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991.
José Manuel Guevara‐Vela Rodrigo Chávez‐Calvillo Prof. Dr. Marco García‐Revilla Prof. Dr. Jesús Hernández‐Trujillo Prof. Dr. Ove Christiansen Prof. Dr. Evelio Francisco Prof. Dr. Ángel Martín Pendás Prof. Dr. Tomás Rocha‐Rinza 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(42):14304-14315
The cooperative effects of hydrogen bonding in small water clusters (H2O)n (n=3–6) have been studied by using the partition of the electronic energy in accordance with the interacting quantum atoms (IQA) approach. The IQA energy splitting is complemented by a topological analysis of the electron density (ρ( r )) compliant with the quantum theory of atoms‐in‐molecules (QTAIM) and the calculation of electrostatic interactions by using one‐ and two‐electron integrals, thereby avoiding convergence issues inherent to a multipolar expansion. The results show that the cooperative effects of hydrogen bonding in small water clusters arise from a compromise between: 1) the deformation energy (i.e., the energy necessary to modify the electron density and the configuration of the nuclei of the isolated water molecules to those within the water clusters), and 2) the interaction energy (Eint) of these contorted molecules in (H2O)n. Whereas the magnitude of both deformation and interaction energies is enhanced as water molecules are added to the system, the augmentation of the latter becomes dominant when the size of the cluster is increased. In addition, the electrostatic, classic, and exchange components of Eint for a pair of water molecules in the cluster (H2O)n?1 become more attractive when a new H2O unit is incorporated to generate the system (H2O)n with the last‐mentioned contribution being consistently the most important part of Eint throughout the hydrogen bonds under consideration. This is opposed to the traditional view, which regards hydrogen bonding in water as an electrostatically driven interaction. Overall, the trends of the delocalization indices, δ(Ω,Ω′), the QTAIM atomic charges, the topology of ρ( r ), and the IQA results altogether show how polarization, charge transfer, electrostatics, and covalency contribute to the cooperative effects of hydrogen bonding in small water clusters. It is our hope that the analysis presented in this paper could offer insight into the different intra‐ and intermolecular interactions present in hydrogen‐bonded systems. 相似文献
992.
993.
Abstract Computer modelling studies have been carried out on the interaction of 18-crown-6 with a variety of guest molecules, including urea, thiourea and substituted ureas. The five known crystal structures of these host/guest systems were used as models. We were interested to establish whether the arrangement of guest molecules around a host molecule in the crystal was indicative of the lowest energy configuration for a host/guest fragment or was a consequence of packing effects. Two models were therefore considered for each structure and the structures minimised via molecular mechanics. In the first mode, the structure consisted of one unit cell and periodic boundary conditions were used in the calculation. Coulombic effects were calculated using the Ewald summation. In the second model, the structure consisted of an 18-crown-6 molecule surrounded by two hydrogen-bonded guest molecules. Both models were minimised using the CERIUS package using the DreidingII forcefield. The crystal structure minimisations reproduced the structures very well with an average change in cell volume of 3.6% and a mean r.m.s. positional deviation of 0.20 Å. The fits for the fragment models were significantly larger for all structures (mean 0.30 Å) but even so it can be concluded that the arrangement in the crystal gave a good indication of the lowest energy configuration of the host/guest in vacuo. 相似文献
994.
Jian‐Biao Liu Prof. Dr. W. H. Eugen Schwarz Prof. Dr. Jun Li 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(44):14758-14767
Experimentally and theoretically derived interatomic distances (D) and ionic radii (R) of more than a hundred monomeric (AX), dimeric (A2X2, ABXY), and crystalline ([AX]) alkali halide species (A=Li, Na, K, Rb, Cs, Fr; X=H, F, Cl, Br, I, At) have been analyzed. Chemists use the word “atomic radius” for two antithetic concepts. Let DCiEE′jj′ be the “billion” of distances i between two adjacent atoms in the millions of known compounds C from a hundred different elements E in bonding states j. The common chemical aim is partitioning D approximately into increments REj+RE′j′. This can be achieved with a few (say <thousand) predictive constants REj. An antipodal aim is specifying in hindsight an electron density feature in the “billion” of different bonds i, by partitioning them into “two billions” of exact bonded radii ${R{{{\rm C}i\hfill \atop {\rm E}\hfill}}}$ +${R{{{\rm C}i\hfill \atop {\rm E}{^\prime}\hfill}}}$ . The constant incremental and the variable bonded radii concepts with the same generic name are useful in different fields of research. Different concepts should be well distinguished, since they have different meaning, different numerical values, and different purposes. 相似文献
995.
Pt µdisc electrodes have been modified by mesoporous organosilica thin films by electrochemically assisted self‐assembly (EASA) of mercaptopropyltrimethoxysilane (MPTMS), tetraethoxysilane (TEOS), and the surfactant cetyltrimethylammonium bromide (CTAB). The EASA process involves the generation of hydroxide ions at the electrode/solution interface, upon the application of a cathodic current density, leading to TEOS and MPTMS polycondensation around the CTAB template and concomitant growing of a thiol‐functionalized mesoporous film onto the electrode surface. The experimental conditions (current density, deposition time, silane concentration and molar ratio between surfactant template and silane) were optimised to form a thin and permeable film likely to be used in preconcentration electroanalysis. The morphology of the film electrodes were characterised by scanning electron microscopy. The permeability properties of the modified Pt µdisc electrodes have been evidenced by cyclic voltammetry using Ru(NH3)63+ as a redox probe. The best parameters identified for the film preparation are a current density of ? 8 mA cm?2 applied for 15 s in a solution containing 110 mM of hydrolysed silane precursors and 70.4 mM of CTAB. Pt µdisc electrodes modified in these conditions were used for the open‐circuit preconcentration of Hg(II) species prior to their detection by anodic stripping voltammetry in a mercury‐free solution. In the optimized conditions, a sensitivity of 14.3 mA cm?2 µM?1 was obtained for the 0.02–0.08 µM concentration range. The analytical performance of such organosilica films could decay by up to two orders of magnitude for the materials prepared in conditions other than the optimized ones, highlighting the need for a fine control of the deposition parameters to elaborate sensors based on such modified ultramicroelectrodes. 相似文献
996.
