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排序方式: 共有452条查询结果,搜索用时 390 毫秒
81.
Will R. Henderson Ajeet Kumar Dr. Khalil A. Abboud Prof. Ronald K. Castellano 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(72):17588-17597
Reported here is the synthesis and self-assembly characterization of [n.n]paracyclophanes ( [n.n]pCps , n=2, 3) equipped with anilide hydrogen bonding units. These molecules differ from previous self-assembling [n.n]paracyclophanes ( [n.n]pCps ) in the connectivity of their amide hydrogen bonding units (C-centered/carboxamide vs. N-centered/anilide). This subtle change results in a ≈30-fold increase in the elongation constant for the [2.2]pCp -4,7,12,15-tetraanilide ( [2.2]pCpNTA ) compared to previously reported [2.2]pCp -4,7,12,15-tetracarboxamide ( [2.2]pCpTA ), and a ≈300-fold increase in the elongation constant for the [3.3]pCp -5,8,14,17-tetraanilide ( [3.3]pCpNTA ) compared to previously reported [3.3]pCp -5,8,14,17-tetracarboxamide ( [3.3]pCpTA ). The [n.n]pCpNTA monomers also represent the reversal of a previously reported trend in solution-phase assembly strength when comparing [2.2]pCpTA and [3.3]pCpTA monomers. The origins of the assembly differences are geometric changes in the association between [n.n]pCpNTA monomers—revealed by computations and X-ray crystallography—resulting in a more favorable slipped stacking of the intermolecular π-surfaces ( [n.n]pCpNTA vs. [n.n]pCpTA ), and a more complementary H-bonding geometry ( [3.3]pCpNTA vs. [2.2]pCpNTA ). 相似文献
82.
83.
A procedure for the colorimetric determination of molybdenum with disodium-1, 2-dihydroxybenzene-3,5-disulfonate (Tiron) has been presented. The maximum absorbancy of the yellow colored complex is at 390 mμ the optimum pH range is 6.6-7.5; and the Tiron concentration is 1.5-2%.The color reaction conforms to Beer's law and has a practical sensitivity of 0.1 p.p.m. molybdenum. The color formation is instantaneous and stable for at least three weeks. There is no temperature effect over the range of 15° — 40° C.The tolerances of the colored complex to many diverse ions have been established. The separation of molybdenum from interfering ions is accomplished by precipitation of the former with α-benzoinoxime. Molybdenum and tungsten are separated photometrically.The colorimetric determination of molybdenum with Tiron was successfully applied to a variety of steels and other materials. 相似文献
84.
Canoy WL Cooley BE Corona JA Lovelace TC Millar A Weber AM Xie S Zhang Y 《Organic letters》2008,10(6):1103-1106
A one-step diastereoselective (up to 98:2) synthesis of the bis-furan alcohol of Darunavir and other HIV drug candidates has been achieved utilizing the novel cyclization of glycolaldehyde and 2,3-dihydrofuran. The cycloaddition was catalyzed by a variety of catalysts including those formed from tin(II) triflate and common chiral ligands such as BINAP and Evans's box ligands. An efficient and unique enzymatic process enhanced the enantiomeric purity to provide the target in optically pure form. 相似文献
85.
High-resolution electron energy loss spectroscopy (HREELS), temperature-programmed desorption (TPD), and density functional theory (DFT) calculations were used to study the adsorption and reaction of 1-epoxy-3-butene (EpB) on Pt(111). These investigations were conducted to help elucidate mechanisms for improving olefin hydrogenation selectivity in reactions of unsaturated oxygenates. EpB dosed to Pt(111) at 91 K adsorbs molecularly on the surface through the vinyl group with apparent rehybridization to a di-sigma-bound state. By 233 K, however, EpB undergoes epoxide ring opening to form an aldehyde intermediate, which further decomposes upon heating to yield gas phase products CO, H2, and propylene. Comparison of the HREELS and TPD data to experiments performed with 2-butenal (crotonaldehyde) shows that EpB and 2-butenal decompose through related pathways. However, the EpB-derived aldehyde intermediate clearly has a unique structure, features of which have been elucidated by DFT calculations. In conjunction with previous surface science studies of EpB chemistry, these results can help explain selectivity trends for reactions of EpB on Pt catalysts and bimetallic PtAg catalysts, with indications that the enhanced olefin hydrogenation selectivity of PtAg catalysts likely originates from a bifunctional effect. 相似文献
86.
