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131.
Darses B Michaelides IN Sladojevich F Ward JW Rzepa PR Dixon DJ 《Organic letters》2012,14(7):1684-1687
A synthetic strategy for the construction of the [7-5-5] all-carbon tricyclic core of numerous calyciphylline A-type Daphniphyllum alkaloids has been developed using a key intramolecular Pauson-Khand reaction. A subsequent base-mediated double-bond migration and a regio- and stereoselective radical late stage allylic oxygenation provide access to the substitution patterns of daphnilongeranin B and daphniyunnine D. 相似文献
132.
A new cascade reaction involving an intramolecular Michael addition followed by an alkyne carbocyclization is presented. The reaction is promoted by a substoichiometric amount of KHMDS and represents one of the rare examples where the carbocyclization of an unactivated alkyne is mediated by an alkali metal base, under mild conditions. The reaction allows the generation of functionally dense, stereochemically defined, tricyclic structures possessing three adjacent stereocenters in good yields and with high stereoselectivity. 相似文献
133.
On-line comprehensive two-dimensional liquid chromatography techniques promise to resolve samples that current one-dimensional liquid chromatography methods cannot adequately deal with. To make full use of the potential of two-dimensional liquid chromatography, optimization is required. Optimization of two-dimensional liquid chromatography is a relatively new yet important research topic the aim of which is to predict combinations of stationary and mobile phases, column formats, and chromatographic conditions that maximize resolving power and minimize analysis time. In on-line two-dimensional liquid chromatography, dilution-related issues play also an important role and these should be taken into account when developing optimization strategies. In this work, state-of-the-art strategies that support method development for on-line two-dimensional liquid chromatography through a rigorous choice of chromatographic parameters are critically reviewed. The final aim is to provide practitioners with a clear understanding of which aspects can be optimized using current on-line two-dimensional liquid chromatography strategies (and which ones cannot). In two-dimensional liquid chromatography, maximizing resolving power for a given analysis time and dilution requires optimizing efficiency, selectivity and retention. While great strides forward have been made in the optimization of efficiency-related issues, considerable effort needs still to be made in terms of (1) developing models that can predict the retention factors that given stationary/mobile phase systems can provide and (2) using this information for choosing the two ones that maximize two-dimensional liquid chromatography orthogonality. Because of this limitation, in two-dimensional liquid chromatography, this aspect is typically dealt with a posteriori through examining chromatograms. This review clearly shows that important progress in the optimization of on-line two-dimensional liquid chromatography has recently been made. 相似文献
134.
Crobu M Rossi A Mangolini F Spencer ND 《Analytical and bioanalytical chemistry》2012,403(5):1415-1432
The surface chemistry of amorphous zinc polyphosphates of different compositions (ranging from zinc metaphosphate to zinc
orthophosphate) has been investigated by means of X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary-ion
mass spectroscopy (ToF-SIMS). The identification of the chain length of zinc polyphosphates by XPS was on the basis of the
integrated intensity ratio of the bridging (P–O–P) and nonbridging (P = O and P–O–M) oxygen peaks used for fitting the oxygen
1s signal, the shift of the P 2p3/2 signal towards lower binding energies and the modified Auger parameter towards higher values as the zinc content increases.
The discrimination of the polyphosphate chain lengths was also achieved by ToF-SIMS, by comparing the intensities of selected
characteristic phosphate fragments. Both techniques appear to be suitable for the investigation of polyphosphate glasses in
applications such as tribology, where there is a need to identify the chain length present in the outermost monolayer of the
film. Fourier-transform infrared (FT-IR) spectroscopy was used to characterize the bulk compounds. The FT-IR studies showed
that long-chain structures linked through P–O–P bonds predominate in the metaphosphate composition, while when the zinc content
is increased, the chains become shorter, ultimately being replaced by PO4 monomers in the orthophosphate composition. 相似文献
135.
136.
