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951.
T Lund D D Wayner M Jonsson A G Larsen K Daasbjerg 《Journal of the American Chemical Society》2001,123(50):12590-12595
Oxidation potentials, E1/2(ox) of alpha-hydroxyalkyl radicals of the type R(1)R(2)C(*)OH (denoted K(1)H(*)) have been obtained in acetonitrile by photomodulated voltammetry. The values of E1/2(ox) increase as the R(1) and R(2) groups are changed from alkyl to aryl and, in particular, strong electron-withdrawing functionalities such as CN and CF3. Using rate data available in the literature for the pinacol photoexchange reaction K + K(1)H(*) --> KH(*) + K(1), it is found that as the difference in the standard potential of the ketone K, EK degrees and the oxidation potential of K(1)H(*), E1/2(ox), increases there is a modest increase in the exchange rate constant, k(ex). This indicates that even if some charge transfer may occur between the hydroxyalkyl radical and the ketone in the transition state, it is certainly not to the extent of a complete electron transfer. If the exchange reaction is treated as a simple hydrogen atom transfer process within the Marcus model, the intrinsic barrier is found to be 8-13 kcal mol(-1) due to the changes occurring in bonds, hybridizations, and bond angles. Finally, acid dissociation constants for K(1)H(*) are provided by means of a thermochemical cycle. 相似文献
952.
953.
Dr. Anne Ugleholdt Petersen Dr. Christina Kjær Cecilie Jensen Prof. Dr. Mogens Brøndsted Nielsen Prof. Dr. Steen Brøndsted Nielsen 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(47):21132-21141
While many key photophysical features are understood for electronic communication between chromophores in neutral compounds, there is limited information on the effect of charges in practically relevant ionic chromo/fluorophores. Here we have chosen positively charged rhodamines and prepared a selection of homo- and heterodimers with alkyl or π-conjugated, acetylenic bridges. Protonated molecules were transferred as isolated ions to gas phase where there is no solvent screening of charges, and fluorescence spectra were measured with a custom-made ion-trap setup. Our work reveals strong polarization of the π-spacer (induced dipole/quadrupole) when it experiences the electric field from one/ two dyes. Hence, π-spacers provide efficient shielding of charges by reducing the Coulomb interaction, whereas two dye cations polarize each other when connected by an alkyl. The screening influences the Förster resonance energy transfer efficiency that relies on the dipole–dipole interaction. 相似文献
954.
Dr. Dennis Larsen Prof. Dr. Sophie R. Beeren 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(48):11032-11038
Enzyme-mediated dynamic combinatorial chemistry combines the concept of thermodynamically controlled covalent self-assembly with the inherent biological relevance of enzymatic transformations. A system of interconverting cyclodextrins has been explored, in which the glycosidic linkage is rendered dynamic by the action of cyclodextrin glucanotransferase (CGTase). External factors, such as pH, temperature, solvent, and salinity are reported to modulate the composition of the dynamic cyclodextrin library. Dynamic libraries of cyclodextrins (CDs) could be obtained in wide ranges of pH (5.0–9.0), temperature (5–37 °C), and salinity (up to 7.5 m NaNO3), and with high organic solvent content (50 % by volume of ethanol), showing that enzyme-mediated dynamic systems can be robust and not limited to physiological conditions. Furthermore, it is demonstrated how strategic choice of reaction conditions can enhance template effects, in this case, to achieve highly selective production of α-CD, an otherwise challenging target due to competition from the structurally similar β-CD. 相似文献
955.
956.
