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271.
Stabilisation of transition states prior to and following eudesmane cation in aristolochene synthase
The mechanistic details of the cyclisation of farnesylpyrophosphate (FPP) by aristolochene synthase (AS) from Penicillium roqueforti have only recently begun to emerge, mainly through the analysis of the reaction products generated by AS-mutants. The reaction proceeds through several intermediates including germacrene A and eudesmane cation. Previous work suggested that the side chain of phenylalanine 178 promoted the conversion of eudesmane cation to aristolochene. We now report that the catalytic function of this residue during the conversion of eudesmane cation to aristolochene is mainly due to the large size of its side chain, which facilitates the hydride shift from C2 to C3, rather than its aromatic character. In addition, F178 appears to control the regioselectivity of the final deprotonation step and, together with F112, helps stabilise the developing positive charge on C1 after the expulsion of pyrophosphate from the substrate. These results complete a screen of likely active-site aromatic residues and establish their respective roles in the conversion of FPP to aristolochene. 相似文献
272.
Gribov EN Cocina D Spoto G Bordiga S Ricchiardi G Zecchina A 《Physical chemistry chemical physics : PCCP》2006,8(10):1186-1196
The adsorption of Ar, H2, O2, N2 and CO on (H,Na)-Y zeolite (Si/Al = 2.9, H+/Na+ approximately 5) has been studied at variable-temperature (90-20 K) and sub-atmospheric pressure (0-40 mbar) by FTIR spectroscopy. Unprecedented filling conditions of the zeolite cavities were attained, which allowed the investigation of very weakly adsorbed species and of condensed, liquid-like or solid-like, phases. Two pressure regimes were singled out, characterized by: (i) specific interaction at low pressure of the probe molecules (P) with the internal Br?nsted and Lewis sites, and (ii) multilayer adsorption at higher pressure. In the case of CO the perturbation of the protonic sites located inside the sodalite cages was also observed. As the molecule is too large to penetrate the sodalite cage, the perturbation is thought to involve a proton jump tunneling mechanism. The adsorption energy for the (HF)OH...P (P = Ar, H2, O2, N2 and CO) specific interaction involving the high frequency Br?nsted acid sites exposed in the supercages was derived following the VTIR (variable temperature infrared spectroscopy) method described by E. Garrone and C. Otero Areán (Chem. Soc. Rev., 2005, 34, 846). 相似文献
273.
Determination of antimicrobial residues and metabolites in the aquatic environment by liquid chromatography tandem mass spectrometry 总被引:2,自引:0,他引:2
Antimicrobials are used in large quantities in human and veterinary medicine. Their environmental occurrence is of particular
concern due to the potential spread and maintenance of bacterial resistance. After intake by the organisms, the unchanged
drug and its metabolized forms are excreted and enter wastewater treatment plants where they are mostly incompletely eliminated,
and are therefore eventually released into the aquatic environment. The reliable detection of several antimicrobials in different
environmental aqueous compartments is the result of great improvements achieved in analytical chemistry. This article provides
an overview of the more outstanding analytical methods based on liquid chromatography tandem mass spectrometry, developed
and applied to determine antimicrobial residues and metabolites present in surface, waste, and ground waters.
相似文献
274.
Nanostructured electrochemical DNA biosensors for detection of the effect of berberine on DNA from cancer cells 总被引:2,自引:0,他引:2
Ovádeková R Jantová S Letasiová S Stepánek I Labuda J 《Analytical and bioanalytical chemistry》2006,386(7-8):2055-2062
Multi walled carbon nanotubes (MWNT) in dimethylformamide (DMF) or aqueous sodium dodecyl sulfate (SDS) solution, colloidal
gold nanoparticles (GNP) in phosphate buffer solution (PBS), and a GNP–MWNT mixture in aqueous SDS solution have been investigated
for chemical modification of a screen-printed carbon electrode used as the signal transducer of a dsDNA-based biosensor. Differential
pulse voltammetry of the DNA redox marker and the guanine moiety anodic oxidation and cyclic voltammetry with K3[Fe(CN)6] as indicator revealed substantial enhancement of the response of the biosensor, particularly when MWNT in SDS solution was
used. The biosensor was used in testing of berberine, an isoquinoline plant alkaloid with significant antimicrobial and anticancer
activity. Berberine had a very strong, concentration-dependent, effect on the structural stability of DNA from the human cancer
cells (U937 cells) whereas non-cancer cells were changed only when berberine concentrations were relatively high 75 and 50 μg
mL−1.
Figure Schematic illustration of preparation of the nanostructured films: (a) layer-to-layer coverage (DNA/nanomaterial/SPE); (b) mixed coverage (DNA-nanomaterial/SPE) 相似文献
275.
