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排序方式: 共有193条查询结果,搜索用时 31 毫秒
181.
Isonitrile derivatives of crosslinked polyacrylamide beads (Biogell P-100) were prepared by a two-step procedure: a. N-hydroxymethylation
(methylolation) of amide groups on the polymer by treatment with formaldehyde; and b. Attachment of side chains, containing
isonitrile functional groups by a displacement reaction involving 1-tosyloxy-3-isocyanopropane (p-CH3-C6H4·SO2·O·(CH2)3 NC) and alkoxide ions generated on methylolated polyacrylamide by treatment with a strong base in a polar aprotic solvent.
The modified polyacrylamide beads were tested as support for the immobilization of proteins, and low mol wt ligands by four
component condensation (4CC) reactions.
Trypsin-polyacrylamide acting on N-benzoyl-L-arginine ethylester exhibited nonlinear Michaelis Menten kinetics and distorted
pH activity profiles. The kinetic anomalies could be reduced by increasing the concentration of buffer. The data were consistent
with a model assuming “buffer facilitated proton transport” in a diffusionally constrained system. 相似文献
182.
Combinations of alkylated polyethylenimine and polyoxometalates yield water-soluble synzymes with hydrophobic regions that allowed the aqueous biphasic selective oxidation of very hydrophobic, water-insoluble substrates with hydrogen peroxide. With the alkylated PEI/{PO4[WO(O2)2]4}3- highly effective C-C bond cleavage of alkenes to aldheydes was observed. The synzymes have both tertiary and quaternary amine centers as shown by a combination of 15N-1H HMBC and XPS measurements. The existence of hydrophobic regions was concluded from the measurement of contact angles and a hypsochromic shift of a fluorescent probe. 相似文献
183.
Cecillon S Lazar A Danylyuk O Suwinska K Rather B Zaworotko MJ Coleman AW 《Chemical communications (Cambridge, England)》2005,(19):2442-2444
A calix[4]arene functionalized at one phenolic group with a pendant ethoxy acetate group, forms an inclusion complex that is stable even in the presence of other potential guest molecules. 相似文献
184.
Gorteau V Perret F Bollot G Mareda J Lazar AN Coleman AW Tran DH Sakai N Matile S 《Journal of the American Chemical Society》2004,126(42):13592-13593
Design, synthesis, and multifunctionality of p-octiphenyl beta-barrel pores with external LRL triads and internal HH dyads are described. Molecular recognition of anionic fullerenes > calixarenes > pyrenes by guanidinium arrays at the outer pore surface is shown to result in pore opening, whereas alpha-helix recognition within the topologically matching internal space is shown to result in noncompetitive pore blockage. This experimental evidence for multifunctionality is supported by comparison with pertinent control pores and blockers, by structural studies using FRET from p-octiphenyl donors in the pore to BODIPY acceptors in the bilayer, and by molecular mechanics simulations. Practical usefulness of ligand-gated synthetic multifunctional pores is exemplified with the continuous detection of chemical processes. 相似文献
185.
A quantitative method for the analysis of EPR spectra from dinuclear Mn(II) complexes is presented. The complex [(Me(3)TACN)(2)Mn(II)(2)(mu-OAc)(3)]BPh(4) (1) (Me(3)TACN=N, N('),N(")-trimethyl-1,4,7-triazacyclononane; OAc=acetate(1-); BPh(4)=tetraphenylborate(1-)) was studied with EPR spectroscopy at X- and Q-band frequencies, for both perpendicular and parallel polarizations of the microwave field, and with variable temperature (2-50K). Complex 1 is an antiferromagnetically coupled dimer which shows signals from all excited spin manifolds, S=1 to 5. The spectra were simulated with diagonalization of the full spin Hamiltonian which includes the Zeeman and zero-field splittings of the individual manganese sites within the dimer, the exchange and dipolar coupling between the two manganese sites of the dimer, and the nuclear hyperfine coupling for each manganese ion. All possible transitions for all spin manifolds were simulated, with the intensities determined from the calculated probability of each transition. In addition, the non-uniform broadening of all resonances was quantitatively predicted using a lineshape model based on D- and r-strain. As the temperature is increased from 2K, an 11-line hyperfine pattern characteristic of dinuclear Mn(II) is first observed from the S=3 manifold. D- and r-strain are the dominate broadening effects that determine where the hyperfine pattern will be resolved. A single unique parameter set was found to simulate all spectra arising for all temperatures, microwave frequencies, and microwave modes. The simulations are quantitative, allowing for the first time the determination of species concentrations directly from EPR spectra. Thus, this work describes the first method for the quantitative characterization of EPR spectra of dinuclear manganese centers in model complexes and proteins. The exchange coupling parameter J for complex 1 was determined (J=-1.5+/-0.3 cm(-1); H(ex)=-2JS(1).S(2)) and found to be in agreement with a previous determination from magnetization. The phenomenon of exchange striction was found to be insignificant for 1. 相似文献
186.
Teodora Cătinaş 《Mediterranean Journal of Mathematics》2014,11(4):1171-1183
We introduce an improved bivariate thin-plate spline quasi-interpolation operator obtained by means of Bernoulli bivariate operator. We study this combined operator and give some error bounds in terms of the modulus of continuity of high order and also with Peano’s theorem. Finally, we make extensive comparison with other existing methods and give some numerical examples. 相似文献
187.
