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481.
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483.
Peter H. Plesch James C. Austin 《Journal of polymer science. Part A, Polymer chemistry》2008,46(13):4265-4284
The content of this article is indicated by what could be its full title: “An Explanation of the dependence of the rate of the cationic polymerizations of alkenes and of the DP of their products, on the reaction variables, especially the size of the anionic moiety of the initiator.” We continue here the discussion started in 1965 and show mathematically how the theory of dieidic polymerizations by unpaired and paired cations can explain why some of these polymerizations become faster with falling temperature, why the Arrhenius plot of the DP of the polymers obtained from most such systems shows a discontinuity or kink, and also how the temperature of minimum rate, TM, and that at which the kink occurs, TK, depend on the reaction variables, namely the concentrations of monomer, m, and of initiator, c, and the a, D, and T (interionic distance in the ion‐pair, dielectric constant of the reaction mixture and temperature). Our treatment explains why the most effective way of achieving the economically desirable aim, to make the longest polymers at the highest possible temperatures, is by maximizing the product a.D, so as to increase the TK, preferably by the use of polar solvents and initiators with large anions. The choice of such combinations by several investigators, but for other, vaguer, reasons, is given here a theoretical basis. Our argument is illustrated by Literature examples and is presented in the form of a new diagram (the Plesch‐Austin plot) which shows the TK as a function of a.D for several systems. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4265–4284, 2008 相似文献
484.
L.M. Lund P.M.L. Sandercock G.J. Basara C.C. Austin 《Journal of Analytical and Applied Pyrolysis》2008,82(1):129-133
The precision and long-term stability of pyrolysis probe set-point temperature calibration of a commercially available coiled-filament pyrolyzer were assessed for a variety of polymers, including Kraton® D1107, high-density polyethylene (HDPE), and low-density polyethylene (LDPE). While plots of peak area ratios for Kraton® and HDPE versus pyrolysis set-point temperatures produced statistically significant linear curves at the 95% confidence level, poor precision was observed at each of the set-point temperatures. Plots of peak area ratios for LDPE, in particular for n-C16 alkyldiene/n-C16 alkene peak area ratios, also exhibited good linearity but showed significant improvements in precision at each set-point temperature. In addition, replicate analysis over a 10-month period of peak area ratios for polymers pyrolyzed at a set-point temperature of 900 °C confirmed the improved method precision obtained from pyrolysis of LDPE and analysis of the n-C16 alkyldiene/n-C16 alkene ratio when compared to the precision obtained from pyrolysis of Kraton® D1107 or high-density polyethylene. 相似文献
485.
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487.
B J Brazeau R N Austin C Tarr J T Groves J D Lipscomb 《Journal of the American Chemical Society》2001,123(48):11831-11837
Norcarane is a valuable mechanistic probe for enzyme-catalyzed hydrocarbon oxidation reactions because different products or product distributions result from concerted, radical, and cation based reactions. Soluble methane monooxygenase (sMMO) from Methylosinus trichosporium OB3b catalyzes the oxidation of norcarane to afford 3-hydroxymethylcyclohexene and 3-cycloheptenol, compounds characteristic of radical and cationic intermediates, respectively, in addition to 2- and 3-norcaranols. Past single turnover transient kinetic studies have identified several optically distinct intermediates from the catalytic cycle of the hydroxylase component of sMMO. Thus, the reaction between norcarane and key reaction intermediates can be directly monitored. The presence of norcarane increases the rate of decay of only one intermediate, the high-valent bis-mu-oxo Fe(IV)(2) cluster-containing species compound Q, showing that it is responsible for the majority of the oxidation chemistry. The observation of products from both radical and cationic intermediates from norcarane oxidation catalyzed by sMMO is consistent with a mechanism in which an initial substrate radical intermediate is formed by hydrogen atom abstraction. This intermediate then undergoes either oxygen rebound, intramolecular rearrangement followed by oxygen rebound, or loss of a second electron to yield a cationic intermediate to which OH(-) is transferred. The estimated lower limit of 20 ps for the lifetime of the putative radical intermediate is in accord with values determined from previous studies of sterically hindered sMMO probes. 相似文献
488.