Arnbjerg J Johnsen M Frederiksen PK Braslavsky SE Ogilby PR 《The journal of physical chemistry. A》2006,110(23):7375-7385
Singlet molecular oxygen, O2(a1Deltag), can be produced upon resonant two-photon excitation of a photosensitizer. In the present study, two molecules that have received recent attention in studies of nonlinear organic materials were characterized for use as standard two-photon sensitizers: 2,5-dicyano-1,4-bis(2-(4-diphenylaminophenyl)vinyl)-benzene, CNPhVB, and 2,5-dibromo-1,4-bis(2-(4-diphenylaminophenyl)vinyl)-benzene, BrPhVB. Absolute two-photon absorption cross sections, delta, were independently determined for these molecules using two techniques that have heretofore not been applied to this problem: an optical technique (time-resolved detection of O2(a1Deltag) phosphorescence) and a nonoptical technique (a time-resolved laser-induced optoacoustic experiment). For experiments performed in toluene, a solvent commonly used for such nonlinear optical studies, appreciable absorption by the solvent itself complicates the measurements. In cyclohexane, however, delta values could be obtained without the interfering effects of solvent absorption. On the basis of these results, we discuss key aspects of the respective techniques used to quantify values of delta. The information reported herein provides some explanation for the lack of consensus that is routinely observed in published values of delta, certainly for experiments performed in aromatic solvents such as toluene and benzene. 相似文献
276.
The acid-base equilibria in 5,10,15,20-tetrakis(4-sulfonatophenyl)chlorin were studied in aqueous solution and compared with the respective data for the corresponding porphyrin. The reduction of the pyrrole ring in the tetrapyrrolic macrocycle noticeably influences both free base/monoprotonated and mono-/diprotonated species equilibria. In strong acidic solutions protonation of 4-sulfonatophenyl groups takes place in addition to protonation of the macrocycle core. The photophysical properties of all ionic forms are influenced by an enhanced rate of internal S1 --> S0 conversion, leading to about 50% and 90% deactivation through this channel for the free base and diprotonated species, respectively. The enhancement of the rate of the radiationless transitions is explained by an increased conformational flexibility of the chlorin macrocycle with respect to that of a porphyrin. Structural volume change measurements with laser-induced optoacoustic spectroscopy support this explanation. The contraction upon triplet state formation of the free base is about one-half of that measured for the corresponding porphyrin. This contraction should be due to intramolecular structural rearrangements of the macrocycle to adopt a minimum energy conformation in case of the chlorin. On the contrary, for the more rigid porphyrin macrocycle the interactions of the molecule with the solvent environment play a more important role. The diprotonated forms of both porphyrin and chlorin show a high radiationless S1 --> S0 conversion rate and seem to have a similar conformational flexibility. In agreement with previous calculations, the conformational flexibility of the diprotonated forms appears to be higher than that of the free base molecule. 相似文献
277.
Triulzi RC Micic M Giordani S Serry M Chiou WA Leblanc RM 《Chemical communications (Cambridge, England)》2006,(48):5068-5070
An immunoassay based upon photoluminescent gold quantum dots aimed at detecting human IgG in aqueous solution from micromolar to nanomolar concentrations is described. 相似文献
278.
279.
Silvia A. Centeno Polonca Ropret Eleonora Del Federico Jacob Shamir Boris Itin Alexej Jerschow 《Journal of Raman spectroscopy : JRS》2010,41(4):445-451
The complexes between Al(III) and hematein, the main coloring matter in alum logwood inks, were characterized by Raman and 27Al NMR (nuclear magnetic resonance) spectroscopies. Raman spectra of the crystallized complexes and of the compounds applied on a paper substrate are presented and assigned based on published data for the parent compounds. These Raman spectra show that the coordination of the hematein to the Al(III) ions takes place in both cases through the carbonyl and hydroxyl groups in the molecule, and that the aromatic rings are also involved in the interaction. The Raman spectra of the pure hematein–Al(III) complexes were found to be consistent with those recorded for a logwood ink prepared following a late 19th century recipe, using logwood chips instead of pure hematein, and applied on a paper substrate. These spectra can be used as references for the noninvasive identification of the compounds in works of art. 27Al solid‐state NMR showed that the coordination of the Al(III) atoms in the crystallized powder is predominantly octahedral, while when applied on a paper substrate the colorant is present mainly as a tetrahedral complex, with an octahedral coordination also present in a smaller proportion. The fact that the predominant coordinations for the complexes in the crystallized material and for the ones present on the paper substrate are different is relevant for the study of the lightfastness and thermal stability of works of art bearing these media. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
280.
Paolo Manganotti Michele Acler Emanuela Formaggio Mirko Avesani Franco Milanese Andrea Baraldo Silvia Francesca Storti Anna Gasparini Roberto Cerini Roberto Pozzi Mucelli Antonio Fiaschi 《Magnetic resonance imaging》2010