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231.
232.
Vivier V Aguey F Fournier J Lambert JF Bedioui F Che M 《The journal of physical chemistry. B》2006,110(2):900-906
UV-visible and Raman spectroscopies as well as electrochemical techniques have been used to characterize cis- and trans-[Co(III)(en)2Cl2]Cl (en=ethylenediamine) complexes and the gamma-alumina-supported cis-Co((III)) complex. It is shown that the electrochemical reduction of these complexes occurs according to a multistage mechanism involving two electrochemical steps, with the formation of a dimer that was characterized by UV-visible spectroscopy (intervalence band at 670 nm). The apparent standard redox potential for each step has been determined, and experimental results reveal that cis and trans complexes present similar electrochemical characteristics. It is also shown that the deposition of trans-[Co(III)(en)2Cl2]+ on gamma-alumina leads to an inner-sphere complex (ISC) in a cis configuration in which Cl- ligands are substituted by OH or O- surface groups of alumina. These changes in the coordination sphere of the complex induce a substantial decrease of its apparent redox potential since it is -0.63 V/SCE (saturated calomel electrode) for the gamma-alumina-supported cis-Co(III) complex, whereas values of -0.17 and -0.35 V/SCE were determined in dimethyl sulfoxide (DMSO) for the trans and cis precursor complexes, respectively. 相似文献
233.
Jean-François Le Gall 《Acta Mathematica》2010,205(2):287-360
We study geodesics in the random metric space called the Brownian map, which appears as the scaling limit of large planar
maps. In particular, we completely describe geodesics starting from the distinguished point called the root, and we characterize
the set S of all points that are connected to the root by more than one geodesic. The set S is dense in the Brownian map and homeomorphic to a non-compact real tree. Furthermore, for every x in S, the number of distinct geodesics from x to the root is equal to the number of connected components of S\{x}. In particular, points of the Brownian map can be connected to the root by at most three distinct geodesics. Our results
have applications to the behavior of geodesics in large planar maps. 相似文献
234.
The Bjerrum association model, which has already been applied successfully to volumes and enthalpies of dilution of electrolyte solutions, has now been extended to apparent molar heat capacities and compressibilities of these systems. The proposed method of calculation, which takes into account the relaxation effect observed in second derivatives of the excess Gibbs free energy, can be used to extrapolate to infinite dilution the experimental data for systems showing a wide range of association constants in acetonitrile, propylene carbonate, and water. The concentration dependence of the thermodynamic properties can be reproduced quantitatively by the addition of one or two virial coefficients. Literature data for C
P,2, and K
S,2, of electrolytes in aprotic solvents were refitted with this equation. For dissociated or slightly associated systems (K
A < 10), the standarY
2
o
d infinite dilution quantities () are in excellent agreement with literature values. For systems with high K
A, Y
2
o
obtained by the model are systematically lower than those reported in the literature. This is not surprising, since the traditional method of extrapolation using the Debye–Hückel limiting law or the Pitzer equation does not take association into account. A computer software that performs the calculations for the application of the Bjerrum model to thermodynamic properties has been designed and is presented in the appendix. 相似文献
235.
Impedance measurements around grazing incidence for nonlocally reacting thin porous layers 总被引:1,自引:0,他引:1
Allard JF Henry M Gareton V Jansens G Lauriks W 《The Journal of the Acoustical Society of America》2003,113(3):1210-1215
For locally reacting materials with a constant surface impedance, a classical method based on the work of Chien and Soroka [J. Sound Vib. 43, 9-20 (1975)] for measuring this impedance in situ around grazing incidence is currently used. A generalization of this work to include thin nonlocally reacting materials with a surface impedance noticeably dependent on the angle of incidence is performed. It is shown that the model by Chien and Soroka can be used, though the constant surface impedance must be replaced by the impedance at grazing incidence for the evaluation of the numerical distance. Measurements performed on a thin porous layer using this method are compared with measurements performed using the near-field acoustical holography method [M. Tamura, J. Acoust. Soc. Am. 88, 2259-2264 (1990)]. Other measurements performed on a fibrous layer are in good agreement with the predicted values of the impedance at grazing incidence. 相似文献
236.
[reaction: see text] The nucleoside dimer linked by a butadiynediyl group at C-3'beta may serve as a building block for the preparation of backbone-modified oligonucleotides for DNA repair or mutation in functional genomics. We prepared this type of dimer by an Eglington or Sonogashira coupling reaction. The unsymmetrical dimer was synthesized by coupling the acetylene with the bromoacetylene. Only marginal cytotoxicity was detected for one of the dimers. 相似文献
237.
