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81.
We describe the construction and operation of a variable temperature (VT) system for a high field fast magic angle spinning (MAS) probe. The probe is used in NMR investigations of biological macromolecules, where stable setting and continuous measurement of the temperature over periods of several days are required in order to prevent sample overheating and degradation. The VT system described is used at and below room temperature. A vortex tube is used to provide cooling in the temperature range of -20 to 20 degrees C, while a liquid nitrogen-cooled heat exchanger is used below -20 degrees C. Using this arrangement, the lowest temperature that is practically achievable is -140 degrees C. Measurement of the air temperature near the spinning rotor is accomplished using a fiber optic thermometer that utilizes the temperature dependence of the absorption edge of GaAs. The absorption edge of GaAs also has a magnetic field dependence that we have measured and corrected for. This dependence was calibrated at several field strengths using the well-known temperature dependence of the (1)H chemical shift difference of the protons in methanol. 相似文献
82.
Karin Erdmann Miles Holloway Rachel Taillefer Nicole Snashall Øyvind Solberg 《K-Theory》2004,33(1):67-87
Support varieties for any finite dimensional algebra over a field were introduced in (Proc. London Math. Soc. 88 (3) (2004) 705–732) using graded subalgebras of the Hochschild cohomology ring. We mainly study these varieties for selfinjective algebras under appropriate finite generation hypotheses. Then many of the standard results from the theory of support varieties for finite groups generalize to this situation. In particular, the complexity of the module equals the dimension of its corresponding variety, all closed homogeneous varieties occur as the variety of some module, the variety of an indecomposable module is connected, the variety of periodic modules are lines and for symmetric algebras a generalization of Webbs theorem is true. As a corollary of a more general result we show that Webbs theorem generalizes to finite dimensional cocommutative Hopf algebras.Received November 2003Mathematics Subject Classifications (2000) Primary: 16E40, 16G10, 16P10, 16P20; Secondary: 16G70. 相似文献
83.
Meallet-Renault R Pansu R Amigoni-Gerbier S Larpent C 《Chemical communications (Cambridge, England)》2004,(20):2344-2345
A fluorescent sensor for Cu(2+) at the nanomolar level in water has been designed by associating a BODIPY fluorophore and a selective ligand (cyclam) in ultrafine polymer nanoparticles. 相似文献
84.
Carter EB Culver SL Fox PA Goode RD Ntai I Tickell MD Traylor RK Hoffman NW Davis JH 《Chemical communications (Cambridge, England)》2004,(6):630-631
The anions of the sweeteners saccharin and acesulfame form ionic liquids when paired with a variety of organic cations. 相似文献
85.
The order in a single layer of spherical domains of a block copolymer melt is investigated as a function of distance from the edges of the 15 microm wide, 30 nm deep wells that confine it along a substrate. At 255 degrees C the edge induces the formation of a hexatic phase whose orientational and translational order decreases slowly away from the edge until in the center of the well the block copolymer spheres have liquidlike order. 相似文献
86.
Botta M Aime S Barge A Bobba G Dickins RS Parker D Terreno E 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(9):2102-2109
The (1)H and (17)O NMR relaxometric properties of two cationic complexes formed by Gd(III) with a macrocyclic heptadentate triamide ligand, L(1), and its Nmethylated analogue, L(2), have been investigated in aqueous media as a function of pH, temperature and magnetic field strength. The complexes possess two water molecules in their inner coordination sphere for which the rate of exchange has been found to be sensibly faster for the Nmethylated derivative and explained in terms of electronic effects (decrease of the charge density at the metal center) and perturbation of the network of hydrogen-bonded water molecules in the outer hydration sphere. The proton relaxivity shows a marked dependence from pH and decreases of about six units in the pH range 6.5 to 9.0. This has been accounted for by the displacement of the two water molecules by dissolved carbonate which acts as a chelating anion. The formation of ternary complexes with lactate, malonate, citrate, acetate, fluoride and hydrogenphosphate has been monitored by (1)H NMR relaxometric titrations at 20 MHz and pH 6.3 and the value of the affinity constant, K, and of the relaxivity of the adducts could be obtained. Lactate, malonate and citrate interact strongly with the complexes (log K > or =3.7) and coordinate in a bidendate mode by displacing both water molecules. Larger affinity constants have been measured for GdL(2). Acetate, fluoride and hydrogenphosphate form monoaqua ternary complexes which were investigated in detail with regard to their relaxometric properties. The NMR dispersion (NMRD) profiles indicate a large contribution to the relaxivity of the adducts from water molecules belonging to the second hydration shell of the complexes and hydrogen-bonded to the anion. A VT (17)O NMR study has shown a marked increase of the rate of water exchange upon binding which is explained by coordination of the anion in an equatorial site, thus leaving the water molecule in an apical position, more accessible for interactions with the solvent molecules of the second hydration shell which facilitate the exchange process. 相似文献
87.
