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141.
Amashukeli X Gruhn NE Lichtenberger DL Winkler JR Gray HB 《Journal of the American Chemical Society》2004,126(47):15566-15571
Inner-sphere electron-transfer reorganization energies of Zn(protoporphyrin IX) and Zn(octaethylporphyrin) are determined from band-shape analyses of the first ionization obtained by gas-phase valence photoelectron spectroscopy. The experimentally determined total inner-sphere reorganization energies for self-exchange (120-140 meV) indicate that structural changes upon oxidation are largely confined to the porphyrin ring, and substituents on the ring or solvent and other environmental factors make smaller contributions. Computational estimates by different models vary over a wide range and are sensitive to numerical precision factors for these low reorganization energies. Of current computational models that are widely available and practical for molecules of this size, functionals that contain a mixture of Hartree-Fock exchange and DFT exchange-correlation appear to be the most applicable. 相似文献
142.
Rajapakshe A Gruhn NE Lichtenberger DL Basta R Arif AM Ernst RD 《Journal of the American Chemical Society》2004,126(43):14105-14116
Molecules of the form Cp(6,6-dmch)ZrX(2) (Cp = eta(5)-cyclopentadienyl, X = Cl, Br, I; 6,6-dmch = eta(5)-6,6-dimethylcyclohexadienyl) have been synthesized, and the molecular and electronic structures have been investigated. These molecules allow direct comparison of the bonding and properties of pentadienyl and cyclopentadienyl ligands in the same high-oxidation-state metal complexes. Unlike the well-known Cp(2)ZrX(2) analogues, these Cp(6,6-dmch)ZrX(2) molecules are intensely colored, indicating significantly different relative energies of the frontier orbitals. Also unusual, the average Zr-C distances to the 6,6-dmch pentadienyl ligand are about 0.1 A longer than the average Zr-C distances to the cyclopentadienyl ligand for these Zr(IV) complexes, opposite of what is observed for the Zr(II) complex Cp(2,6,6-tmch)Zr(PMe(3))(2) (tmch = eta(5)-2,6,6-trimethylcyclohexadienyl), reflecting a dramatic reversal in the favorability of the bonding depending on the metal oxidation state. The experimental and computational results indicate that the color of the Cp(6,6-dmch)ZrX(2) complexes is due to a 6,6-dmch ligand-to-metal charge-transfer band. Compared to the Cp(2)ZrX(2) analogues, the Cp(6,6-dmch)ZrX(2) molecules have a considerably less stable HOMO that is pentadienyl-based and an essentially unchanged metal-based LUMO. Also, the lowest unoccupied orbital of pentadienyl is stabilized relative to cyclopentadienyl and becomes a better potential delta electron acceptor, thus contributing to the differences in structure and reactivity of the low-valent and high-valent metal complexes. 相似文献
143.
Bian Y Li L Dou J Cheng DY Li R Ma C Ng DK Kobayashi N Jiang J 《Inorganic chemistry》2004,43(23):7539-7544
Three (1,8,15,22-tetrasubstituted phthalocyaninato)lead complexes Pb[Pc(alpha-OR)(4)] [H(2)Pc(alpha-OC(5)H(11))(4) = 1,8,15,22-tetrakis(3-pentyloxy)phthalocyanine; H(2)Pc(alpha-OC(7)H(15))(4) = 1,8,15,22-tetrakis(2,4-dimethyl-3-pentyloxy)phthalocyanine; H(2)Pc(alpha-OC(10)H(7))(4) = 1,8,15,22-tetrakis(2-naphthyloxy)phthalocyanine] (1-3) have been prepared as racemic mixtures by treating the corresponding metal-free phthalocyanines H(2)Pc(alpha-OR)(4) (4-6) with Pb(OAc)(2).3H(2)O in refluxing n-pentanol. The molecular structure of Pb[Pc(alpha-OC(5)H(11))(4)] (1) in the solid state has been determined by single-crystal X-ray diffraction analysis. This compound, having a nonplanar structure, crystallizes in the monoclinic system with a P2(1)/c space group. Each unit cell contains two pairs of enantiomeric molecules, which are linked by weak coordination of the Pb atom of one molecule with an aza nitrogen atom and its neighboring oxygen atom from the alkoxy substituent of another molecule, forming a pseudo-double-decker supramolecular structure in the crystals with a short ring-to-ring separation, 2.726 A, and thus a strong ring-ring pi-pi interaction. The decreased molecular symmetry for these complexes has also been revealed by the NMR spectra of 1 and 2. The methyl protons of the 3-pentyloxy and 2,4-dimethyl-3-pentyloxy side chains of 1 and 2, respectively, are chemically inequivalent. In addition to the elemental analysis and various spectroscopic characterizations, these compounds have also been electrochemically studied. Two one-electron oxidations and up to five one-electron reductions have been revealed by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) methods. 相似文献
144.
