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排序方式: 共有268条查询结果,搜索用时 15 毫秒
151.
David Trummer Anna Giorgia Nobile Pierre-Adrien Payard Anton Ashuiev Yuya Kakiuchi Daniel Klose Gunnar Jeschke Christophe Copret 《Chemical science》2022,13(37):11091
The Union Carbide (UC) ethylene polymerization catalysts, based on chromocene dispersed on silica, show distinct features from the Phillips catalysts, but share the same heated debate regarding the structure of their active sites. Based on a combination of IR, EPR spectroscopies, labeling experiments, and DFT modeling, we identified monomeric surface-supported Cr(iii) hydrides, ( SiO)Cr(Cp)–H, as the active sites of the UC catalyst. These sites are formed in the presence of grafted and adsorbed chromocene as well as residual surface OH groups, only possible at high Cr loading, and involve a C–H activation of the Cp ring. These Cr-hydrides initiate polymerization, yielding Cr(iii) alkyl species that insert ethylene through a Cossee–Arlman-type mechanism, as evidenced by spectroscopic studies. These insights inspired the design of a well-defined analog, CpCr(CH(SiMe3)2)2 grafted on partially dehydroxylated silica, that shows similar spectroscopic and polymer structure to the UC catalyst, further supporting the proposed active site structure.A combined SOMC and DFT studies on the Union Carbide catalyst enable to propose that Cr(iii)-H, formed at high-loading, are active in ethylene polymerization. We therefore design a well-defined supported CpCr(iii)-R ethylene polymerization catalyst. 相似文献
152.
153.
Hikaru Kondo Dr. Takuya Kochi Prof. Dr. Fumitoshi Kakiuchi 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(8):1737-1741
We report here a method for in situ generation of various ruthenium carbonyl phosphine catalysts for arylation via cleavage of inert aromatic carbon–oxygen bonds. The use of catalyst systems consisting of [RuCl2(CO)(p-cymene)], CsF, styrene, and phosphines enabled facile screening of phosphine ligands. Asymmetric C–O arylation was also achieved for atropo-enantioselective biaryl synthesis using a chiral monodentate phosphine ligand. 相似文献
154.
We study the origin of a sudden change of the radius of oxygen isotopes at 23O in terms of a coupled-channel model approach. We use the cluster-orbital shell model approach for describing the system. Qualitatively, the sudden change of the radius can be understood as the competition of occupation of the valence neutrons and the component of the core configuration. For the 23O case, the effect of the partially Pauli-forbidden states of the 1s 1/2-orbit is small in the excited-core configuration. 相似文献
155.
Yusuke MizutaniHiroki Tanimoto Tsumoru MorimotoYasuhiro Nishiyama Kiyomi Kakiuchi 《Tetrahedron letters》2012,53(44):5903-5906
Double nucleophilic N-alkylation of α-oxime-esters, affording N,N-dialkyl-α-amino acids is herein described. Grignard reagents accomplished double N-alkylations via umpolung and various N,N-dialkylated α-amino acids were successfully synthesized in 15 min. Both electron-withdrawing sulfonyl groups and electron-donating silyl and methyl groups on oximes were available. Alkylmagnesium species and (E)-configuration of α-oxime-ester were essential to this cascade reaction. 相似文献
156.
Yukinori Yasui Yuki Kitazumi Hiroyuki Mizunuma Naoya Nishi Takashi Kakiuchi 《Electrochemistry communications》2010,12(11):1479-1482
The ultraslow relaxation, on the order of a few seconds or longer, of the structure of the electrical double layer in response to the change in the phase-boundary potential across the ionic liquid (IL)|water(W) interface, which was recently reported for trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide, has been confirmed in two new hydrophobic ionic liquids, trihexyltetradecylphosphonium bis(nonafluorobutanesulfonyl)amide and trihexyltetradecylphosphonium tetrakis(pentafluorophenyl)borate. A comparison of the degree of the hysteresis in electrocapillary curves for these ILs with those for trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide demonstrates that the degree of the hysteresis is not correlated with the viscosity of these ILs. The ultraslow relaxation of the electrical double layer seems to be a general feature of ILs at electrified interfaces. 相似文献
157.
Kakiuchi F Kochi T Mizushima E Murai S 《Journal of the American Chemical Society》2010,132(50):17741-17750
Mechanistic studies of the ruthenium-catalyzed reaction of aromatic ketones with olefins are presented. Treatment of the original catalyst, RuH(2)(CO)(PPh(3))(3), with trimethylvinylsilane at 90 °C for 1-1.5 h afforded an activated ruthenium catalyst, Ru(o-C(6)H(4)PPh(2))(H)(CO)(PPh(3))(2), as a mixture of four geometric isomers. The activated complex showed high catalytic activity for C-H/olefin coupling, and the reaction of 2'-methylacetophenone with trimethylvinylsilane at room temperature for 48 h gave the corresponding ortho-alkylation product in 99% isolated yield. The activated catalyst was thermally robust and showed excellent catalytic activity under refluxing toluene conditions. (1)H and (31)P NMR studies of the C-H/olefin coupling at room temperature suggested that an ortho-ruthenated complex, P,P'-cis-C,H-cis-Ru(2'-(6'-MeC(6)H(4)C(O)Me))(H)(CO)(PPh(3))(2), participated in the reaction as a key intermediate. Isotope labeling studies using acetophenone-d(5) indicated that the rate-limiting step was the C-C bond formation, not the C-H bond cleavage, and that each step prior to the reductive elimination was reversible. The rate of C-H/olefin coupling was found to exhibit pseudo first-order kinetics and to show first-order dependence on the ruthenium complex concentration. 相似文献
158.
Hiroaki Kakiuchi Hiromasa Ohmi Kei Nakamura Yoshihito Yamaguchi Kiyoshi Yasutake 《Plasma Chemistry and Plasma Processing》2010,30(5):579-590
We have investigated the structure and stability of SiN
x
films deposited with very high rates (>50 nm/s) in atmospheric-pressure (AP) He-based plasma excited by a 150 MHz very high-frequency
(VHF) power using a cylindrical rotary electrode at room temperature. The SiN
x
films are prepared on Si(001) substrates with varying VHF power density (P
VHF), H2 concentration and source ratio (NH3/SiH4). Infrared absorption spectroscopy is used to analyze the bonding configurations in the films. The results show that increasing
H2 concentration under the supply of a moderately large P
VHF, together with the adjustment of NH3/SiH4 ratio, enables us to prepare SiN
x
showing reasonable stability against a buffered hydrofluoric acid solution in spite of the very high deposition rate of 130 nm/s.
The achievement of such a high-rate deposition at room temperature is primarily due to the significant enhancement of both
gas-phase and surface-phase reactions in AP-VHF plasma. 相似文献
159.