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排序方式: 共有493条查询结果,搜索用时 15 毫秒
91.
Jurriaan M Zwier Albert M Brouwer Tamás Keszthelyi Gurusamy Balakrishnan Jesper F Offersgaard Robert Wilbrandt Frédérique Barbosa Urs Buser Jér?me Amaudrut Georg Gescheidt Stephen F Nelsen Charles D Little 《Journal of the American Chemical Society》2002,124(1):159-167
The radical cation of 1,3,6,8-tetraazatricyclo [4.4.1.1(3,8)]dodecane (TTD) has been studied using magnetic resonance and optical spectroscopic methods and computational techniques. With the help of deuterated isotopomers, assignments of EPR and resonance Raman spectra could be unequivocally established. The results demonstrate that the radical cation has D(2d) symmetry, and instantaneous electron delocalization over the four equivalent nitrogen atoms occurs. This extensive delocalization in a completely saturated system is a unique feature of the TTD radical cation. The spectroscopy of TTD, in contrast to that of simpler diamines such as 1,4-diaza[2.2.2]bicyclooctane, simultaneously reveals the consequences of orbital interactions through space and through bonds. The relationship between nitrogen pyramidalization and hyperfine coupling constants in nitrogen-centered radical cations with a number of different bonding arrangements is reviewed. 相似文献
92.
Sune Nygaard Amar H. Flood Jan O. Jeppesen Andrew D. Bond 《Acta Crystallographica. Section C, Structural Chemistry》2006,62(12):o677-o680
The isomeric title compounds, 2,7‐bis(2‐cyanoethylsulfanyl)‐3,6‐(decane‐1,10‐diyldithio)tetrathiafulvalene and 2,6‐bis(2‐cyanoethylsulfanyl)‐3,7‐(decane‐1,10‐diyldithio)tetrathiafulvalene, both C22H28N2S8, comprise bis(2‐cyanoethylsulfanyl)tetrathiafulvalene units tethered by a saturated decamethylenedithio linker attached in either a cis or a trans manner. The tetrathiafulvalene (TTF) group is planar in the cis isomer, but distorted significantly from planarity and twisted about its long axis in the trans isomer. In both structures, intermolecular interactions are segregated into regions in which TTF units are brought into close contact and regions where the polymethylene chains are brought into close contact. In the cis isomer, TTF units exhibit π–π stacking interactions, while in the trans isomer they do not. 相似文献
93.
Mikael Borg Catherine Stampfl Anders Mikkelsen Johan Gustafson Edvin Lundgren Matthias Scheffler Jesper N Andersen 《Chemphyschem》2005,6(9):1923-1928
The structural phases of Al(x)Na(1-x) surface alloys have been investigated theoretically and experimentally. We describe the system using a lattice-gas Hamiltonian, determined from density functional theory, together with Monte Carlo (MC) calculations. The obtained phase diagram reproduces the experiment on a quantitative level. From calculation of the (configurational) density of states by the recently introduced Wang-Landau MC algorithm, we derive thermodynamic quantities, such as the free energy and entropy, which are not directly accessible from conventional MC simulations. We accurately reproduce the stoichiometry, as well as the temperature at which an order-disorder phase transition occurs, and demonstrate the crucial role, and magnitude, of the configurational entropy. 相似文献
94.
Hansen PM Dreyer JK Ferkinghoff-Borg J Oddershede L 《Journal of colloid and interface science》2005,287(2):561-571
We present an experimental method based on video microscopy to perform nanometer scale position detection of a micrometer bead in the direction along the propagation of the detection light. Using the same bead for calibration and detection significantly improves the in depth resolution in comparison to video microscopy methods from literature. This method is used together with an optical trap to measure interaction potentials between a glass surface and colloids made of polystyrene or silica at different electrolyte concentrations. The results are confirmed by an independent method where the optical trap is used in connection with a quadrant photodiode. Also, we present a maximum likelihood analysis method which considerably improves the spatial resolution of interaction potentials by optimizing the underlying potential function to fit all observed position distributions. The measured interaction potentials agree well with DLVO theory for small electrolyte concentrations; however, for larger electrolyte concentrations the potentials differ qualitatively from both DLVO and Lifshitz theory. 相似文献
95.
Synthesis of three different amide-linked LNA-type dinucleotides and their incorporation into 9-mer and 17-mer oligodeoxynucleotides is described; compared to the reference DNA-RNA duplex, incorporation of one of the three dimers (5'-DNA*LNA dimer) induced significantly increased duplex thermostabilities. 相似文献
96.
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99.
Let X be a quasiprojective smooth surface defined over an algebraically closed field of positive characteristic. In this note we
show that if X is Frobenius split then so is the Hilbert scheme Hilb of n points inX. In particular, we get the higher cohomology vanishing for ample line bundles on Hilb when X is projective and Frobenius split.
Received November 2, 1999 / Published online March 12, 2001 相似文献
100.
Donghong Yu Kim Steffensen Jesper Tranholm Anne Louise Nielsen Reinhard Wimmer Kim Lambertsen Larsen 《Journal of inclusion phenomena and macrocyclic chemistry》2007,57(1-4):333-338
The regioselective transesterification of native α- and β-Cyclodextrins (CDs) with vinyl acetate and vinyl laurate is presented
in this paper. The reactions were carried out in dimethyl sulfoxide by using low molecular weight salts (Na2HPO4 and NaCl, respectively). MALDI-TOF mass spectrometry and NMR analysis were applied to investigate the chemical structure
and the degree of the modification of the final products. Results show that these salts catalyze regioselective transesterification
between β-CDs and vinyl laurate aiming for the secondary C-2 hydroxyl group. The synthesis and separation of β-CD-laurate
derivatives leads to a mixture of mono-2-O-dodecanoyl-β-CD and native β-CDs. Unreacted β-CDs could be selectively removed enzymatically by cyclodextrin glucosyl-transferase
(CGTase) and β-amylase. 相似文献