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91.
Zoltan Füredi Dhruv Mubayi Oleg Pikhurko 《Journal of Combinatorial Theory, Series A》2008,115(8):1552-1560
A 4-graph is odd if its vertex set can be partitioned into two sets so that every edge intersects both parts in an odd number of points. Let
92.
Agod A Nagy N Hórvölgyi Z 《Langmuir : the ACS journal of surfaces and colloids》2007,23(10):5445-5451
Two-dimensional molecular dynamics computer simulation has been developed to model the compression of Langmuir films composed of spherical nanoparticles with arbitrary size distribution. We demonstrate that the usual assumption in the determination of interparticle potentials from the surface pressure vs area isotherms (i.e., monodisperse particles in perfect hexagonal order) leads to a systematic overestimation of the characteristic length of the interaction. On the basis of the results of the simulation, we propose a correction method to improve the traditional way of determining the interparticle potentials. We use the corrected particle-particle interactions to explore the correlation between the broadness of the size distribution and several structural parameters (decay length of pair-correlation function, global orientational order parameter, mean, and standard deviation of number of neighbors). Due to the uniaxial compression and the stiffness of the particulate layer, the surface pressure is not a scalar field. We investigate the effect of polydispersity on the anisotropy and the fluctuation of the surface pressure tensor in Langmuir films during uniaxial compression. 相似文献
93.
94.
Vipond J Woods M Zhao P Tircsó G Ren J Bott SG Ogrin D Kiefer GE Kovacs Z Sherry AD 《Inorganic chemistry》2007,46(7):2584-2595
Interest in macrocyclic lanthanide complexes such as DOTA is driven largely through interest in their use as contrast agents for MRI. The lanthanide tetraamide derivatives of DOTA have shown considerable promise as PARACEST agents, taking advantage of the slow water exchange kinetics of this class of complex. We postulated that water exchange in these tetraamide complexes could be slowed even further by introducing a group to sterically encumber the space above the water coordination site, thereby hindering the departure and approach of water molecules to the complex. The ligand 8O2-bridged DOTAM was synthesized in a 34% yield from cyclen. It was found that the lanthanide complexes of this ligand did not possess a water molecule in the inner coordination sphere of the bound lanthanide. The crystal structure of the ytterbium complex revealed that distortions to the coordination sphere were induced by the steric constraints imposed on the complex by the bridging unit. The extent of the distortion was found to increase with increasing ionic radius of the lanthanide ion, eventually resulting in a complete loss of symmetry in the complex. Because this ligand system is bicyclic, the conformation of each ring in the system is constrained by that of the other; in consequence, inclusion of the bridging unit in the complexes means only a twisted square, antiprismatic coordination geometry is observed for lanthanide complexes of 8O2-bridged DOTAM. 相似文献
95.
Zoltan Varga 《Structural chemistry》2017,28(2):297-301
The carbocation CH5 + has a unique structure of C s symmetry with three 2-center-2-electron bonds and one 3-center-2-electron bond. This ground-state geometry is different from the prediction by the VSEPR model. We have investigated this carbocation along with related systems, among them, BH5, BLi5, CLi5 +, NH5 2+, NLi5 2+, to seek possible trends and regularities in the variations of their ground-state geometries. 相似文献
96.
Chen X Wang J Paszti Z Wang F Schrauben JN Tarabara VV Schmaier AH Chen Z 《Analytical and bioanalytical chemistry》2007,388(1):65-72
Electrostatic interactions between negatively charged polymer surfaces and factor XII (FXII), a blood coagulation factor,
were investigated by sum frequency generation (SFG) vibrational spectroscopy, supplemented by several analytical techniques
including attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), quartz crystal microbalance (QCM),
ζ-potential measurement, and chromogenic assay. A series of sulfonated polystyrenes (sPS) with different sulfonation levels
were synthesized as model surfaces with different surface charge densities. SFG spectra collected from FXII adsorbed onto
PS and sPS surfaces with different surface charge densities showed remarkable differences in spectral features and especially
in spectral intensity. Chromogenic assay experiments showed that highly charged sPS surfaces induced FXII autoactivation.
ATR-FTIR and QCM results indicated that adsorption amounts on the PS and sPS surfaces were similar even though the surface
charge densities were different. No significant conformational change was observed from FXII adsorbed onto surfaces studied.
Using theoretical calculations, the possible contribution from the third-order nonlinear optical effect induced by the surface
electric field was evaluated, and it was found to be unable to yield the SFG signal enhancement observed. Therefore it was
concluded that the adsorbed FXII orientation and ordering were the main reasons for the remarkable SFG amide I signal increase
on sPS surfaces. These investigations indicate that negatively charged surfaces facilitate or induce FXII autoactivation on
the molecular level by imposing specific orientation and ordering on the adsorbed protein molecules.
Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users. 相似文献
97.
