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931.
The isotopic difference between the diamagnetic shielding of a positive muon (+) in + HBr and H2O and the analogous shielding of protons is computed from first principles in free space and in Br2 solution, using a self-consistent cellular cluster multiple scattering method (SC-CMS) for condensed matter and for the free molecules. The isotope shift of the chemical shift is at the + in these molecules dissolved in liquid Br2 is evaluated with the eigenfunctions and eigenvalues obtained using Ramsey's formalism. For HBr the computed chemical shifts is are comparable with experiment and with the calculations of Breskman and Kanofsky and of Williams, and the solvent effect has the correct sign and order of magnitude. For H2O, is has also the correct sign and order of magnitude when compared with +SR experiments. 相似文献
932.
933.
O. Heil J. Kemmler K. Kroneberger A. Kövér Gy. Szabó L. Gulyás R. DeSerio S. Lencinas N. Keller D. Hofmann H. Rothard D. Berényi K. O. Groeneveld 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1988,9(3):229-234
We have measured the cusp electron yield in coincidence with the transmitted charge state (He0, He+ and He++) when3He+ collides with He and Ne under single collision conditions. For the first time this enables the electron capture to the continuum (ECC) yield to be directly compared with that from electron loss to continuum (ELC). While the ECC contribution is smaller than that from ELC at high projectile velocities (V p >3 au) the data suggest that ECC will dominate belowV p =2.8 au. The relevance of the results to the projectile velocity dependence of existing capture theories is discussed. 相似文献
934.
AgCo3H2(AsO4)3 and AgZn3H2(AsO4)3. Preparation and Crystal Structure. Another New Structure Type of an Arsenate AgCo3H2(AsO4)3 ( 1 ) and AgZn3H2(AsO4)3 ( 2 ) were prepared by heating of As2O5, AgNO3, CoSO4 · 7H2O, ZnSO4 · 7H2O, respectively, and water in a sealed tube at 300°C and investigated with X-rays. Both compounds are isotypic and crystallize in the monoclinic space group C2/c with 4 formula units per cell. The lattice parameters are ( 1 ): a = 1215.9(6), b = 1243.8(7), c = 678.2(3) pm, β = 113.16(3)° ( 2 ): a = 1216.9(2), b = 1249.5(3), c = 675.5(1) pm, β = 112.77(1)°. The structure contains chains of edge-shared CoO6 or ZnO6 octahedra, respectively, which are connected by AsO4 tetrahedra and silver oxygen ribbons with square planar coordinated silver forming a framework. Based on the charge balances derived from the geometrical data and the IR spectra the occurence of hydrogen bonds is discussed. 相似文献
935.
The defects in atomic monopole models of molecular charge distribution have been analyzed for several model-blocked peptides and compared with accurate quantum chemical values. The results indicate that the angular characteristics of the molecular electrostatic potential around functional groups capable of forming hydrogen bonds can be considerably distorted within various models relying upon isotropic atomic charges only. It is shown that these defects can be corrected by augmenting the atomic point charge models by cumulative atomic multipole moments (CAMMs). Alternatively, sets of off-center atomic point charges could be automatically derived from respective multipoles, providing approximately equivalent corrections. For the first time, correlated atomic multipoles have been calculated for N-acetyl, N'-methylamide-blocked derivatives of glycine, alanine, cysteine, threonine, leucine, lysine, and serine using the MP2 method. The role of the correlation effects in the peptide molecular charge distribution are discussed. 相似文献
936.
Bringmann G Price Mortimer AJ Keller PA Gresser MJ Garner J Breuning M 《Angewandte Chemie (International ed. in English)》2005,44(34):5384-5427
A rotationally hindered and thus stereogenic biaryl axis is the structurally and stereochemically decisive element of a steadily growing number of natural products, chiral auxiliaries, and catalysts. Thus, it is not surprising that significant advances have been made in the asymmetric synthesis of axially chiral biaryl compounds over the past decade. In addition to the classic approach (direct stereoselective aryl-aryl coupling), innovative concepts have been developed in which the asymmetric information is introduced into a preformed, but achiral-that is, symmetric or configurationally labile-biaryl compound, or in which an aryl--C single bond is stereoselectively transformed into an axis. This Review classifies these strategies according to their underlying concepts and critically evaluates their scope and limitations with reference to selected model reactions and applications. Furthermore, the preconditions required for the existence of axial chirality in biaryl compounds are discussed. 相似文献
937.