Site of protonation of nicotine and nornicotine in the gas phase: pyridine or pyrrolidine nitrogen? 总被引:1,自引:0,他引:1
Graton J Berthelot M Gal JF Girard S Laurence C Lebreton J Le Questel JY Maria PC Naus P 《Journal of the American Chemical Society》2002,124(35):10552-10562
The gas-phase basicities (GBs) of nornicotine, nicotine, and model pyrrolidines have been measured by FT-ICR. These experimental GBs are compared with those calculated (for the two sites of protonation in the case of nicotine and nornicotine) at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d,p) level, or those estimated from substituent effects on the GBs of 2-substituted pyrrolidines, 2-substituted N-methylpyrrolidines, and 3-substituted pyridines. It is found that, in contrast to the Nsp(3) protonation in water, in the gas phase nornicotine is protonated on the pyridine nitrogen, because the effects of an intramolecular CH.Nsp(3) hydrogen bond and of the polarizability of the 3-(pyrrolidin-2-yl) substituent add up on the Nsp(2) basicity, while the polarizability effect of the 2-(3-pyridyl) substituent on the Nsp(3) basicity is canceled by its field/inductive electron-withdrawing effect. The same structural effects operate on the Nsp(3) and Nsp(2) basicities of nicotine, but here, the polarizability effect of the methyl group puts the pyrrolidine nitrogen basicity very close to that of pyridine. Consequently, protonated nicotine is a mixture of the Nsp(3) and Nsp(2) protonated forms. 相似文献
238.
Marcel Graffeuil Jean-François Labarre Claude Leibovici Eliane Taillandier 《Journal of Molecular Structure》1973,15(3):367-375
CNDO/2 calculations are performed in order to determine the preferred conformations of 1,1-dimethylhydrazine and 1,1-dimethylhydrazine-boron trifluoride complex. In each case, the calculations reveal the existence of an equilibrium between gauche and trans forms (25 % of trans form in the free molecule, 65 % in the complex). In agreement with experiment, the results confirm that BF3 is attached to the nitrogen atom bearing the methyl groups. 相似文献
239.
Maria PC Gal JF Massi L Burk P Tammiku-Taul J Tamp S 《Rapid communications in mass spectrometry : RCM》2005,19(4):568-573
A concerted theoretical (density-functional theory) and experimental electrospray mass spectrometry study was conducted on the formation of cesium cation adducts with small molecules taken as models of specific interactions sites in humic substances. Electrospray experiments with phenol, benzoic acid, salicylic acid, and phthalic acid, in methanolic solution containing cesium nitrate, were performed using a quadrupole ion trap. The formation of positively charged mixed clusters, [Cs(CsNO3)(n)(CsA1)(m)(Cs2A2)(p)]+ (A1 = benzoate, salicylate, and hydrogenophthalate, A2 = phthalate), was observed. Calculations of structures and bonding energetics of Cs+ in simple adducts formed with NO3-, CsNO3, A-, AH, and CsA are reported. The observation of variable cluster stoichiometry (n, m and p values) was interpreted in terms of more or less favorable interaction energies between Cs+ and the neutral species constituting the clusters. Phenol did not form clusters in significant abundances, despite a strong calculated interaction between Cs+ and cesium phenolate. This was attributed to its weak acid dissociation in the electrospray solution. 相似文献
240.
Armaroli N Accorsi G Felder D Nierengarten JF 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(10):2314-2323
The rhenium(I) and ruthenium(II) complexes of a fullerene-substituted bipyridine ligand have been prepared. Electrochemical studies indicate that some ground state electronic interaction between the fullerene subunit and the metal-complexed moiety are present in the Re(I) but not the Ru(II) complex. The photophysical properties have been investigated by steady-state and time-resolved UV/Vis-NIR luminescence spectroscopy and nanosecond laser flash photolysis in CH2Cl2 solution, and compared to those of the corresponding model compounds. Excitation of the methanofullerene moiety in the dyads does not lead to excited state intercomponent interactions. Instead, excitation of the metal-complexed unit shows that the lowest triplet metal-to-ligand-charge-transfer excited state ((3)MLCT) centered on the Re(I)- or Ru(II)-type unit is quenched with a rate constant of about 2.5 x 10(8) s(-1). The quenching is attributed to an electron-transfer (ElT) process leading to the reduction of the carbon sphere, as determined by luminescence spectroscopy for the Ru(II) dyad. Experimental detection of electron transfer in the Re(I) dyad is prevented due to the unfavorable absorption of the metal-complexed moiety relative to the fullerene unit. However, it can be postulated on the basis of energetic/kinetic arguments and by comparison with the Ru(II)-type array. The primary ElT process is followed by charge-recombination to give the lowest-lying fullerene triplet excited state (3C60) with quantitative yield, as determined by sensitized singlet oxygen luminescence experiments. Direct (3)MLCT-->3C60 triplet-triplet energy-transfer (EnT) does not successfully compete with ElT since it is highly exoergonic and located in the Marcus inverted region. The quantum yield of singlet oxygen sensitization (Phi(delta)) of the Re(I)-based dyad is found to be lower (0.80) than for the corresponding Ru(II) derivative (1.0). This is likely to be the consequence of different conformational structures for the two dyads, rather than a different yield of 3C60 formation. 相似文献