Morozan A Jégou P Jousselme B Palacin S 《Physical chemistry chemical physics : PCCP》2011,13(48):21600-21607
One of the major limitations yet to the global implementation of polymer electrolyte membrane fuel cells (PEMFCs) is the cathode catalyst. The development of efficient platinum-free catalysts is the key issue to solve the problem of slow kinetics of the oxygen reduction reaction (ORR) and high cost. We report a promising catalyst for ORR prepared through the annealing treatment under inert conditions of the cobalt-benzotriazole (Co-BTA) complex supported on carbon nanotubes (CNTs). The N-rich benzotriazole precursor was chosen based on its ability to complex Co(II) ions and generate under annealing highly reactive radicals able to tune the physicochemical properties of CNTs. X-Ray photoelectron spectroscopy (XPS) was used to follow the surface structure changes and highlight the active electrocatalytic sites towards the ORR. To achieve further evaluation of the catalysts in acidic medium, voltamperometry, rotating disk electrode (RDE), rotating ring-disk electrode (RRDE) and half-cell measurements were performed. The resulting catalysts (Co/N/CNTs) all show catalytic activity towards the ORR, the most active one resulting from annealing at 700 °C. The overall electron transfer number for the catalyzed ORR was determined to be ~3.7 with no change upon the catalyst loading, suggesting that the ORR was dominated by a 4e(-) transfer process. The results indicate a promising alternative cathode catalyst for ORR in fuel cells, although its performance is still lower (overpotential around 110 mV evaluated by RDE and RRDE) than the reference Pt/C catalyst. 相似文献
188.
Catalin Borcia Karin Popa Claudiu C. Pavel Adina Dascălu Cristina Viţelaru Bogdan A. Apetrăchioaei 《Journal of Radioanalytical and Nuclear Chemistry》2011,288(1):25-30
The sorption of the thallous ion from aqueous acidic solution (pH = 1.5) onto as-synthesized and modified ETS-10 titanosilicates
was studied by using an isotope dilution method and a batch-mode technique. The present results show that the thallium(I)
sorption was effective onto all three considered materials and is enhanced by the porosity and acidity modification of the
ETS-10 titanosilicate. The best uptake performance was achieved by the meso-ETS-10. This behavior is explained based on the
newly created additional mesoporous system and enriching the external surface with silanol groups. Also, the presence of phosphorus
enhanced the inherent porosity allowing thus better internal diffusion properties of crystalline material. However, the chemically
modified surface seems to have a negative contribution to the kinetic uptake of thallous ion as shown by the positive value
of the activation energy E
a, in comparison with the processes more favorable energetically for ETS-10 and meso-ETS-10 materials. 相似文献
189.
Wang BY Turner DA Zujović T Hadad CM Badjić JD 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(32):8870-8881
Cup‐shaped porphyrin 1 a has four norbornane rings for encircling space and this type of host could be of interest in supramolecular and catalytic chemistry. We used 1H NMR spectroscopy to investigate the acid‐catalyzed (pTsOH in CHCl3 and TFA in CH3CN) condensation of racemic, enantioenriched (80–85 % enantiomeric excess (ee)), and enantiopure (99 % ee) pyrromethanecarbinol 7 into 1 a . We found that the oligomerization of racemic 7 rac would give 1 a–d in the ratio different from the statistical one, though a minuscule quantity of 1 a (<5 %) formed. The oligomerization of enantioenriched 7 (80–85 % ee), however, led to the formation of greater amounts of 1 a (31–47 %) along with other stereoisomers 1 b–d . Importantly, pTsOH catalyzed the conversion of enantiopure 7 (99 % ee) into 1 a (>95 % diastereomeric excess (de), 25 % overall yield) in CHCl3 although prolonged reaction times or greater concentration of the catalytic acid gave rise to 1 b–d at the expense of 1 a . The metallation of 1 a with Zn(OAc)2 led to the formation of ZnII‐ 1 a and we used computational (DFT: RI‐BP86/SV(P),TZVP) and experimental (1H NMR spectroscopy) methods to study the partitioning of smaller N‐methylimidazole 13 (94 Å3) and bigger 1,5‐dicyclohexylimidazole 14 (268 Å3) between the inner and outer side of the host. We found that bigger 14 was mostly encapsulated (90 %) inside ZnII‐ 1 a at 298.0 K, whereas smaller 13 would equally partition between the two sides of the host. Furthermore, the out/in equilibrium was, in the case of 14 ‐ZnII‐ 1 a , favored by entropy (TΔS°out/in=3.5±0.1 kcal mol?1) indicating that perhaps differential solvation of the coordinated ligand assisted the encapsulation. 相似文献
190.
Katarina avikin Jelena
ivkovi Teodora Jankovi Nada uji-Nikoli Gordana Zduni Neboja Menkovi Zorica Drini 《Molecules (Basel, Switzerland)》2021,26(13)
In this study we define the optimal conditions for ultrasound-assisted extraction of bioactive polyphenols from S. raeseri aerial parts using response surface methodology. The influence of ethanol concentration (10–90%), extraction temperature (20–80 °C), extraction time (10–60 min), and solid-to-solvent ratio (1:10–1:50) on total phenolic content as well as on content of individual flavonoids, and hypolaetin and isoscutellarein derivatives was studied. For the experimental design, a central composite design was chosen. In the obtained extracts, the following ranges of targeted compounds were detected: total phenol from 19.32 to 47.23 mg GAE/g dw, HYP from 1.05 to 11.46 mg/g dw, ISC 1 from 0.68 to 10.68 mg/g dw, and ISC 2 from 0.74 to 15.56 mg/g dw. The optimal extraction conditions were set as: ethanol concentration of 65%, extraction time of 50 min, extraction temperature of 63 °C, and solid-to-solvent ratio of 1:40. Contents of TP, HYP, ISC 1, and ISC 2 in optimal extracts were 47.11 mg GAE/g dw, 11.73 mg/g dw, 9.54 mg/g dw, and 15.40 mg/g dw, respectively. Experimentally set values were in good agreement with those predicted by the response surface methodology model, indicating suitability of the used model, as well as the success of response surface methodology in optimizing the conditions of the extraction. 相似文献