Green KN Viswanathan S Rojas-Quijano FA Kovacs Z Sherry AD 《Inorganic chemistry》2011,50(5):1648-1655
A series of new 1,4,7,10-tetraazacyclododecane-derivatives having a combination of amide and ketone donor groups as side-arms were prepared, and their complexes with europium(III) studied in detail by high resolution NMR spectroscopy. The chemical shift of the Eu(3+)-bound water resonance, the chemical exchange saturation transfer (CEST) characteristics of the complexes, and the bound water residence lifetimes (τ(m)) were found to vary dramatically with the chemical structure of the side-arms. Substitution of ketone oxygen donor atoms for amide oxygen donor atoms resulted in an increase in residence water lifetimes (τ(m)) and a decrease in chemical shift of the Eu(3+)-bound water molecule (Δω). These experimental results along with density functional theory (DFT) calculations demonstrate that introduction of weakly donating oxygen atoms in these complexes results in a much weaker ligand field, more positive charge on the Eu(3+) ion, and an increased water residence lifetime as expected for a dissociative mechanism. These results provide new insights into the design of paramagnetic CEST agents with even slower water exchange kinetics that will make them more efficient for in vivo imaging applications. 相似文献
98.
Bogdan?Ti?aEmail author Adriana?Fulia? Zoltan?Szabadai Gerlinde?Rusu Geza?Bandur Dumitru?Ti?a 《Journal of Thermal Analysis and Calorimetry》2011,105(2):517-527
The thermal techniques of analysis were used to assess the compatibility between ibuprofen (IB) and some excipients used in
the development of extended released formulations. This study is a part of a systematic study undertaken to find and optimizes
a general method of detecting the drug–excipient interactions, with the aim of predicting rapidly and assuring the long-term
stability of pharmaceutical product and speeding up its marketing. The thermal properties of IB and its physical association
as binary mixtures with some common excipients were evaluated by thermogravimetry/derivative thermogravimetry (TG/DTG) and
differential scanning calorimetry. FT-IR spectroscopy and X-ray powder diffraction (XRPD) were used as complementary techniques
to adequately implement and assist in interpretation of the thermal results. Based on their frequent use in preformulations
nine different excipients: starch; microcrystalline cellulose (PH 101 and PH 102); colloidal silicon dioxide; lactose (monohydrate
and anhydre); polyvinylpyrrolidone; magnesium stearate and talc were blended with IB. The samples were prepared by mixing
the analyte and excipients in a proportion of 1:1 (w:w). The TG/DSC curves of the IB have shown a single stage of mass loss
between 175 and 290 °C, respectively, an endothermic peak at 78.5 °C, which corresponds to the melting (literature T
m = 75–78 °C). 相似文献
99.
Lumata L Merritt ME Malloy CR Sherry AD Kovacs Z 《The journal of physical chemistry. A》2012,116(21):5129-5138
Hyperpolarized [1-(13)C]pyruvate has become an important diagnostic tracer of normal and aberrant cellular metabolism for in vitro and in vivo NMR spectroscopy (MRS) and imaging (MRI). In pursuit of achieving high NMR signal enhancements in dynamic nuclear polarization (DNP) experiments, we have performed an extensive investigation of the influence of Gd(3+) doping, a parameter previously reported to improve hyperpolarized NMR signals, on the DNP of this compound. [1-(13)C]Pyruvate samples were doped with varying amounts of Gd(3+) and fixed optimal concentrations of free radical polarizing agents commonly used in fast dissolution DNP: trityl OX063 (15 mM), 4-oxo-TEMPO (40 mM), and BDPA (40 mM). In general, we have observed three regions of interest, namely, (i) a monotonic increase in DNP-enhanced nuclear polarization P(dnp) upon increasing the Gd(3+) concentration until a certain threshold concentration c(1) (1-2 mM) is reached, (ii) a region of roughly constant maximum P(dnp) from c(1) until a concentration threshold c(2) (4-5 mM), and (iii) a monotonic decrease in P(dnp) at Gd(3+) concentration c > c(2). Of the three free radical polarizing agents used, trityl OX063 gave the best response to Gd(3+) doping, with a 300% increase in the solid-state nuclear polarization, whereas addition of the optimum Gd(3+) concentration on BDPA and 4-oxo-TEMPO-doped samples only yielded a relatively modest 5-20% increase in the base DNP-enhanced polarization. The increase in P(dnp) due to Gd(3+) doping is ascribed to the decrease in the electronic spin-lattice relaxation T(1e) of the free radical electrons, which plays a role in achieving lower spin temperature T(s) of the nuclear Zeeman system. These results are discussed qualitatively in terms of the spin temperature model of DNP. 相似文献
100.
Szilvia A. Nagy Mihaly Aradi Gergely Orsi Gabor Perlaki David Olayinka Kamson Andrea Mike Hedvig Komaromy Attila Schwarcz Arpad Kovacs Jozsef Janszky Zoltan Pfund Zsolt Illes Peter Bogner 《Magnetic resonance imaging》2013