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81.
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
82.
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
83.
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
84.
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
85.
86.
87.
Thomsen AE Jensen H Jorgensen L van de Weert M Ostergaard J 《Colloids and surfaces. B, Biointerfaces》2008,63(2):243-248
Protein adsorption represents a considerable challenge in the development and production of macromolecular drugs. From an analytical point of view the adsorption process is difficult to study in an efficient way using currently available techniques. In this work potential and time dependent adsorption and adsorption kinetics of human insulin at an 1,2-dichloroethane-aqueous interface were studied using a novel electroanalytical approach based on measurements of interfacial capacitance. Two different types of measurements were performed; potential scans and time scans. In the potential scans, the capacitance was measured over a range of applied potential differences across the interface. The interfacial potential difference is linked to the charge at the interface. Adsorption of human insulin was detectable at a bulk phase insulin concentration as low as 0.1 μM as a negative shift in the potential of zero charge (pzc). Adsorption kinetics were further studied using time scans in which the interfacial capacitance was measured at a fixed applied interfacial potential difference. Using this approach it was possible to study how the adsorption kinetics and the shape of the time scan curves were related to the bulk concentration of insulin and the interfacial potential difference. The changes in capacitance could be described phenomenologically by pseudo-first-order kinetics at low concentrations of insulin except at positive interfacial potential differences and high insulin concentrations (≥0.25 μM) where a more complex shape of the time scans curves was observed. 相似文献
88.
Nielsen KM Petersen M Håkansson AE Wengel J Jacobsen JP 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(13):3001-3009
We have used NMR and CD spectroscopy to study and characterise two alpha-L-LNA:DNA duplexes, a nonamer that incorporates three alpha-L-LNA nucleotides and a decamer that incorporates four alpha-L-LNA nucleotides, in which alpha-L-LNA is alpha-L-ribo-configured locked nucleic acid. Both duplexes adopt right-handed helical conformations and form normal Watson-Crick base pairing with all nucleobases in the anti conformation. Deoxyribose conformations were determined from measurements of scalar coupling constants in the sugar rings, and for the decamer duplex, distance information was derived from 1H-1H NOE measurements. In general, the deoxyriboses in both of the alpha-L-LNA:DNA duplexes adopt S-type (B-type structure) sugar puckers, that is the inclusion of the modified alpha-L-LNA nucleotides does not perturb the local native B-like double-stranded DNA (dsDNA) structure. The CD spectra of the duplexes confirm these findings, as these display B-type characteristic features that allow us to characterise the overall duplex type as B-like. The 1H-1H NOE distances which were determined for the decamer duplex were employed in a simulated annealing protocol to generate a model structure for this duplex, thus allowing a more detailed inspection of the impact of the alpha-L-ribo-configured nucleotides. In this structure, it is evident that the malleable DNA backbone rearranges in the vicinity of the modified nucleotides in order to accommodate them and present their nucleobases in a geometry suitable for Watson-Crick base pairing. 相似文献
89.
Hedin J Löfroth JE Nydén M 《Langmuir : the ACS journal of surfaces and colloids》2007,23(11):6148-6155
The adsorption behavior of ethyl(hydroxyethyl) cellulose EHEC and hydrophobically modified EHEC (HM-EHEC) at hydrophilic and hydrophobic surfaces has been studied using surface plasmon resonance (SPR) and quartz crystal microbalance with dissipation monitoring (QCM-D) methods. The adsorbed amounts measured with the different methods were different due to large amounts of water in the films. The slow adsorption process made it reasonable to assume a continuous polymer reconfiguration process at the surface. This was mostly seen for HM-EHEC at the hydrophobic surface, where a more flexible structure was adopted during the adsorption process. A cross-linking agent was seen to truly interpolymer cross-link EHEC at the hydrophilic surface and HM-EHEC at the hydrophobic surface. For EHEC at a hydrophobic surface and for HM-EHEC at a hydrophilic surface, the polymers adsorbed in an individually phase-separated manner, making an interpolymer cross-linking reaction unsuccessful. 相似文献
90.
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