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11.
The half-life period (6.6 mm.) for the hydrogenation of Z-methyl-a-acetamidocinnamate (catalyzed by a neutral DIOPRh complex) was found to be the same when the Me ester reduction was performed in the presence of equimolar quantities of the corresponding i-Pr or t-Bu ester unsaturated stbstrates. Neither the Me nor the i-Pr or t-Bu esters underwent appreciable Z,E-isomerization. The formation of N-acetylphenylalanine methyl ester product suffered inhibition when the hydrogenation reaction was performed in the presence of the corresponding bornyl or 1-adamantyl unsaturated esters (half-life period of the Me ester: 27 ~ 40 mm., respectively. The greater the inhibition of the Me ester unsaturated substrate, the more the bulky inhibitor itself underwent Z,E-isomerization. In the presence of inhibitors, the Me unsaturated substrate did not undergo appreciable Z,E-isomerization. 相似文献
12.
Hofmann C Boll R Heitmann B Hauser G Dürr C Frerich A Weitnauer G Glaser SJ Bechthold A 《Chemistry & biology》2005,12(10):1137-1143
The oligosaccharide antibiotic avilamycin A is composed of a polyketide-derived dichloroisoeverninic acid moiety attached to a heptasaccharide chain consisting of six hexoses and one unusual pentose moiety. We describe the generation of mutant strains of the avilamycin producer defective in different sugar biosynthetic genes. Inactivation of two genes (aviD and aviE2) resulted in the breakdown of the avilamycin biosynthesis. In contrast, avilamycin production was not influenced in an aviP mutant. Inactivation of aviGT4 resulted in a mutant that accumulated a novel avilamycin derivative lacking the terminal eurekanate residue. Finally, AviE2 was expressed in Escherichia coli and the gene product was characterized biochemically. AviE2 was shown to convert UDP-D-glucuronic acid to UDP-D-xylose, indicating that the pentose residue of avilamycin A is derived from D-glucose and not from D-ribose. Here we report a UDP-D-glucuronic acid decarboxylase in actinomycetes. 相似文献
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14.
On the Preparation of Arsenic Dichloride Trifluoride, AsCl2F3 AsCl2F3 is prepared by pyrolysis of [AsCl4]+ [AsF6]?. The vibrational spectra of the new low temperature compound are reported and the valence force constants are calculated. The formation of AsCl2F3 by ligand exchange reactions is discussed. 相似文献
15.
Novel ruthenium and osmium silylene species containing Si-H bonds have been synthesized and characterized by NMR spectroscopy. The ruthenium complex [Cp*(iPr3P)(H)2Ru=Si(H)Ph.Et2O][B(C6F5)4] catalyzes the hydrosilyation of alkenes with excellent substrate selectivity for primary silanes and exclusive anti-Markovnikov regiochemistry. Evidence for a novel mechanism involving direct addition of an alkene to the silylene Si-H bond is presented. 相似文献
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17.
Solid state (19)F NMR revealed the conformation and alignment of the fusogenic peptide sequence B18 from the sea urchin fertilization protein bindin embedded in flat phospholipid bilayers. Single (19)F labels were introduced into nine distinct positions along the wild-type sequence by substituting each hydrophobic amino acid, one by one, with L-4-fluorophenylglycine. Their anisotropic chemical shifts were measured in uniaxially oriented membrane samples and used as orientational constraints to model the peptide structure in the membrane-bound state. Previous (1)H NMR studies of B18 in 30% TFE and in detergent micelles had shown that the peptide structure consists of two alpha-helical segments that are connected by a flexible hinge. This helix-break-helix motif was confirmed here by the solid-state (19)F NMR data, while no other secondary structure (beta-sheet, 3(10)-helix) was compatible with the set of orientational constraints. For both alpha-helical segments we found that the helical conformation extends all the way to the respective N- and C-termini of the peptide. Analysis of the corresponding tilt and azimuthal rotation angles showed that the N-terminal helix of B18 is immersed obliquely into the bilayer (at a tilt angle tau approximately 54 degrees), whereas the C-terminus is peripherally aligned (tau approximately 91 degrees). The azimuthal orientation of the two segments is consistent with the amphiphilic distribution of side-chains. The observed 'boomerang'-like mode of insertion into the membrane may thus explain how peptide binding leads to lipid dehydration and acyl chain perturbation as a prerequisite for bilayer fusion to occur. 相似文献
18.