Anna?AmatEmail author Simona?FantacciEmail author Filippo?De?Angelis Benedetta?Carlotti Fausto?Elisei 《Theoretical chemistry accounts》2012,131(5):1218
Tetracyclines are a class of derivatives of polycyclic naphthacene carboxamide, which have attracted wide interest in the
pharmaceutical field for their use as antibiotics. These molecules are characterized by a substantial conformational flexibility
and by the presence of different binding sites which endow tetracycline with a noticeable capability in binding biological
targets. A salient property of tetracyclines is the presence of multiple acidic groups: four equilibrium constants have been
measured for the fully protonated tetracycline (TCH3
+) but so far no clear information concerning the pKas of the various sites has been reported. We present here a computational investigation on the correlation between the acid–base
and the spectroscopic properties of this important class of compounds. Starting from the TCH3
+ species, the pKa of all the possible deprotonation sites has been computed by DFT calculations. The computed pKas nicely compare with the experimental data, within 1 pKa unit, allowing us to individuate the products of the first deprotonation. This procedure has been iteratively repeated using
as starting species the products singled out from the previous deprotonation, thus individuating the stepwise products of
each deprotonation step. Then, the optical absorption spectra have been computed for all the species involved in the protonation/deprotonation
equilibria, comparing the results with the experimental data. The good agreement between theory and experiment has allowed
us to rationalize the correlation between the solution pH and the absorption spectra. 相似文献
137.
Dealing with the material microstructure an analytical multiscale model has recently been developed by Sih. Physically, the different orders of the stress singularities are related to the different constraints associated with the defect thought as a microscopic V-notch at the tip of the main crack. Irregularities of the material microstructure tend to curl the crack tip being the clamped-free boundary conditions the more realistic and general representation of what occurs on the microscopic V-notch. As a result, mixed mode conditions are always present along the V-notch bisector line.It is known for a long time that at the antisymmetric (mode II) stress distribution ahead of the crack tip generates a coupled out-of-plane singular mode. Recent theoretical and numerical analyses have demonstrated that this out-of-plane mode due to three-dimensional effects occurs also in the case of large V-notches where the mode II stress field is no longer singular. In addition, when the notch opening angle is non-zero, the three-dimensional singular stress state is strongly influenced by the plate thickness.The aim of this paper is to investigate the effect of free-fixed boundary conditions along the notch edges in three dimensional plates weakened by pointed V-notches and quantify the intensity of the out-of-plane singularity occurring under this constraint configuration. For the sake of simplicity a macronotch is considered rather than a micronotch. A synthesis of the magnitude of the stress state through the plate thickness is carried out by using the mean value of the strain energy density over a given control volume embracing the notch tip. The capability of the strain energy density to capture all the combined effects due to the out-of-plane mode make it a powerful parameter at every scale levels. 相似文献
138.
139.
Savarese M Aliberti A De Santo I Battista E Causa F Netti PA Rega N 《The journal of physical chemistry. A》2012,116(28):7491-7497
Although lifetimes and quantum yields of widely used fluorophores are often largely characterized, a systematic approach providing a rationale of their photophysical behavior on a quantitative basis is still a challenging goal. Here we combine methods rooted in the time-dependent density functional theory and fluorescence lifetime imaging microscopy to accurately determine and analyze fluorescence signatures (lifetime, quantum yield, and band peaks) of several commonly used rhodamine and pyronin dyes. We show that the radiative lifetime of rhodamines can be correlated to the charge transfer from the phenyl toward the xanthene moiety occurring upon the S(0) ← S(1) de-excitation, and to the xanthene/phenyl relative orientation assumed in the S(1) minimum structure, which in turn is variable upon the amino and the phenyl substituents. These findings encourage the synergy of experiment and theory as unique tool to design finely tuned fluorescent probes, such those conceived for modern optical sensors. 相似文献
140.
We compare two rational polyhedral admissible decompositions of the cone of positive definite quadratic forms: the perfect cone decomposition and the 2nd Voronoi decomposition. We determine which cones belong to both the decompositions, thus providing a positive answer to a conjecture of Alexeev and Brunyate (Invent. Math. doi:10.1007/s00222-011-0347-2, 2011). As an application, we compare the two associated toroidal compactifications of the moduli space of principal polarized abelian varieties: the perfect cone compactification and the 2nd Voronoi compactification. 相似文献