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111.
The internally folded density or reciprocal form factor B( r ) of many‐electron systems is tightly estimated from the knowledge of a small discrete set of values of the Compton profile J( q ). In doing so, the minimum cross‐entropy technique is employed. A numerical analysis of the approximations is carried out for the Helium atom. © 2002 John Wiley & Sons, Inc. Int J Quantum Chem, 2002  相似文献   
112.
It is well known that many nonlinear phenomena such as bifurcations and chaotic behavior occur in DC–DC converters mainly due to the switching action among all the different topologies of the circuit. Such behavior has been described with detail numerically, and also mathematical reasoning has been provided. In this paper we focuss on the experimental side of a DC–DC Buck converter controlled with two different strategies: classical Pulse Width Modulation (PWM) with a ramp and a more recently described Zero Average Dynamics (ZAD). We show some nonsmooth events and we explain with detail the experimental set-ups. In one of them, we use a FPGA card to obtain on-line results. In the other we use Virtual Instrumentation to generate an experimental two-dimensional bifurcation diagram, which will be compared to the numerical data. After the data acquisition of the system state variables some elaborated post-processing must be made. This is done through LabView. Although the main application of these results is centered in avoiding non-periodic or high-amplitude periodic behavior, they can also be applied to reducing the generated electro-magnetic interference and to the information transmission.Partially funded by SICONOS.  相似文献   
113.
Crime is a negative phenomenon that affects the daily life of the population and its development. When modeling crime data, assumptions on either the spatial or the temporal relationship between observations are necessary if any statistical analysis is to be performed. In this paper, we structure space–time dependency for count data by considering a stochastic difference equation for the intensity of the space–time process rather than placing structure on a latent space–time process, as Cox processes would do. We introduce a class of spatially correlated self-exciting spatio-temporal models for count data that capture both dependence due to self-excitation, as well as dependence in an underlying spatial process. We follow the principles in Clark and Dixon (2021) but considering a generalized additive structure on spatio-temporal varying covariates. A Bayesian framework is proposed for inference of model parameters. We analyze three distinct crime datasets in the city of Riobamba (Ecuador). Our model fits the data well and provides better predictions than other alternatives.  相似文献   
114.
The incidence of nosocomial infections, as well as the high mortality and drug resistance expressed by nosocomial pathogens, especially in immunocompromised patients, poses significant medical challenges. Currently, the efficacy of plant compounds with antimicrobial potential has been reported as a promising alternative therapy to traditional methods. Isoespintanol (ISO) is a monoterpene with high biological activity. Using the broth microdilution method, the antibacterial activity of ISO was examined in 90 clinical isolates, which included 14 different species: (Escherichia coli (38), Pseudomonas aeruginosa (12), Klebsiella pneumoniae (13), Acinetobacter baumannii (3), Proteus mirabilis (7), Staphylococcus epidermidis (3), Staphylococcus aureus (5), Enterococcus faecium (1), Enterococcus faecalis (1), Stenotrophomonas maltophilia (2), Citrobacter koseri (2), Serratia marcescens (1), Aeromonas hydrophila (1), and Providencia rettgeri (1). MIC90 minimum inhibitory concentration values ranged from 694.3 to 916.5 µg/mL and MIC50 values from 154.2 to 457.3 µg/mL. The eradication of mature biofilms in P. aeruginosa after 1 h of exposure to ISO was between 6.6 and 77.4%, being higher in all cases than the percentage of biofilm eradication in cells treated with ciprofloxacin, which was between 4.3 and 67.5%. ISO has antibacterial and antibiofilm potential against nosocomial bacteria and could serve as an adjuvant in the control of these pathogens.  相似文献   
115.
The one-particle density in momentum space γ(p) is studied for diatomic molecules by using the maximum-entropy technique. The knowledge of one or more momentum expectation values <p"> provides approximations on the density γ(p) for any value of the momentum, which are convergent when increasing the number of known moments. Other unknown expectation values are estimated in terms of the constructed maximum-entropy densities. A numerical study of the quality of the approximations is carried out by means of experimental and theoretical data for the momentum expectation values involved. Experimental errors are also taken into account to have an idea of the sensibility of the results to the information from which they are obtained. © 1997 John Wiley & Sons, Inc.  相似文献   
116.