An efficient route to various sugar lactones has been developed. Key to the overall transformation is the sequential osmium-catalyzed dihydroxylation of 2,4-dienoates. The simplest (one-step/racemic) example of this reaction occurs when the dihydroxylation is performed with aqueous NMO in MeOH. When the first dihydroxylation is performed using the AD-mix procedure, an enantioselective variant results. When a matched AD-mix procedure is used for the second dihydroxylation, an exceedingly diastereo- and enantioselective synthesis of galacto-1,4-lactone results. [Reaction: see text] 相似文献
145.
Elise A. Dennis Alexander W. Gundlach–Graham Christie G. Enke Steven J. Ray Anthony J. Carado Charles J. Barinaga David W. Koppenaal Gary M. Hieftje 《Journal of the American Society for Mass Spectrometry》2013,24(5):690-700
Resolution in time–of–flight mass spectrometry (TOFMS) is ordinarily limited by the initial energy and space distributions within an instrument’s acceleration region and by the length of the field–free flight zone. With gaseous ion sources, these distributions lead to systematic flight–time errors that cannot be simultaneously corrected with conventional static–field ion–focusing devices (i.e., an ion mirror). It is known that initial energy and space distributions produce non–linearly correlated errors in both ion velocity and exit time from the acceleration region. Here we reinvestigate an old acceleration technique, constant–momentum acceleration (CMA), to decouple the effects of initial energy and space distributions. In CMA, only initial ion energies (and not their positions) affect the velocity ions gain. Therefore, with CMA, the spatial distribution within the acceleration region can be manipulated without creating ion–velocity error. The velocity differences caused by a spread in initial ion energy can be corrected with an ion mirror. We discuss here the use of CMA and independent focusing of energy and space distributions for both distance–of–flight mass spectrometry (DOFMS) and TOFMS. Performance characteristics of our CMA–DOFMS and CMA–TOFMS instrument, fitted with a glow–discharge ionization source, are described. In CMA–DOFMS, resolving powers (FWHM) of greater than 1000 are achieved for atomic ions with a flight length of 285 mm. In CMA–TOFMS, only ions over a narrow range of m/z values can be energy–focused; however, the technique offers improved resolution for these focused ions, with resolving powers of greater than 2000 for a separation distance of 350 mm. 相似文献
146.
147.
A method for measuring local current density, not requiring segmentation of the electrode or spatial scanning, is presented. The motion of colloidal particles in response to local current density, characterized by the intensity of the light they scatter, is the fundamental phenomenon of the technique. The scattering was produced and measured with the electrochemical total internal reflection microscope, a tool that places an electrochemical cell within a total internal reflection apparatus. The electrolysis of water and the oxidation of ferrocene monocarboxylic acid were used as test reactions. Light scattered by a probe particle produced an "image" of current density; scattered light was converted to local current density by a function derived herein. Numerical simulations supplemented experimental evidence that local current density controlled the probe particle's vertical motion. The spatial resolution of the method was approximately the length scale of the probe particle, in this case 5.7 μm. The resolution of current density was better than 100 nA cm(-2). The method might find use in high throughput screening of electrocatalysts. 相似文献
148.