Andrei Blasko Alana Leahy-Dios William O. Nelson Steven A. Austin Robert B. Killion Gary C. Visor Ian J. Massey 《Monatshefte für Chemie / Chemical Monthly》2001,132(7):789-798
Summary. The kinetic and thermodynamic solubilities of Roche (Ro) pharmaceutical compounds were determined by HPLC, titrimetry, and UV/Vis spectroscopy in aqueous buffers and in non-buffered
systems. For kinetic solubility, a turbidimetric method that allows the rapid determination of solubilities using small amounts
of compounds (5–50 mg) was used. Two types of precipitation were observed during the kinetic solubility determinations: i) a disperse precipitation where the solution became foggy with very small particles uniformly distributed in the solution,
and ii) discrete precipitation characterized by formation of crystals that rapidly sediment. The thermodynamic solubility was determined
by shake flask and titrimetrically using a pH-STAT. The pH-STAT titrimetric method for the pH-thermodynamic solubility profile determination eliminates the buffer species and represents a new way to approach the solubility
characterization of pharmaceutical compounds. The strengths of the turbidimetric method for determining the kinetic solubility
are its rapidity, minimal compound requirements, and suitability for high throughput screening. The limitations are that the
maximum solubility is limited to less than 100 mg · cm−3, and the precipitation of trace impurities cannot be distinguished from precipitation of the analyte. The pH-STAT titrimetric approach for the thermodynamic solubility has a lower throughput and is suitable for the characterization
of the lead candidate. It is not limited in its solubility range and provides a common basis for the comparison of the solubility
values at different pH values in contrast to traditional buffered systems.
Received August 21, 2000. Accepted (revised) February 5, 2001 相似文献
489.
Austin P. Lanquist Sayak Gupta Kathlyn F. Al-Afyouni Malik Al-Afyouni Jeremy J. Kodanko Claudia Turro 《Chemical science》2021,12(36):12056
A series of five ruthenium complexes containing triphenyl phosphine groups known to enhance both cellular penetration and photoinduced ligand exchange, cis-[Ru(bpy)2(P(p-R-Ph)3)(CH3CN)]2+, where bpy = 2,2′-bipyridine and P(p-R-Ph)3 represent para-substituted triphenylphosphine ligands with R = –OCH3 (1), –CH3 (2) –H (3), –F (4), and –CF3 (5), were synthesized and characterized. The photolysis of 1–5 in water with visible light (λirr ≥ 395 nm) results in the substitution of the coordinated acetonitrile with a solvent molecule, generating the corresponding aqua complex as the single photoproduct. A 3-fold variation in quantum yield was measured with 400 nm irradiation, Φ400, where 1 is the most efficient with a Φ400 = 0.076(2), and 5 the least photoactive complex, with Φ400 = 0.026(2). This trend is unexpected based on the red-shifted metal-to-ligand charge transfer (MLCT) absorption of 1 as compared to that of 5, but can be correlated to the substituent Hammett para parameters and pKa values of the ancillary phosphine ligands. Complexes 1–5 are not toxic towards the triple negative breast cancer cell line MDA-MB-231 in the dark, but 3 and 5 are >4.2 and >19-fold more cytotoxic upon irradiation with blue light, respectively. A number of experiments point to apoptosis, and not to necrosis or necroptosis, as the mechanism of cell death by 5 upon irradiation. These findings provide a foundation for understanding the role of phosphine ligands on photoinduced ligand substitution and show the enhancement afforded by –CF3 groups on photochemotherapy, which will aid the future design of photocages for photochemotherapeutic drug delivery.Ru(ii) complexes exhibit photoinduced exchange of coordinated CH3CN and photocytotoxicity against breast cancer cells highly dependent on the substituents of the ancillary triphenylphospine ligand. 相似文献
490.
In this paper attempts have been made to prepare superionic glassy electrolytes in the mixed system 30(Cu(1−x)AgxI)–46.66(Ag2O)–23.33(P2O5), where x=0.05, 0.1, 0.15, 0.2 and 0.25, respectively, using the melt quenching process. The solid samples were characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC), Fourier transform infrared (FT-IR) spectroscopy and silver ionic transport number studies. XRD analysis and FT-IR spectroscopic results have provided details regarding the various phases present in the new system and also indicated the formation of composite materials consisting of glassy and crystalline phases. The transport number measurements have indicated the formation of superionic solids having Ag+ ions as the mobile species in the present system. 相似文献