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1INTRODUCTIONInrecentyears,ourresearchgrouphasstudiedsystematicalycycloadditionreac tionsoftheC=Ndoublebondsinthebenzoheteroa... 相似文献
93.
Gd,-.Na.CuO#, an interesting compound with complex magnetic behavior, has beensynthesized by a PbO-based flux method (l) and a solid state reaction (2) at temperaturesabout 1000"C. Its X-ray diffraction pattern has been indexed with tetragonal latticeparameters (l,3). Here we report a low temperature synthetic route in which the newmonoclinic form of Gd,-.Na.CuO# is obtained directly by precipitation from a moltenKOH/NaOH/KNO, solution. The SEM, EDX, XRD, XPS and magnetic characte… 相似文献
94.
A series of novel cis-4-substituted proline analogues were designed and synthesized. Highly stereoselective alkylations at the γ-position of glutamic ester 2 were achieved, followed by reduction, mesylation, and cyclization to afford the title compounds 1 in good yields and high diastereoselectivity. 相似文献
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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
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Geometry optimisation and metal ion affinities (MIAs) of the binding configurations of Be2+, Mg2+ and Ca2+ to L ‐proline were calculated using the hybrid Density Functional Theory (DFT‐B3LYP) and second order Møllet?Plesset perturbation theory (MP2) methods. Be2+ was found to bind preferentially in a charge transfer type arrangement through the carbonyl oxygen (? C?O) and the lone pair of the imino‐group nitrogen atom (? NH? ). On the contrary Mg2+ and Ca2+ were found to prefer binding in a bi‐dentate manner through the carboxylate group of L ‐proline (OCO) in a zwitterion form. The main types of interactions found to influence the binding preference of M2+ ions to L ‐proline were (i) charge transfer in the case of Be2+ ions and (ii) electrostatic interactions in the case of Mg2+ and Ca2+ ions. Inspection of the IR stretching of the N? H and the O? H groups of L ‐proline with M2+ ions in a chelating configuration (to both O and N atoms) indicated a considerable shift to higher frequency with decreasing MIA. On the other hand, the MIA for the zwitterion L ‐proline with M2+ tracks the reciprocal distance of the M2+? OCO bond further confirming that the nature of the bond is mainly electrostatic. Comparison with other molecules containing the carboxylic function is also included in order to gain more insight on the types of interaction of this amino acid with metal ions in general. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
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Results of classical molecular dynamics simulations are presented for the re-orientational dynamics of water hydrogen bonded to perchlorate anion. Different mechanisms of bond formation are presented. Due to its regular tetrahedron geometry the perchlorate anion can make classical as well as bifurcated and trifurcated hydrogen bonds. The angular variation of water in the first solvation shell of perchlorate suggests the transitional character of multi-centred hydrogen bonds. As a result water molecules can slide around the anion. 相似文献