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991.
Falvey P Lim CW Darcy R Revermann T Karst U Giesbers M Marcelis AT Lazar A Coleman AW Reinhoudt DN Ravoo BJ 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(4):1171-1180
A family of amphiphilic cyclodextrins (6, 7) has been prepared through 6-S-alkylation (alkyl=n-dodecyl and n-hexadecyl) of the primary side and 2-O-PEGylation of the secondary side of alpha-, beta-, and gamma-cyclodextrins (PEG=poly(ethylene glycol)). These cyclodextrins form nonionic bilayer vesicles in aqueous solution. The bilayer vesicles were characterized by transmission electron microscopy, dynamic light scattering, dye encapsulation, and capillary electrophoresis. The molecular packing of the amphiphilic cyclodextrins was investigated by using small-angle X-ray diffraction of bilayers deposited on glass and pressure-area isotherms obtained from Langmuir monolayers on the air-water interface. The bilayer thickness is dependent on the chain length, whereas the average molecular surface area scales with the cyclodextrin ring size. The alkyl chains of the cyclodextrins in the bilayer are deeply interdigitated. Molecular recognition of a hydrophobic anion (adamantane carboxylate) by the cyclodextrin vesicles was investigated by using capillary electrophoresis, thereby exploiting the increase in electrophoretic mobility that occurs when the hydrophobic anions bind to the nonionic cyclodextrin vesicles. It was found that in spite of the presence of oligo(ethylene glycol) substituents, the beta-cyclodextrin vesicles retain their characteristic affinity for adamantane carboxylate (association constant K(a)=7.1 x 10(3) M(-1)), whereas gamma-cyclodextrin vesicles have less affinity (K(a)=3.2 x 10(3) M(-1)), and alpha-cyclodextrin or non-cyclodextrin, nonionic vesicles have very little affinity (K(a) approximately 100 M(-1)). Specific binding of the adamantane carboxylate to beta-cyclodextrin vesicles was also evident in competition experiments with beta-cyclodextrin in solution. Hence, the cyclodextrin vesicles can function as host bilayer membranes that recognize small guest molecules by specific noncovalent interaction. 相似文献
992.
Tobias Schmidt 《Journal of solid state chemistry》2003,173(2):259-272
The title compounds were isolated in well-crystallized form from samples with a substantial excess of antimony, annealed at temperatures slightly below the melting point of that element. Their crystal structures were determined from single-crystal diffractometer data. Pr9-xSb21-y and Nd9-xSb21-y crystallize with a new monoclinic structure type, Pearson symbol mS(62-5.4), space group Cm, Z=2 with a=2859.1(4) pm, b=426.3(1) pm, c=1356.1(2) pm, β=95.52(1)°, R=0.034 for 4351 structure factors and 188 variable parameters for Pr9-xSb21-y and a=2845(2) pm, b=424.7(8) pm, c=1345.9(9) pm, β=95.42(7)°, R=0.069 for 2928 F values and 188 variables for Nd9-xSb21-y. Of the 30 atomic sites, three show fractional occupancy corresponding to the compositions Pr8.303(5)Sb20.03(1) and Nd8.30(2)Sb19.98(9), respectively. A model for the order of occupied atomic sites with a tripled b-axis is proposed resulting in the ideal compositions Pr5Sb12 and Nd5Sb12. The holmium compound Ho2Sb5 has a Dy2Sb5-type structure: mP28, P21/m, a=1301.8(3) pm, b=414.9(1) pm, c=1451.1(2) pm, β=102.14(1)°, R=0.028 for 2573 F values and 86 variables. In both structure types most rare earth atoms have nine antimony neighbors forming tricapped trigonal prisms. The coordination polyhedra of the antimony atoms show a great variety, with a trigonal prism of rare earth atoms as one extreme case. The other extreme coordination of an antimony atom is a distorted octahedron formed by six antimony atoms. The differences and similarities of both structures are discussed. Chemical bonding within the antimony polyanions is analyzed on the basis of an extended Zintl-Klemm concept using bond-length-bond-strength relationships. 相似文献
993.
Reaction of enolates derived from esters and ketones to an easily prepared alkylidene[1,3]dithiane-1,3-dioxide afforded the respective adducts with good yields and selectivities generally exceeding 85:15. The base used for enolate addition played no significant role for the reaction outcome, and addition of a silyl enole ether gave similar results. The thus formed oxygenated S,S-acetals were transformed into the corresponding 1,4-dicarbonyls by a reduction/oxidation sequence with 84% yield. [reaction: see text] 相似文献
994.
Hong BH Lee JY Beetz T Zhu Y Kim P Kim KS 《Journal of the American Chemical Society》2005,127(44):15336-15337
We report the growth of ultralong (>10 cm) multi-walled and single-walled carbon nanotubes such that the length is limited by the size of the furnace rather than by the termination of growth. The disturbance of microscale laminar flows results in disordered or shorter growth of carbon nanotubes. By downsizing reaction pipes, reaction gas flows are stabilized with low Reynolds numbers. In this way, the catalyst nanoparticles at the end of growing carbon nanotubes can travel a longer distance to grow ultralong nanotubes. 相似文献
995.
996.
For a monotonically advancing front, the arrival time is the time when the front reaches a given point. We show that it is twice differentiable everywhere with uniformly bounded second derivative. It is smooth away from the critical points where the equation is degenerate. We also show that the critical set has finite codimensional 2 Hausdorff measure. For a monotonically advancing front, the arrival time is equivalent to the level set method, a~priori not even differentiable but only satisfying the equation in the viscosity sense . Using that it is twice differentiable and that we can identify the Hessian at critical points, we show that it satisfies the equation in the classical sense. The arrival time has a game theoretic interpretation. For the linear heat equation, there is a game theoretic interpretation that relates to Black‐Scholes option pricing. From variations of the Sard and ?ojasiewicz theorems, we relate differentiability to whether singularities all occur at only finitely many times for flows.© 2016 Wiley Periodicals, Inc. 相似文献
997.
