on XC6H5Cr(CO)3 complexes is more influenced by the conformation of the Cr(CO)3 group than by the nature of X substituent. This is supported both by experimental and theoretical results. 相似文献
2.
Abstract— A transformation system in Escherichia coli was employed to verify the extent of the lesions caused by ultraviolet (UV) and ionizing radiations. DNA inactivated at 280 nm could be reactivated to some extent by exposing the transforming DNA solutions at 240 nm. This reactivation has been tested using more than one strain of E. coli as recipient. Transforming DNA inactivated by ionizing radiations (90 Sr beta rays and 60 Co gamma rays) was not reactivable. Low doses of beta rays, however, reactivated the DNA inactivated by 280 nm UV to a slight but significant extent. 相似文献
3.
Arlette Solladi-Cavallo Marie-Claude Simon-Wermeister Dariouch Farkhani 《Helvetica chimica acta》1991,74(2):390-396
An asymmetric synthesis of the analogue 4a of isoproterenol and of di-o-pyvaloylepinephrine 3 which provides both enantiomers in optically pure state is reported. The key step of the method is the highly diastereo-selective reduction (using DIBAL or DIBAL/ZnCl2) of a β-ketosulfoxide 7 which leads, as desired, to the (S)- or the (R)-configuration at C(1) (Schemes 4 and 5). 相似文献
4.
[structure: see text] The first enantioselective total synthesis of (-)-Centrolobine is described. The key reaction is the synthesis of the cis-disubstituted tetrahydropyran framework by intramolecular cyclization of the enantiopure hydroxyketone 3 with Et3SiH and TMSOTf. The stereoselective reduction of the beta-ketosulfoxide 4 is the source of chirality. Revision of the absolute configuration of (-)-Centrolobine is proposed. 相似文献
5.
Engyodontochones,Antibiotic Polyketides from the Marine Fungus Engyodontium album Strain LF069 下载免费PDF全文
Prof. Dr. Bin Wu Dr. Jutta Wiese Arlette Wenzel‐Storjohann Susan Malien Dr. Rolf Schmaljohann Prof. Dr. Johannes F. Imhoff 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(22):7452-7462
Six new ( 2 , 4 – 8 ) and two known polyketides with a basic structure of an anthraquinone‐xanthone were isolated from mycelia and culture broth of the fungus Engyodontium album strain LF069. The structures and relative configurations of these compounds were established by spectroscopic means, and their absolute configurations were defined mainly by comparison of quantum chemical TDDFT calculated and experimental ECD spectra. Compounds 2 and 4 – 8 were given the trivial names engyodontochone A ( 2 ) and B–F ( 4 – 8 ). Compounds 5 – 8 represent the first example of a 23,28 seco‐beticolin carbon skeleton. The relative and absolute configurations of two known substances JBIR‐97/98 ( 1 ) and JBIR‐99 ( 3 ) were determined for the first time. All isolated compounds were subjected to bioactivity assays. Compounds 1 – 4 exhibited inhibitory activity against methicillin‐resistant Staphylococcus aureus (MRSA) that was 10‐fold stronger than chloramphenicol. 相似文献
6.
Arlette Solladi-Cavallo Charles Mioskowski 《Magnetic resonance in chemistry : MRC》1981,16(4):273-279
Lithium and magnesium enolates of an α-sulphinyl ester 83% 13C enriched at C-1 and C-2 are studied by 13C NMR at different temperatures. It is shown that two metaliated species are present in the case of lithium which exchange at ?60°C, ΔG≠ ?60° = 37.8 KJ mol?1 (9.05 Kcal mol?1) and that there is no rapid exchange between the non-metallated ester and the metallated species (on the NMR time scale). In the case of magnesium, two or three metallated species are formed, according to the temperature which do not exchange up to 0 °C. 相似文献
7.
Arlette Clayer Louis Agneray Gérard Vandenbussche et Pierre Petel 《Fresenius' Journal of Analytical Chemistry》1968,236(1):240-249
Résumé Le procédé de séparation des isotopes de l'hydrogène par chromatographie en lit mobile présenté est un procédé continu.Le principe en est le suivant: dans une colonne verticale, un lit mobile de silicagel descend par gravité. Le mélange à séparer (protium/deutérium) est introduit au milieu de cette colonne. La séparation du mélange en ses deux constituants s'effectue par échange à contre-courant entre la phase gazeuse et la phase adsorbée. Le protium, moins adsorbé, est soutiré en tête de colonne, le deutérium est soutiré en pied. Le reflux en bas de colonne est obtenu de 2 façons différentes qui dépendent du type de désorption employé: désorption du silicagel par élévation de températuré et élution du deuté rium, par de l'hélium désorption du silicagel par élévation de température et déplacement du deutérium, par de l'azote.Dans le ler cas, les gaz produits sont dilués dans l'hélium, dans le 2ème cas ils sortent non dilués. Le silicagel est remonté en tête de colonne par un transport pneumatique. L'expérimentation présentée à été faite avec gaz de déplacement azote.
Summary The preparation-process of hydrogen isotopes by chromatography in a moving bed in a continuous process is described.The principle is as follows: In a vertical column silicagel flows downward, by gravity, in a moving bed. The gaseous mixture to be separated (protium/deuterium) is introduced into the central portion of this column. Separation of the mixture into its two constituents is achieved by a counter-current exchange between the gaseous phase and the adsorbed phase. Protium less adsorbed is withdrawn at the top, deuterium is withdrawn at the bottom. The reflux is produced, at the bottom of the column, by two differents means, depending on the type of desorption used: Silicagel-desorption by rise of temperature and deuterium-elution by helium or displacing of deuterium by nitrogen.The gases produced are mixed with helium (1st case), or not (2nd case). Silicagel is carried again onto the top of the column by pneumatic transport. The experiments presented were carried out by means of nitrogen as displacing gas.相似文献
8.
250 MHz 1H NMR of two monosubstituted [2.2]paracyclophanes shows that whether the substituent is an electron releasing group, OMe, or a withdrawing group, CO2Me, the transannular effect is deshielding. 62.86 MHz 13C NMR shows that among the six transannular effects, only one has a sign which changes with the nature of the substituent (–4.6 ppm for OMe but +3.3 ppm for CO2Me). 相似文献
9.
Guy Solladié Christine Greck Gilles Demailly Arlette Solladié-Cavallo 《Tetrahedron letters》1982,23(48):5047-5050
It is shown that reductions of β-ketosulfoxides of identical chirality (R) at sulfur, lead to reduction products of opposite stereochemistry according to the reducing agent used. The high enantiomeric excesses obtained (80 to 100%) provide a general route to both enantiomers of methylcarbinols from the corresponding esters. 相似文献
10.
Trokiner A Bessière A Thouvenot R Hau D Marko J Nardello V Pierlot C Aubry JM 《Solid state nuclear magnetic resonance》2004,25(4):209-215
In order to get some insight into the mechanism of the disproportionation of hydrogen peroxide catalyzed by calcium hydroxide, 43Ca NMR spectra of enriched samples of calcium peroxides and of their precursors have been studied in both solution and solid state. This study demonstrates that no well-defined peroxidized calcium species are formed in solution, showing that the catalytic role of calcium is likely restricted to the solid state. Most of the calcium compounds that could be involved in the catalytic process have been investigated with solid state NMR. The shift and quadrupolar parameters of Ca(OH)2, CaO2.8H2O and CaO2.2H2O2 are reported for the first time. These parameters are different enough to allow the quantitative analysis of a complex mixture of these compounds by NMR. 相似文献