Eann A. Patterson Erwin Hack Philippe Brailly Richard L. Burguete Qasim Saleem Thorsten Siebert Rachel A. Tomlinson Maurice P. Whelan 《Optics and Lasers in Engineering》2007,45(5):550
The design and testing of a reference material for the calibration of optical systems for strain measurement is described, together with the design and testing of a standardized test material that allows the evaluation and assessment of fitness for purpose of the most sophisticated optical system for strain measurement. A classification system for the steps in the measurement process is also proposed and allows the development of a unified approach to diagnostic testing of components or sub-systems in an optical system for strain measurement based on any optical technique. The results described arise from a European study known as SPOTS whose objectives were to begin to fill the gap caused by a lack of standards. 相似文献
88.
Rachel Grainger Dr. Josep Cornella Dr. David C. Blakemore Dr. Igor Larrosa Dr. Josep M. Campanera 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(50):16680-16687
A combined experimental and computational investigation on the Ag‐catalysed decarboxylation of benzoic acids is reported herein. The present study demonstrates that a substituent at the ortho position exerts dual effects in the decarboxylation event. On one hand, ortho‐substituted benzoic acids are inherently destabilised starting materials compared to their meta‐ and para‐substituted counterparts. On the other hand, the presence of an ortho‐electron‐withdrawing group results in an additional stabilisation of the transition state. The combination of both effects results in an overall reduction of the activation energy barrier associated with the decarboxylation event. Furthermore, the Fujita–Nishioka linear free energy relationship model indicates that steric bulk of the substituent can also exert a negative effect by destabilising the transition state of decarboxylation. 相似文献
89.
Clayden J Turnbull R Helliwell M Pinto I 《Chemical communications (Cambridge, England)》2004,(21):2430-2431
Thiophene-3-carboxamides bearing allyl or benzyl substituents at nitrogen undergo dearomatising cyclisation on treatment with LDA. Rearrangements transform the dearomatised products into pyrrolinones, azepinones or partially saturated azepinothiophenes. 相似文献
90.
Simon G. Patching Rachel Edwards David A. Middleton 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2009,199(2):242-246
We demonstrate that individual H–C–C–H torsional angles in uniformly labelled organic solids can be estimated by selective excitation of 13C double-quantum coherences under magic-angle spinning at rotational resonance. By adapting a straightforward one-dimensional experiment described earlier [T. Karlsson, M. Eden, H. Luhman, M.H. Levitt, J. Magn. Reson. 145 (2000) 95–107], a double-quantum filtered spectrum selective for Cα and Cβ of uniformly labelled l-[13C,15N]valine is obtained with 25% efficiency. The evolution of Cα–Cβ double-quantum coherence under the influence of the dipolar fields of bonded protons is monitored to provide a value of the Hα–Cα–Cβ–Hβ torsional angle that is consistent with the crystal structure. In addition, double-quantum filtration selective for C6 and C1′ of uniformly labelled [13C,15N]uridine is achieved with 12% efficiency for a 13C–13C distance of 2.5 Å, yielding a reliable estimate of the C6–H and C1′–H projection angle defining the relative orientations of the nucleoside pyrimidine and ribose rings. This procedure will be useful, in favourable cases, for structural analysis of fully labelled small molecules such as receptor ligands that are not readily synthesised with labels placed selectively at structurally diagnostic sites. 相似文献