The trinuclear Cu(II) complex [(talen)Cu(II)(3)] (1) using the new triplesalen ligand H(6)talen has been synthesized and structurally characterized. The three Cu(II) ions are bridged in a m-phenylene linkage by the phloroglucinol backbone of the ligand. This m-phenylene bridging mode results in ferromagnetic couplings with an S(t) = (3)/(2) spin ground state, which has been analyzed by means of EPR spectroscopy and DFT calculations. The EPR spectrum exhibits an unprecedented pattern of 10 hyperfine lines due to the coupling of three Cu(II) ions (I = (3)/(2)). Resonances around g = 4 in both perpendicular and parallel mode EPR spectra demonstrate a zero-field splitting of D approximately 74 x 10(-4) cm(-1) arising from anisotropic/antisymmetric exchange interactions. The DFT calculations show an alteration in the sign of the spin densities of the central benzene ring corroborating the spin-polarization mechanism as origin for the ferromagnetic coupling. 相似文献
19.
Saalfrank RW Glaser H Demleitner B Hampel F Chowdhry MM Schünemann V Trautwein AX Vaughan GB Yeh R Davis AV Raymond KN 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(2):493-497
In a one-pot reaction, the tetranuclear iron chelate complex [Fe4(L4)4] 6 was generated from benzene-1,3,5-tricarboxylic acid trichloride (4), bis-tert-butyl malonate (5a), methyllithium, and iron(II) dichloride under aerobic conditions. Alternatively, hexanuclear iron chelate complex [Fe(L5)6] 7 was formed starting from bis-para-tolyl malonate (5b) by employing identical reaction conditions to those applied for the synthesis of 6. The clusters 6 and 7 are present as racemic mixtures of homoconfigurational (delta,delta,delta,delta)/(lambda,lambda,lambda,lambda)-fac or (delta,delta,delta,delta,delta,delta)/(lambda,lambda,lambda,lambda,lambda,lambda)-fac stereoisomers. The structures of 6 and 7 were unequivocally resolved by single-crystal X-ray analyses. The all-iron(III) character of 6 and 7 was determined by M?ssbauer spectroscopy. 相似文献
20.
Alice Merca Jürgen Schnack Joris van Slageren Thorsten Glaser Hartmut Bögge Veronika Hoeke Mechtild Läge Achim Müller Bernt Krebs 《Journal of Cluster Science》2013,24(4):979-988
The gadolinium (Gd) member of a new type of heteropolytungstates that contain one lanthanide and two transition metal ions in a triangular arrangement is reported. The compound NaK6Gd0.33 [((VO)2Gd(H2O)4K2(H2O)2(Na)(H2O)2)(α-B-AsW9O33)2]·24H2O (1) was prepared from acidified aqueous solutions of Na2WO4·2H2O, As2O3 and VOSO4·5H2O to which Gd3+ ions were added. The single crystal X-ray structure analysis (monoclinic, space group P21/m) shows that the anion consists of two [α-B-AsIIIW9O33]9? trilacunary Keggin-type units linked by two VO2+, one Gd3+ as well as weakly by two K+ and one Na+ ions, resulting in a sandwich-type structure with idealized C 2v symmetry. The problem of positioning crystal lattice and special polyoxometalate sites with different cations is discussed also in connection with supramolecular chemistry aspects and as an option for further research. A fit of the magnetic susceptibility yielded exchange coupling constants of J VV = ?2.55 cm?1 (anti-ferromagnetic) between the vanadium ions and J GdV = 0.6 cm?1 (ferromagnetic) between the Gd and each of the two vanadium ions. The complete magnetochemical analysis also revealed a partial occupancy of the Na+ sites in the counter-cation–water system by Gd3+ ions (0.33 Gd3+ ions in total). 相似文献