Many of the existing molecular simulation tools require the efficient identification of the set of nonbonded interacting atoms. This is necessary, for instance, to compute the energy values or the steric contacts between atoms. Cell linked-lists can be used to determine the pairs of atoms closer than a given cutoff distance in asymptotically optimal time. Despite this long-term optimality, many spurious distances are anyway computed with this method. Therefore, several improvements have been proposed, most of them aiming to refine the volume of influence for each atom. Here, we suggest a different improvement strategy based on avoiding to fill cells with those atoms that are always at a constant distance of a given atom. This technique is particularly effective when large groups of the particles in the simulation behave as rigid bodies as it is the case in simplified models considering only few of the degrees of freedom of the molecule. In these cases, the proposed technique can reduce the number of distance computations by more than one order of magnitude, as compared with the standard cell linked-list technique. The benefits of this technique are obtained without incurring in additional computation costs, because it carries out the same operations as the standard cell linked-list algorithm, although in a different order. Since the focus of the technique is the order of the operations, it might be combined with existing improvements based on bounding the volume of influence for each atom.  相似文献   
117.
The possibility of using differential pre-heating prior to supramolecular gelation to control the balance between hydrogen-bonding and aromatic stacking interactions in supramolecular gels and obtain consequent systematic regulation of structure and properties is demonstrated. Using a model aromatic peptide amphiphile, Fmoc-tyrosyl-leucine (Fmoc-YL) and a combination of fluorescence, infrared, circular dichroism and NMR spectroscopy, it is shown that the balance of these interactions can be adjusted by temporary exposure to elevated temperatures in the range 313–365 K, followed by supramolecular locking in the gel state by cooling to room temperature. Distinct regimes can be identified regarding the balance between H-bonding and aromatic stacking interactions, with a transition point at 333 K. Consequently, gels can be obtained with customizable properties, including supramolecular chirality and gel stiffness. The differential supramolecular structures also result in changes in proteolytic stability, highlighting the possibility of obtaining a range of supramolecular architectures from a single molecular structure by simply controlling the pre-assembly temperature.  相似文献   
118.
The phosphoramidite building block synthesis of the thio analogue at the 5,6-dihydropyrimidine C5 position of the thymidylyl(3'-5')thymidine (6-4) photoproduct 1 is presented. This compound was readily obtained from the appropriately protected dinucleotide P-methyl-5'-O-dimethoxytritylthymidilyl(3' --> 5')-4-thiothymidine 2 after irradiation at 366 nm, then S-sulfenylmethylation of the thiol function of the resulting (6-4) adduct, and phosphitylation of the 3'-hydroxyl group.  相似文献   
119.
The biosynthesis of human blood group B antigens is accomplished by a highly specific galactosyltransferase (GTB). On the basis of NMR experiments, we propose a "molecular tweezers mechanism" that accounts for the exquisite stereoselectivity of donor substrate selection. Transferred NOE experiments for the first time reveal the bioactive conformation of the donor substrate UDP-galactose (UDP-Gal) and of its enzymatically inactive analogue, UDP-glucose (UDP-Glc). Both bind to GTB in a folded conformation that is sparsely populated in solution, whereas acceptor ligands bind in a conformation that predominates in solution. The bound conformations of UDP-Gal and UDP-Glc are identical within experimental error. Therefore, GTB must discriminate between the two activated sugars on the basis of a hitherto unknown transition state that can only be formed in the case of UDP-Gal. A full relaxation and exchange matrix analysis of STD NMR experiments reveals that acceptor substrates dissociate significantly faster (k(off) > 100 Hz) from the binding pocket than donor substrates (k(off) approximately 10 Hz). STD NMR experiments also directly show that proper recognition of the hexopyranose rings of the UDP sugars requires bivalent metal cations. At the same time, this analysis furnishes the complete three-dimensional structure of the enzyme with its bound donor substrate UDP-Gal on the basis of a prior crystal structure analysis. We propose that, upon acceptor binding, GTB uses the Asp 302 and Glu 303 side chains as "molecular tweezers" to promote bound UDP-Gal but not UDP-Glc into a transition state that leads to product formation.  相似文献   
120.
The discrepancy among one-electron and two-electron densities for diverse N-electron atomss, enclosing neutral systems (with nuclear charge Z=N) and charge-one ions (|NZ|=1), is quantified by means of mutual information, I, and Quantum Similarity Index, QSI, in the conjugate spaces position/momentum. These differences can be interpreted as a measure of the electron correlation of the system. The analysis is carried out by considering systems with a nuclear charge up to Z=103 and singly charged ions (cations and anions) as far as N=54. The interelectronic correlation, for any given system, is quantified through the comparison of its double-variable electron pair density and the product of the respective one-particle densities. An in-depth study along the Periodic Table reveals the importance, far beyond the weight of the systems considered, of their shell structure.  相似文献   
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