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931.
932.
Geng Y Trajkovska A Culligan SW Ou JJ Chen HM Katsis D Chen SH 《Journal of the American Chemical Society》2003,125(46):14032-14038
Novel nonafluorenes with a varying extent of pendant chirality were synthesized for an investigation of the origins of chiroptical activities in neat films. Thermal annealing of 4-microm-thick sandwiched films and of 90-nm-thick spin-cast films, all on surface-treated substrates, produced monodomain glassy films characterized as a right-handed cholesteric stack with a helical pitch length ranging from 180 to 534 nm and from 252 to 1151 nm, respectively. The observed strong circular dichroism (CD) and g(e) as functions of helical pitch length in single-substrate monodomain glassy cholesteric films were quantitatively interpreted with a circularly polarized fluorescence theory accounting for light absorption, emission, and propagation in a cholesteric stack. Although intertwined molecular helices were likely to be present, cholesteric stacking of rodlike molecules seemed to be the predominant contributor to the strong chiroptical activities. All the cholesteric stacks comprising a polydomain glassy film on an untreated substrate were found to contribute to CD and g(e) largely to the same extent as in a monodomain film. A circularly polarized blue organic light-emitting diode containing a nonafluorene film resulted in a g(e) of 0.35 with a luminance yield of 0.94 cd/A at 20 mA/cm(2), the best performance to date. 相似文献
933.
Ceccacci F Diociaiuti M Galantini L Mancini G Mencarelli P Scipioni A Villani C 《Organic letters》2004,6(10):1565-1568
A CD band in chiral recognition experiments on racemic stereolabile compounds can be ascribed either to deracemization or to a solely induced CD effect. A procedure is presented that allows one to discriminate positively between the two phenomena. The procedure, based on CD spectroscopy, was used in experiments on racemic biphenylic derivatives in aggregates formed by enantiopure surfactants. In addition to demonstrating a deracemization event, the procedure allowed us to measure the enantiomeric excess. 相似文献
934.
Gozzo FC Fernandes SA Rodrigues DC Eberlin MN Marsaioli AJ 《The Journal of organic chemistry》2003,68(14):5493-5499
Theoretical calculations and the isomeric product composition for a series of eight meta-substituted allyl aryl ethers confirm the reliability of a new (1)H NMR methodology used to predict aromatic Claisen regioselectivity from ground-state conformational preference of the reactant allyloxy group. Frontier HOMO-LUMO intramolecular orbital interactions, a classical approach in predicting reactivity and selectivity for Claisen rearrangements of allyl vinyl ethers, is shown to fail to mimic transition-state orbital interactions for aromatic Claisen rearrangements of meta-substituted allyl aryl ethers. B3LYP/6-31G(d,p) calculations on reactants and transition states are shown, however, to correctly predict the outcome of such aromatic Claisen rearrangements from either the preferential reactant ground-state conformation (theoretical predictions that agree with the NMR measurements) or the less energetic transition state, or both. 相似文献
935.
Summary The kinetics of chromic acid oxidation of phenylphosphinic acid to phenylphosphonic acid has indicated the formation of an anhydride between HCrO
4
–
and phenylphosphinic acid in its active PhP(OH)2 and inactive PhPH(O)OH forms. The ambiguity about the reactive form of phenylphosphinic acid arises from the fact that protonation of the anhydride leads to the same transition state which disproportionates in the rate-determining step to phosphonium ion and chromium(IV). These, through different reactions in the fast step, yield phenylphosphonic acid and chromium(III) as the final products. That HCrO
4
–
is the reactive species of chromium(VI) is confirmed by the fact that k0 is independent of the inital [CrVI] where k0 is defined by the Equation k0=kobs[CrVI]/[HCrO
4
–
]; kobs is the pseudo first-order rate constant with respect to chromium(VI) ([Phenyl-phosphinic acid][CrVI]).The plot between k0 and [H+] passes through the origin indicating that the reaction does not occur in the absence of H+-ions. Furthermore, the plot between log k0 and –H0, the Hammett acidity function, is linear with a slope value of 1.02±0.02 confirming the protonation of the anhydride prior to its rate-limiting disproportionation.The equilibrium constant for the anhydride formation and the composite rate constant kK, K is the protonation constant of anhydride, are reported. The equilibrium constant is almost independent of temperature.Sen Gupta and Chakaladar(3) reported values of 8.5, 9.2, 11, 12 and 13, respectively, at 26°, 30.4°, 36°, 39.4° and 46° C. The uncertainty limits were not reported. Nevertheless it is apparent from the data that the values are not greatly influenced by temperature. The statistical mean is 11±2 dm3 mol–1, in fair agreement with our values. 相似文献
936.