Dr. Dymytrii Listunov Prof. Ole Hammerich Irving Caballero-Quintana Dr. Albert Poater Cécile Barthes Dr. Carine Duhayon Dr. Mie Højer Larsen Dr. José-Luis Maldonado Dr. Gabriel Ramos-Ortiz Prof. Mogens Brøndsted Nielsen Dr. Valérie Maraval Prof. Remi Chauvin 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(47):10707-10711
carbo-Benzene is an aromatic molecule devised by inserting C2 units within each C−C bond of the benzene molecule. By integrating the corresponding carbo-quinoid core as bridging unit in a π-extended tetrathiafulvalene (exTTF), it is shown that a carbo-benzene ring can be reversibly formed by electrochemical reduction or oxidation. The so-called carbo-exTTF molecule was thus experimentally prepared and studied by UV–visible absorption spectroscopy and cyclic voltammetry, as well as by X-ray crystallography and by scanning tunneling microscopy (STM) on a surface of highly oriented pyrolytic graphite (HOPG). The molecule and its oxidized and reduced forms were subjected to a computational study at the density functional theory (DFT) level, supporting carbo-aromaticity as a driving force for the formation of the dication, radical cation, and radical anion. By allowing co-planarity of the dithiolylidene rings and carbo-quinoidal core, carbo-exTTFs present a promising new class of redox-active systems. 相似文献
957.
Kramer MA Andrews B Hugar DL Jaitli A Larsen SB Kline BP McEllen KN Agrawal N Su SM Dammu SA Kammeyer RM Ivanisevic A 《Langmuir : the ACS journal of surfaces and colloids》2010,26(23):18083-18088
Fourier transform infrared (FT-IR) spectroscopy is utilized to observe adsorbate interactions with a tissue-derived collagen scaffold extracted from the Bruch's membrane of pig eyes. The characterization includes conformational changes in isoleucine, polyisoleucine, collagen-binding peptide, RGD-tagged collagen-binding peptide, and laminin after adsorption onto the substrate. Isotopically labeled isoleucine is further utilized to understand changes in the biomolecular structure upon binding to a tissue-derived surface. The adsorbates associated with the collagen scaffold predominately through hydrophobic interactions and hydrogen bonding. The results of this study can be used to improve our understanding of surface chemistry changes during the engineering of biomimetic scaffolds before and after biomolecule adsorption. 相似文献
958.
Neeraj Sharma René B. Macquart Mogens Christensen Yu-Sheng Chen 《Journal of solid state chemistry》2009,182(6):1312-4494
The floating-zone furnace method was used to synthesize single crystals of the fluorite-related δ-Bi2O3-type phase Bi38Mo7O78 for the first time. Single crystal synchrotron X-ray diffraction data, in conjunction with ab initio (density functional theory) calculations, were used to solve, optimize, and refine the 5×3×3 commensurate superstructure of fluorite-type δ-Bi2O3 in Pbcn (a=28.7058(11) Å, b=16.8493(7) Å and c=16.9376(6) Å, Z=4, RF=11.26%, wRI=21.67%). The structure contains stepped channels of Mo6+ in tetrahedral environments along the b axis and chains of Mo6+ in octahedral environments along the ac plane. The role of the stepped channels in oxide ion conduction is discussed. The simultaneous presence of both tetrahedral and octahedral coordination environments for Mo6+, something not previously observed in Mo6+-doped δ-Bi2O3-type phases, is supported by charge balance considerations in addition to the results of crystallographic and ab initio analysis. 相似文献
959.
Mikkel Juul Larsen Yue Ma Peter B. Lund Eivind M. Skou 《Applied Physics A: Materials Science & Processing》2009,96(3):569-573
In order to develop cheaper and better fuel-cell electrolyte membranes than the polyperfluorosulfonic acids, an effort has
been made to improve the fuel-cell relevant properties of sulfonated styrene/divinylbenzene-grafted poly(ethylene-alt-tetrafluoroethylene) membranes. Thus the influence of the crosslinking agent divinylbenzene has been investigated and its
amount optimized. Substitution of styrene by methylstyrene and t-butylstyrene has been performed with the purpose of improving the chemical stability of the membranes. Grafting with a fraction
of divinylbenzene in the order of 1–2 vol-% of the total monomers has been found to be the best compromise between high grafting
yield, good chemical stability, and high proton conductivity of the final membrane. The use of methylstyrene and t-butylstyrene as grafting monomers instead of styrene results in substantially increased chemical stability, with reasonable
proton conductivity still being possible to obtain. 相似文献
960.