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991.
Rajaraman G Caneschi A Gatteschi D Totti F 《Physical chemistry chemical physics : PCCP》2011,13(9):3886-3895
Here we present DFT calculations based on a periodic mixed gaussians/plane waves approach to study the energetics, structure, bonding of SAMs of simple thiols on Au(111). Several open issues such as structure, bonding and the nature of adsorbate are taken into account. We started with methyl thiols (MeSH) on Au(111) to establish the nature of the adsorbate. We have considered several structural models embracing the reconstructed surface scenario along with the MeS˙-Au(ad)-MeS˙ type motif put forward in recent years. Our calculations suggest a clear preference for the homolytic cleavage of the S-H bond leading to a stable MeS˙ on a gold surface. In agreement with the recent literature studies, the reconstructed models of the MeS˙ species are found to be energetically preferred over unreconstructed models. Besides, our calculations reveal that the model with 1:2 Au(ad)/thiols ratio, i.e. MeS˙-Au(ad)-MeS˙, is energetically preferred compared to the clean and 1:1 ratio models, in agreement with the experimental and theoretical evidences. We have also performed Molecular Orbital/Natural Bond Orbital, MO/NBO, analysis to understand the electronic structure and bonding in different structural motifs and many useful insights have been gained. Finally, the studies have then been extended to alkyl thiols of the RSR' (R, R' = Me, Et and Ph) type and here our calculations again reveal a preference for the RS˙ type species adsorption for clean as well as for reconstructed 1:2 Au(ad)/thiols ratio models. 相似文献
992.
993.
Do H-bond features of silica surfaces affect the H2O and NH3 adsorption? Insights from periodic B3LYP calculations 总被引:1,自引:0,他引:1
The adsorption of a single H(2)O and NH(3) molecule on different fully hydroxylated α-quartz, cristobalite, and tridymite surfaces has been studied at the B3LYP level of theory, within a periodic approach using basis sets of polarized triple-ζ quality and accounting for basis set superposition error (BSSE). Fully hydroxylated crystalline silica exhibits SiOH as terminal groups whose distribution and H-bond features depend on both the considered silica polymorph and the crystallographic plane, which gives rise to isolated, H-bond interacting SiOH pairs or infinitely connected H-bond chains. A key point of the present study is to understand how the H-bond features of a dry crystalline silica surface influence its adsorption properties. Results reveal that the silica-adsorbate (H(2)O and NH(3)) interaction energy anticorrelates with the density of SiOH groups at the surface. This counterintuitive observation arises from the fact that pre-existing H-bonds of the dry surface need to be broken to establish new H-bonds between the surface and the adsorbate, which manifests in a sizable energy cost due to surface deformation. A simple method is also proposed to estimate the strength of the pre-existing H-bonds at the dry surfaces, which is shown to anticorrelate with the adsorbate interaction energy, in agreement with the above trends. 相似文献
994.
Dal Molin M Gasparini G Scrimin P Rastrelli F Prins LJ 《Chemical communications (Cambridge, England)》2011,47(46):12476-12478
(13)C-isotope labelling is presented as a novel tool for the study of complex chemical systems. (13)C-isotope labelling permits the quantification of all 26 members of a dynamic library from a single (13)C NMR spectrum without the need for advanced instrumentation or sophisticated experimental protocols. 相似文献
995.
Federico Ardila 《Journal of Combinatorial Theory, Series A》2011,118(8):2454-2462
Stanley (1986) showed how a finite partially ordered set gives rise to two polytopes, called the order polytope and chain polytope, which have the same Ehrhart polynomial despite being quite different combinatorially. We generalize his result to a wider family of polytopes constructed from a poset P with integers assigned to some of its elements.Through this construction, we explain combinatorially the relationship between the Gelfand-Tsetlin polytopes (1950) and the Feigin-Fourier-Littelmann-Vinberg polytopes (2010, 2005), which arise in the representation theory of the special linear Lie algebra. We then use the generalized Gelfand-Tsetlin polytopes of Berenstein and Zelevinsky (1989) to propose conjectural analogues of the Feigin-Fourier-Littelmann-Vinberg polytopes corresponding to the symplectic and odd orthogonal Lie algebras. 相似文献
996.