Radhey?M.?NaikEmail author Raj?K.?Tewari Pradeep?K.?Singh Anjani?Tewari Surendra?Prasad 《Transition Metal Chemistry》2005,30(8):968-977
The kinetic and mechanistic studies of HgII catalyzed exchange of coordinated cyanide in hexacyanoferrate(II) by pyrazine (Pz) were monitored by following the appearance
of the yellow complex [Fe(CN)5Pz]3− at 440 nm corresponding to metal ligand charge transfer (MLCT) transitions at temp=25.0±0.1 °C, pH=2.5±0.02 and ionic strength,
I=0.1 m (KNO3). The effect of pH, ionic strength and the concentrations of [Fe(CN)6]4− and Pz on the rate of reaction were also studied and explained. To investigate the dependency of catalytic activity of HgII, the initial rates were determined at several concentrations of HgII, keeping the concentration of other reactants constant. The kinetic observations suggest that the substitution follows an
interchange dissociative (Id) mechanism and proceeds via formation of a solvent-bound intermediate. The repetitive spectral scan is also provided as evidence for the exchange of
cyanide ions by pyrazine in [Fe(CN)6]4−. Activation parameters have also been evaluated and provided support for the proposed mechanistic scheme. 相似文献
937.
The addition of acylsilanes to imines catalyzed by neutral carbenes (or zwitterions) generated in situ from readily available thiazolium salts is described. The general reaction successfully utilizes acylsilanes as carbonyl anion precursors and is tolerant of a range of structural diversity on the acylsilane or imine electrophile. The overall reaction utilizes easily available precursors and directly accesses protected alpha-amino ketones in the correct oxidation state. 相似文献
938.
939.
Sreeruttun RK Ramasami P Wannere CS Paul A Schleyer PV Schaefer Iii HF 《The Journal of organic chemistry》2005,70(22):8676-8686
[reaction: see text] The adiabatic electron affinity (EA(ad)) of the CH(3)-C[triple bond]C(*) radical [experiment = 2.718 +/- 0.008 eV] and the gas-phase basicity of the CH(3)-C[triple bond]C:(-) anion [experiment = 373.4 +/- 2 kcal/mol] have been compared with those of their fluorine derivatives. The latter are studied using theoretical methods. It is found that there are large effects on the electron affinities and gas-phase basicities as the H atoms of the alpha-CH(3) group in the propynyl system are substituted by F atoms. The predicted electron affinities are 3.31 eV (FCH(2)-C[triple bond]C(*)), 3.86 eV (F(2)CH-C[triple bond]C(*)), and 4.24 eV (F(3)C-C[triple bond]C(*)), and the predicted gas-phase basicities of the fluorocarbanion derivatives are 366.4 kcal/mol (FCH(2)-C[triple bond]C:(-)), 356.6 kcal/mol (F(2)CH-C[triple bond]C:(-)), and 349.8 kcal/mol (F(3)C-C[triple bond]C:(-)). It is concluded that the electron affinities of fluoropropynyl radicals increase and the gas-phase basicities decrease as F atoms sequentially replace H atoms of the alpha-CH(3) in the propynyl system. The propargyl radicals, lower in energy than the isomeric propynyl radicals, are also examined and their electron affinities are predicted to be 0.98 eV ((*)CH(2)-C[triple bond]CH), 1.18 eV ((*)CFH-C[triple bond]CH), 1.32 eV ((*)CF(2)-C[triple bond] CH), 1.71 eV ((*)CH(2)-C[triple bond]CF), 2.05 eV ((*)CFH-C[triple bond]CF), and 2.23 eV ((*)CF(2)-C[triple bond]CF). 相似文献
940.
Neubert H Halket JM Fernandez Ocaña M Patel RK 《Journal of the American Society for Mass Spectrometry》2004,15(3):336-343
Large signals from alpha-cyano-4-hydroxycinnamic acid (CHCA) matrix complexes with sodium and potassium ions were found to interfere with sensitive matrix-assisted laser desorption/ionization (MALDI) analysis of a hydrochloric acid digest of gelatine preparations. The nature of some selected matrix clusters was investigated by conventional post-source decay and LIFT-TOF/TOF experiments. The matrix clusters fragmented readily by neutral evaporation to give smaller sized matrix cluster species without matrix disintegration. Their characterization distinguished them from peptide signals, in particular from those that had the same nominal mass and differed only in the fractional part of the mass as encountered for gelatine-derived peptides. Knowledge of the molecular composition of these cluster species allowed using them for internal calibration of the MALDI mass spectra. The hydrolytic peptides could be analyzed with increased sensitivity when using 2,5-dihydroxy benzoic acid (DHB) as the MALDI matrix. 相似文献