Thomas Aussieker Hans‐Lothar Keller Thorsten Oldag Yurii Prots Michael Ruck Prof. Dr. Aron Wosylus 《无机化学与普通化学杂志》2007,633(4):603-609
Dark grey (dark red with transmitting light) crystals of heptathallium(I) hexadecaiodo‐tribismuthate(III), Tl7Bi3I16, were obtained by slow cooling of a melt from 800 K to ambient temperature and, with higher crystal quality via solvothermal synthesis in aqueous HI by slowly cooling from 428 to 363 K. The compound is diamagnetic and melts congruently at 630(5) K. X‐ray diffraction on single‐crystals revealed that Tl7Bi3I16 crystallizes in the orthorhombic space group Cmcm with lattice parameters a = 2473.4(5), b = 1441.9(2), c = 3616.9(7) pm. The crystal structure can be interpreted as a layered intergrowth of fragments from the CsNiCl3 and K5Dy3I12 structure types with isolated [BiI6]3? octahedra and [Bi2I10]4? double octahedra. Rotation and distortion of the complex anions establish coordination numbers (c.n.) between 7 and 9 for the Tl+ cations. Dark red crystals of trithallium(I) hexaiodo‐bismuthate(III), Tl3BiI6, are only accessible via hydrothermal synthesis in aqueous HI and slowly cooling from 428 to 363 K. Thermal analysis reveals a peritectoid decomposition at 540(5) K into the neighboring phases Tl7Bi3I16 and TlI. Tl3BiI6 crystallizes in the monoclinic space group P21/c with lattice parameters a = 1352.6(3), b = 899.6(2), c = 1353.8(3) pm, and β = 104.18(3)°. In the crystal structure isolated [BiI6]3? octahedra are arranged according to the motif of a face‐centered pseudo‐cubic packing. Due to the tilted orientation of the [BiI6]3? groups the Tl+ cations have c.n. of 8 and 9. Although the crystal structure of Tl3BiI6 looks like a distorted variant of the elpasolith type, there is no symmetry relation according to a group subgroup formalism. 相似文献
938.
Several examples of 1,7-dioxaspiro[5.5]undecane spiroketal systems have been synthesized from the common bicyclic intermediate 1 via acid-catalyzed rearrangement. Intermolecular ketalization of C(2) symmetric diene diol 3 with ketone 9 and then desymmetrization by ring-closing metathesis rapidly constructs bicyclic acetal 1. The locked conformation and steric bias of 1 allow stereoselective functionalization of one or both double bonds before spiroketalization. 相似文献
939.
P. Bleckmann P. Breitenbach K. U. Dickhut D. Keller C. Schwittek 《Fresenius' Journal of Analytical Chemistry》1997,359(1):115-120
The ab initio energies and force constants of 38 geometrically optimized formamide dimers which differ in the lengths of the hydrogen bonds,
are evaluated using the program GAUSSIAN 90 with the 6–31G** basis set. A potential energy function was fitted simultaneously
to the dimerization energies (including vibrational energy contributions to association energies) and the force constants
of the N-H…O=C bridge. As an application, the broadening of the signals in vibrational spectra of liquid formamide was simulated
by a superposition of spectra of different formamide oligomers.
Received: 12 November 1996 / Revised: 27 May 1997 / Accepted: 27 May 1997 相似文献
940.
Collision-activated dissociation (CAD) has been employed to assess the gas-phase fragmentation behavior of a series of 1:1 oligodeoxynucleotide (ODN):metal complexes over a range of charge states, using several ten-residue ODNs and a wide array of alkali, alkaline earth, and transition metals. For parent species in low to intermediate charge states, complexation with Ca(+2), Sr(+2), or Ba(+2) altered the relative intensity of M-B species, promoting loss of cytosine over loss of guanine. The relative intensities of sequence ions were largely unaffected. This behavior was most prevalent for isomeric sequences with complementary residues at the 5'- and 3'-termini, suggesting that metal complexation may change the gas-phase conformation and/or conformational dynamics for some sequences. In higher charge states, some ODN/Ba(+2) complexes produced abundant fragment ions corresponding to metallated a(n)(-m) species, which are not commonly observed in CAD mass spectra for deprotonated ODNs. The formation of these ions was most favored for complexes between Ba(+2) and ODN sequences with a thymine residue at Position 6. Literature precedent exists for the formation of a(n)(-m) ions from sequences in which covalent modification generates one or more neutral sites along the phosphate backbone. ODN/metal adducts in high charge states possess only a few acidic protons, and the juxtaposition of these neutral phosphate groups near thymine residues and the bound Ba(+2) ion may direct formation of the metallated a(n)(-m) species. 相似文献