Federico Martellosio 《Statistics & probability letters》2011,81(8):1285-1291
Previous results have indicated that the OLS estimator of the vector of regression coefficients can be nearly as efficient as the best linear unbiased estimator when the regression errors follow a spatial process with root in the vicinity of unity. Such results were derived under the assumption of a symmetric weights matrix, which simplifies the analysis considerably, but is very often not satisfied in applications. This paper provides nontrivial generalizations to the important case of nonsymmetric weights matrices. 相似文献
997.
998.
Federico Cozzi Gianpiero Adami Pierluigi Barbieri Edoardo Reisenhofer Pietro Apostoli Massimo Bovenzi 《Central European Journal of Chemistry》2010,8(5):1014-1026
PM10 samples were collected during winter and summer seasons at two different sites in the area of Trieste (Italy). The content
of As, Cd, Cr, Mn, Ni, Pb and V in the PM10 samples was determined by inductively coupled plasma-mass spectrometry, with the purpose of evaluating the relevance of PM10 inhalation as a route of human exposure to these elements. The results showed that the ambient air concentration of the aforementioned
elements were below the limits or target values for both sites and season. Site and season-specific correlation analysis was
conducted for the identification of metals with similar origin: very good correlation for the couple Ni-V was found in both
sites and seasons, showing the influence of combustion of heavy oils in PM10 composition. The inter-site and seasonal variability of both PM10 and metal concentrations were examined. A stronger impact by the vehicular traffic on the PM10 and metal concentrations was found for the urban site in both seasons. Because of the great importance of the “Bora” wind
on the local climate, variability of PM10 and toxic elements concentration with wind were analysed, allowing determination of the effect of wind on dilution of the
pollutants for the urban site during both seasons.
相似文献
999.
Talens-Alesson FI 《Langmuir : the ACS journal of surfaces and colloids》2010,26(22):16812-16817
The critical surface excess of micellization (CSEM) should be regarded as the critical condition for micellization of ionic surfactants instead of the critical micelle concentration (CMC). There is a correspondence between the surface excesses Γ of anionic, cationic, and zwitterionic surfactants at their CMCs, which would be the CSEM values, and the critical association distance for ionic pair association calculated using Bjerrum's correlation. Further support to this concept is given by an accurate method for the prediction of the relative binding of alkali cations onto dodecylsulfate (NaDS) micelles. This method uses a relative binding strength parameter calculated from the values of surface excess Γ at the CMC of the alkali dodecylsulfates. This links both the binding of a given cation onto micelles and the onset for micellization of its surfactant salt. The CSEM concept implies that micelles form at the air-water interface unless another surface with greater affinity for micelles exists. The process would start when surfactant monomers are close enough to each other for ionic pairing with counterions and the subsequent assembly of these pairs becomes unavoidable. This would explain why the surface excess Γ values of different surfactants are more similar than their CMCs: the latter are just the bulk phase concentrations in equilibrium with chemicals with different hydrophobicity. An intriguing implication is that CSEM values may be used to calculate the actual critical distances of ionic pair formation for different cations, replacing Bjerrum's estimates, which only discriminate by the magnitude of the charge. 相似文献
1000.
Seta Noventa Jvan Barbaro Malgorzata Formalewicz Claudia Gion Federico Rampazzo Rossella Boscolo Brusà Massimo Gabellini Daniela Berto 《Analytica chimica acta》2015
This work validated an automated, fast, and low solvent- consuming methodology suited for routine analysis of tributyltin (TBT) and degradation products (dibutyltin, DBT; monobutyltin, MBT) in biota samples. The method was based on the headspace solid-phase microextraction methodology (HS-SPME), coupled with gas chromatographic separation and tandem mass-spectrometry (GC–MS/MS). The effectiveness of the matrix-matched signal ratio external calibration was tested for quantification purposes. The exclusion of matrix influences in the calibration curves proved the suitability of this versatile quantification method. The method detection limits obtained were of 3 ng Sn g−1 dw for all the analytes. The analysis of references materials showed satisfying accuracy under optimum calibration conditions (% recovery between 87–111%; |Z-scores|<2). The repeatability RSD% and intra-laboratory reproducibility RSD% were lower than 9.6% and 12.6%, respectively. The work proved the remarkable analytical performances of the method and its high potential for routine application in monitoring organotin compounds (OTC). 相似文献