Small Cause,Great Impact: Modification of the Guanidine Group in the RGD Motif Controls Integrin Subtype Selectivity 下载免费PDF全文
Tobias G. Kapp Maximilian Fottner Dr. Oleg V. Maltsev Prof. Dr. Horst Kessler 《Angewandte Chemie (International ed. in English)》2016,55(4):1540-1543
Due to its unique role as a hydrogen‐bond donor and its positive charge, the guanidine group is an important pharmacophoric group and often used in synthetic ligands. The chemical modification of the guanidine group is often considered to destroy its function. Herein, we show that the N‐methylation, N‐alkylation, or N‐acylation of the guanidine group can be used to modify the receptor subtype specificity of the integrin ligand cilengitide. Using the αvβ6/α5β1‐biselective ligand c(isoDGRkphg) and the αvβ6‐specific ligand c(FRGDLAFp(NMe)K(Ac) as examples, we show that the binding affinities of the ligands can be fine‐tuned by this method to enhance the selectivity for αvβ6. Furthermore, we describe a new strategy for the functionalization of integrin ligands. By introducing longer N‐alkylguanidine and N‐acylguanidine groups, we are able to simultaneously identify a hitherto unknown anchoring point and enhance the subtype selectivity of the ligand. 相似文献
998.
Frustrated Lewis Pair Catalyzed Dehydrogenative Oxidation of Indolines and Other Heterocycles 下载免费PDF全文
Dipl.‐Chem. Alexander F. G. Maier Dipl.‐Chem. Sebastian Tussing M. Sc. Tobias Schneider Dr. Ulrich Flörke Dr. Zheng‐Wang Qu Prof. Dr. Stefan Grimme Prof. Dr. Jan Paradies 《Angewandte Chemie (International ed. in English)》2016,55(40):12219-12223
An acceptorless dehydrogenation of heterocycles catalyzed by frustrated Lewis pairs (FLPs) was developed. Oxidation with concomitant liberation of molecular hydrogen proceeded in high to excellent yields for N‐protected indolines as well as four other substrate classes. The mechanism of this unprecedented FLP‐catalyzed reaction was investigated by mechanistic studies, characterization of reaction intermediates by NMR spectroscopy and X‐ray crystal analysis, and by quantum‐mechanical calculations. Hydrogen liberation from the ammonium hydridoborate intermediate is the rate‐determining step of the oxidation. The addition of a weaker Lewis acid as a hydride shuttle increased the reaction rate by a factor of 2.28 through a second catalytic cycle. 相似文献
999.
Monofluorination and Trifluoromethylation of BODIPY Dyes for Prolonged Single‐Molecule Detection
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Anh Minh Huynh Johannes Menges Michael Vester Tobias Dier Dr. Volker Huch Prof. Dr. Dietrich A. Volmer Prof. Dr. Gregor Jung 《Chemphyschem》2016,17(3):433-442
Electrophilic monofluorination with Selectfluor and nucleophilic trifluoromethylation with the Ruppert–Prakesh reagent of dimethyl‐, tetramethyl‐ and pentamethyl‐substituted boron dipyrromethenes (BODIPY) are investigated. Monofluorinated dyes are synthesized with low yields (<30 %), however trifluoromethyl derivatives are obtained in moderate to high yields (≈40–90 %). All compounds are characterized by steady‐state and time‐resolved fluorescence spectroscopy, the photostability is investigated with fluorescence correlation spectroscopy (FCS) and total internal reflection fluorescence microscopy (TIRF). Monofluorination hardly affects the spectroscopic parameters of the unsubstituted parent compounds, but distinctly enhances the photostability, whereas trifluoromethylation leads to a hypsochromic shift by up to 17 nm in both absorption and emission, slightly enhanced intersystem crossing, and higher photostability. Further development of soft fluorination and trifluoromethylation methods is therefore highly desired. 相似文献
1000.
Ian Ken Dimzon Xenia Trier Tobias Frömel Rick Helmus Thomas P. Knepper Pim de Voogt 《Journal of the American Society for Mass Spectrometry》2016,27(2):309-318
High resolution mass spectrometry (HRMS) was successfully applied to elucidate the structure of a polyfluorinated polyether (PFPE)-based formulation. The mass spectrum generated from direct injection into the MS was examined by identifying the different repeating units manually and with the aid of an instrument data processor. Highly accurate mass spectral data enabled the calculation of higher-order mass defects. The different plots of MW and the nth-order mass defects (up to n = 3) could aid in assessing the structure of the different repeating units and estimating their absolute and relative number per molecule. The three major repeating units were -C2H4O-, -C2F4O-, and -CF2O-. Tandem MS was used to identify the end groups that appeared to be phosphates, as well as the possible distribution of the repeating units. Reversed-phase HPLC separated of the polymer molecules on the basis of number of nonpolar repeating units. The elucidated structure resembles the structure in the published manufacturer technical data. This analytical approach to the characterization of a PFPE-based formulation can serve as a guide in analyzing not just other PFPE-based formulations but also other fluorinated and non-fluorinated polymers. The information from MS is essential in studying the physico-chemical properties of PFPEs and can help in assessing the risks they pose to the environment and to human health. 相似文献