We demonstrate the utilization of silver/gold nanocages (Ag/Au NCs) deposited onto transparent indium tin oxide (ITO) film glass as the basis of a reagentless, simple and inexpensive mercury probe. The localized surface plasmon resonance (LSPR) peak wavelength was located at ∼800 nm. By utilizing the redox reaction between Hg2+ ions and Ag atoms that existed in Ag/Au NCs, the LSPR peak of Ag/Au NCs was blue-shifted. Thus, we develop an optical sensing probe for the detection of Hg2+ ions. The LSPR peak changes were lineally proportional to the concentration of Hg2+ ions over the range from 10 ppb to 0.5 ppm. The detection limit was ∼5 ppb. This plasmonic probe shows good selectivity and high sensitivity. The proposed optical probe is successfully applied to the sensing of Hg2+ in real samples. 相似文献
997.
The discovery and development of photoelectrochemical sensors with novel principles are of great significance to realize sensitive and low-cost detection. In this paper, a new photoelectrochemial sensor based on the in situ formation of p–n junction was designed and used for the accurate determination of mercury(II) ions. Cysteine-capped ZnS quantum dots (QDs) was assembled on the surface of indium tin oxide (ITO) electrode based on the electrostatic interaction between Poly(diallyldimethylammonium chloride) (PDDA) and Cys-capped ZnS QDs. The in situ formation of HgS, a p-type semiconductor, on the surface of ZnS facilitated the charge carrier transport and promoted electron-hole separation, triggered an obviously enhanced anodic photocurrent of Cys-capped ZnS QDs. The formation of p–n junction was confirmed by P–N conductive type discriminator measurements and current–voltage (I–V) curves. The photoelectrochemical method was used for the sensing of trace mercuric (II) ions with a linear concentration of 0.01 to 10.0 µM and a detection limit of 4.6 × 10−9 mol/L. It is expected that the present study can serve as a foundation to the application of p–n heterojunction to photoelectrochemical sensors and it might be easily extended to more exciting sensing systems by photoelectrochemistry. 相似文献
998.
Maria José da Silva Ana Paula S. Paim Maria Fernanda Pimentel M. Luisa Cervera Miguel de la Guardia 《Analytica chimica acta》2014
Total mercury, at μg kg−1 level, was determined in different types of nuts (cashew nut, Brazil nuts, almond, pistachio, peanut, walnut) using a direct mercury analyser after previous sample defatting and by cold vapour atomic fluorescence spectrometry. There is not enough sensitivity in the second approach to determine Hg in previously digested samples due to the strong matrix effect. Mercury levels in 25 edible nut samples from Brazil and Spain were found in the range from 0.6 to 2.7 μg kg−1 by using the pyrolysis of sample after the extraction of the nut fat. The accuracy of the proposed method was confirmed by analysing certified reference materials of Coal Fly Ash-NIST SRM 1633b, Fucus-IAEA 140 and three unpolished Rice Flour NIES-10. The observed results were in good agreement with the certified values. The recoveries of different amounts of mercury added to nut samples ranged from 94 to 101%. RSD values corresponding to three measurements varied between 2.0 and 14% and the limit of detection and quantification of the method were 0.08 and 0.3 μg kg−1, respectively. 相似文献
999.
Aberrant expression of microRNAs (miRNAs), short non-coding RNA molecules regulating gene expression, is often found in tumor cells, making the miRNAs suitable candidates as cancer biomarkers. Electrochemistry is an interesting alternative to current standard methods of miRNA detection by offering cheaper instrumentation and faster assays times. In this paper, we labeled miRNA in a quick, simple, two-step procedure with electroactive complex of osmium(VI) and 2,2′-bipyridine, Os(VI)bipy, which specifically binds to the ribose at the 3′-end of the miRNA, and hybridized such labeled miRNA with biotinylated capture probe attached to the streptavidin magnetic beads. Labeled miRNA was then detected at hanging mercury drop electrode at femtomole level due to an electrocatalytic nature of the peak from the Os(VI)bipy label. We obtained good selectivity of the assay using elevated hybridization temperatures for better discrimination of perfect duplex from single and double mismatches. After optimization of the protocol, we demonstrated feasibility of our assay by detecting target miRNA in real total RNA samples isolated from human cancer cells. 相似文献
1000.
Using the Hg2+-induced desulfurization reaction of thiosemicarbazide derivative, we designed and synthesized a novel “turn on” coumarin-based fluorescent probe L with a simple structure for detecting mercury ion (II). Spectroscopy revealed that the probe responds selectively to mercury ions over other metal ions with marked fluorescence enhancement. Detection of Hg2+ was effective at pH 7.0–9.5, with high selectivity and significant effect in HeLa cells, human umbilical vein endothelial cells and Escherichia coli, but no cytotoxicity. This probe could be an ideal and practical Hg2+ probe with important biological significance. 相似文献