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排序方式: 共有1319条查询结果,搜索用时 390 毫秒
221.
Lucia De Crescentini Gianfranco Favi Giacomo Mari Gianluca Ciancaleoni Marcello Costamagna Stefania Santeusanio Fabio Mantellini 《Molecules (Basel, Switzerland)》2021,26(21)
Here we report the synthesis of interesting 3-alkyl-4-hydroxy-1-aryl-4-(propa-1,2-dienyl)1H-pyrazol-5(4H)-ones and 9-alkyl-7-aryl-1-oxa-7,8-diazaspiro[4.4]nona-3,8-dien-6-ones, starting from 1,2-diaza-1,3-dienes (DDs) and propargyl alcohol. The reaction proceeds through a sequence Michael-type nucleophilic attack/cyclization/[2,3]-Wittig rearrangement. In the same way, the reaction between the aforementioned DDs and allyl alcohol furnished 4-allyl-4-hydroxy-3-alkyl-1-aryl-1H-pyrazol-5(4H)-ones. A DFT study was also carried out, in order to have decisive clarifications about the mechanism. 相似文献
222.
In this work the kinetics of the isothermal crystallization from the melt of isotactic polyolefins in quiescent conditions as well as after the application of a step-shear flow is investigated by means of rheological measurements. It is shown that the kinetics of the crystallization, as measured by the increase of the storage modulus, is not affected by the strain amplitude and the frequency of the oscillation, once they are properly chosen. A strong enhancement of the crystallization kinetics has been obtained when the step-shear flow was applied at the crystallization temperature TC=92°C for two different molecular weight poly(1-butene) samples (i-PB400 and i-PB200) and at TC=137°C for a polypropylene (i-PP). In particular, the overall-crystallization rate constants of the i-PB400 increased with increasing the applied shear rate at a constant total strain of 60. At higher shear flow temperatures slower kinetics occurred in all the cases until the effect of the applied shear flow was lost. Moreover, the effect of the molecular weight on the flow induced crystallization phenomenon is investigated on the two i-PB samples and the results have clearly shown that the higher molecular weight i-PB200 polymer is much more sensitive than the i-PB400 to the flow history. 相似文献
223.
Riccardo Destro Emanuele Ortoleva Giorgio Modena Lucia Pasquato Vittorio Lucchini 《Helvetica chimica acta》2001,84(4):860-866
Four‐membered ring thiosulfonium ions may be obtained quantitatively and under mild conditions by anionotropic rearrangement of C‐(tert‐butyl)‐substituted thiiranium ion precursors. Thus, t‐4‐(tert‐butyl)‐r‐1,2,2,c‐3‐tetramethylthietanium tetrafluoroborate or hexachloroantimonate ( 2a or 2b , resp.) were formed from thiiranium ion 1 . The thietanium salts 2a and 2b were characterized by X‐ray crystal‐structure analysis. Their cation geometry was also optimized by ab initio calculations at the RHF/6‐31G*//RHF/6‐31G* level, as were those of its stereoisomer 3 and of the unsubstituted S‐methylthietanium ion 5 . Comparison of 2 , 3 , and 5 with 4 – the only other thietanium ion studied by XRD, where the C‐atoms of the thioniacyclobutane ring are part of a trinorbornane skeleton – indicates that, in these systems, relief from substituent overcrowding is easily achieved by a folding of the four‐membered ring along the line connecting the two opposite C‐atoms. The corresponding ring‐deformation normal mode has a calculated frequency as low as 109 cm−1 in ion 5 , to be compared with a frequency of 138 cm−1 in methylcyclobutane. For thietanium ion 2 , the frequencies of the two normal modes involving such ring deformation have calculated values of 61 and 85 cm−1. 相似文献
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Jin-Shuang Zhao Yan-Fei Mu Li-Yuan Wu Zhi-Mei Luo Lucia Velasco Maxime Sauvan Dooshaye Moonshiram Jia-Wei Wang Min Zhang Tong-Bu Lu 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2024,136(21):e202401344
The development of high-performance photocatalytic systems for CO2 reduction is appealing to address energy and environmental issues, while it is challenging to avoid using toxic metals and organic sacrificial reagents. We here immobilize a family of cobalt phthalocyanine catalysts on Pb-free halide perovskite Cs2AgBiBr6 nanosheets with delicate control on the anchors of the cobalt catalysts. Among them, the molecular hybrid photocatalyst assembled by carboxyl anchors achieves the optimal performance with an electron consumption rate of 300±13 μmol g−1 h−1 for visible-light-driven CO2-to-CO conversion coupled with water oxidation to O2, over 8 times of the unmodified Cs2AgBiBr6 (36±8 μmol g−1 h−1), also far surpassing the documented systems (<150 μmol g−1 h−1). Besides the improved intrinsic activity, electrochemical, computational, ex-/in situ X-ray photoelectron and X-ray absorption spectroscopic results indicate that the electrons photogenerated at the Bi atoms of Cs2AgBiBr6 can be directionally transferred to the cobalt catalyst via the carboxyl anchors which strongly bind to the Bi atoms, substantially facilitating the interfacial electron transfer kinetics and thereby the photocatalysis. 相似文献
229.
Dario Braga Lucia Maini Fabrizia Grepioni 《Angewandte Chemie (International ed. in English)》1998,37(16):2240-2242
Cooperative strong negatively charged and neutral O–H⋅⋅⋅O as well as weak charge-assisted C–Hδ+⋅⋅⋅Oδ− hydrogen-bonding interactions have been utilized for the crystal engineering of organometallic FeII/CoIII and FeII/CrI sandwich complexes. A section of the structure of [(η6-C6H6)2Cr]+{[(η5-C5H4COOH)(η5-C5H4COO)Fe][(η5-C5H4COOH)2Fe]0.5}− is shown in the picture. 相似文献
230.