Igarashi RY Laryukhin M Dos Santos PC Lee HI Dean DR Seefeldt LC Hoffman BM 《Journal of the American Chemical Society》2005,127(17):6231-6241
We here show that the iron-molybdenum (FeMo)-cofactor of the nitrogenase alpha-70(Ile) molybdenum-iron (MoFe) protein variant accumulates a novel S = (1)/(2) state that can be trapped during the reduction of protons to H(2). (1,2)H-ENDOR measurements disclose the presence of two protons/hydrides (H(+/)(-)) whose hyperfine tensors have been determined from two-dimensional field-frequency (1)H ENDOR plots. The two H(+/)(-) have large isotropic hyperfine couplings, A(iso)( )() approximately 23 MHz, which shows they are bound to the cofactor. The favored analysis for these plots indicates that the two H(+/)(-) have the same principal values, which indicates that they are chemically equivalent. The tensors are further related to each other by a permutation of the tensor components, which indicates an underlying symmetry of binding relative to the cofactor. At present, no model for the structure of the iron-molybdenum (FeMo)-cofactor in the S = (1)/(2) state trapped during the reduction of H(+) can be shown unequivocally to satisfy all of the constraints generated by the ENDOR analysis. The data disfavors any model that involves protonation of sulfides, and thus suggests that the intermediate instead contains two chemically equivalent bound hydrides; it appears unlikely that these are terminal monohydrides. 相似文献
149.
The electron distributions and bonding in Ru3(CO)9(
3-
2,
2,
2-C6H6) and Ru3(CO)9(
3-
2,
2,
2-C60) are examined via electronic structure calculations in order to compare the nature of ligation of benzene and buckminsterfullerene to the common Ru3(CO)9 inorganic cluster. A fragment orbital approach, which is aided by the relatively high symmetry that these molecules possess, reveals important features of the electronic structures of these two systems. Reported crystal structures show that both benzene and C60 are geometrically distorted when bound to the metal cluster fragment, and our ab initio calculations indicate that the energies of these distortions are similar. The experimental Ru–Cfullerene bond lengths are shorter than the corresponding Ru–Cbenzene distances and the Ru–Ru bond lengths are longer in the fullerene-bound cluster than for the benzene-ligated cluster. Also, the carbonyl stretching frequencies are slightly higher for Ru3(CO)9(
3-
2,
2,
2-C60) than for Ru3(CO)9(
3-
2,
2,
2-C6H6). As a whole, these observations suggest that electron density is being pulled away from the metal centers and CO ligands to form stronger Ru–Cfullerene than Ru–Cbenzene bonds. Fenske-Hall molecular orbital calculations show that an important interaction is donation of electron density in the metal–metal bonds to empty orbitals of C60 and C6H6. Bonds to the metal cluster that result from this interaction are the second highest occupied orbitals of both systems. A larger amount of density is donated to C60 than to C6H6, thus accounting for the longer metal–metal bonds in the fullerene-bound cluster. The principal metal–arene bonding modes are the same in both systems, but the more band-like electronic structure of the fullerene (i.e., the greater number density of donor and acceptor orbitals in a given energy region) as compared to C6H6 permits a greater degree of electron flow and stronger bonding between the Ru3(CO)9 and C60 fragments. Of significance to the reduction chemistry of M3(CO)9(
3-
2,
2,
2-C60) molecules, the HOMO is largely localized on the metal–carbonyl fragment and the LUMO is largely localized on the C60 portion of the molecule. The localized C60 character of the LUMO is consistent with the similarity of the first two reductions of this class of molecules to the first two reductions of free C60. The set of orbitals above the LUMO shows partial delocalization (in an antibonding sense) to the metal fragment, thus accounting for the relative ease of the third reduction of this class of molecules compared to the third reduction of free C60. 相似文献
150.