Andrew J. Greener Patrycja Ubysz Will Owens-Ward George Smith Ivan Ocaa Adrian C. Whitwood Victor Chechik Michael J. James 《Chemical science》2021,12(43):14641
The design and development of an oxime-based hydroxylation reagent, which can chemoselectively convert aryl halides (X = F, Cl, Br, I) into phenols under operationally simple, transition-metal-free conditions is described. Key to the success of this approach was the identification of a reducing oxime anion which can interact and couple with open-shell aryl radicals. Experimental and computational studies support the proposed radical-nucleophilic substitution chain mechanism.The design and development of an oxime-based hydroxylation reagent, which can chemoselectively convert aryl halides (X = F, Cl, Br, I) into phenols under operationally simple, transition-metal-free conditions is described. 相似文献
87.
Andrew N. Cammidge Victoria H.M. Goddard Geoffrey Will Michael J Cook 《Tetrahedron letters》2009,50(25):3013-321
Mixed cyclisations have been performed to give phthalocyanine-naphthalocyanine hybrids bearing solubilising substituents. Reactivity differences between the two phthalonitrile precursors result in inefficient mixed-macrocyclisation under standard, non-templating conditions leading to predominant formation of symmetrical phthalocyanine. Templated mixed-macrocyclisation leads to the hybrids. However, the reaction proceeds with unexpected selectivity with only one of the possible 2:2 products observed. 相似文献
88.
89.
Influence of Rare Earth Ho3+ Doping on Structural,Microstructure and Magnetic Properties of ZnO Bulk and Thin Film Systems
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Ghulam Murtaza Rai Muhammad Azhar Iqbal Yongbing Xu Iain Gordon Will Wen Zhang 《化学物理学报(中文版)》2011,24(3):353-357
We have investigated the doping behavior of rare earth element holmium (Ho3+) in ZnO semiconductor. The structural, microstructure, and magnetic properties of Zn1-xHoxO (x=0.0, 0.04, and 0.05) thin films deposited on Si(100) substrate by thermal evaporation technique were studied. The ceramic targets were prepared by conventional solid state ceramic technique. The pallets used as target were final sintered at 900 oC in the presence of N2 atmosphere. The experimental results of X-ray diffraction (XRD) spectra, surface morphology, and magnetic properties show that the Ho3+ doped ZnO thin films has a strong influence on the materials properties. The higher angle shift in peak position and most preferred (101) orientation were observed in XRD pattern. These spectra confirmed the substitution of Ho3+ in ZnO lattice. The surface morphology and stoichiometry for both bulk and thin films were analyzed by scanning electron microscopy and energy dispersive spectroscopy. It was observed that grain size decreases with the increase of Ho3+. Room temperature ferromagnetism was observed for Zn0.95Ho0.05O films. The ferromagnetism might be attributed to the substitution of Ho ions for Zn2+ in ZnO lattices. 相似文献
90.
Julian J. Schuster Stefan Will Alfred Leipertz Andreas Braeuer 《Journal of Raman spectroscopy : JRS》2014,45(3):246-252
The paper reports on a routine to extract the composition of multi‐component mixtures from their Raman spectra at elevated pressures. The strategy is based on fitting weighted Raman spectra of the pure compounds to the measured Raman spectrum of the mixture, also considering the effects of intermolecular interactions onto the Raman spectra by applying Gaussian and Voigt profile deconvolution of the Raman peaks. Thereby, an improved accuracy compared to previous evaluation strategies could be obtained. The more accurate data of the ternary mixtures of carbon dioxide, water and organic solvents are presented. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献