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  • Wiley-Blackwell  (2,393)
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  • 1
    ISSN: 0192-8651
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: The relative stabilities of the keto and enol forms [Δ E0 (enol-keto)] and the energy barriers to enolization of the keto forms [Δ E≠ (transition state-keto)] for CH3COR (R = CH3, H, F, and CN) and CH3CHY (Y = CH2, NH, and S) are investigated theoretically by Hartree-Fock and Möoller-Plesset second-order calculations with 6-31G** basis sets. Specific and bulk solvent effects are considered by incorporating one water molecule and applying the self-consistent reaction field (SCRF) method to the reaction system, respectively. The Δ E0MP2 values are all positive, in agreement with the lower stability of the enol form in the gas phase as well as in solution. In contrast to a relatively small effect of specific as well as bulk solvation on Δ E0, there is a large lowering of Δ E≠ (by ca. 30 kcal/mol) when solvent effects are accounted for. In general, both Δ E0 and Δ E≠ are depressed in solution and hence enolization is favored thermodynamically as well as kinetically. The keto form is strongly stabilized by a π donor, whereas the enol isomer is stabilized by a π as well as a σ-acceptor substituent, R. As a result, substituent R = F is the most unfavorable whereas R = CN is the most favorable for the enolization. The water catalyzed enolization in the neutral water proceeds concertedly, but carbon deprotonation is more important than carbonyl-oxygen protonation by water in the rate determining step. © 1997 by John Wiley & Sons, Inc.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
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  • 2
    Publication Date: 2018-01-16
    Description: DOI: 10.1002/pmic.201700302 As demonstrated by Yae Eun Park et al. in article number 1700302, lectin arrays in conjunction with label-free quantitative mass spectrometry allowed for discovering cell surface glycoproteins to differentiate low- and high-grade glioma cells (Hs683 and T98G). The cell surface glycoproteins showing significant increases in the high-grade glioma cells could be valuable diagnostic markers and therapeutic targets for malignant gliomas.
    Print ISSN: 1615-9853
    Electronic ISSN: 1615-9861
    Topics: Medicine
    Published by Wiley-Blackwell
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  • 3
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The kinetics of reactions between 2-phenylethyl benzenesulphonates (2-PEB) and benzylamines in acetonitrile at 65·0°C have been studied; the mechanism was examined on the basis of the sign and magnitude of cross-interaction constants ρij and βij. In contrast to the reactions of 2-PEB with anilines in methanol, participation of the aryl-assisted pathway was negligible, with a strong indication that the reaction proceeds largely by an intermolecular SNi mechanism with a four-centre transition state (TS). The effect of substituents on the TS variation was in accord with the predictions of the quantum mechanical model.
    Additional Material: 6 Tab.
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  • 4
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Nucleophilic substitution reactions of benzoic anhydrides, in which one of the rings is substituted, with anilines were investigated in methanol. The product-formation step coincides with the rate-limiting step so that the two rate constants, kXZ and kXZ, for the competitive reaction pathways can be dissected. The two cross-interaction constants, ρXY and ρXY, especially an unusually large magnitude of the latter, indicate that the reaction proceeds by a frontside SN2 attack on either one of the caronyl carbon with a strong interaction between the nucleophile (X) and the leaving group (Z). The mechanism is also supposed by the trends in the activation parameters.
    Additional Material: 1 Ill.
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  • 5
    ISSN: 0887-624X
    Keywords: dinuclear metallocene ; polysiloxane bridge ; heterogenation ; ethylene polymerization ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The polymerization of ethylene was studied by using a series of polysiloxane-bridged dinuclear zirconocenes [(SiMe2O)nSiMe2(C5H4)2][(C9H7)ZrCl2]2 (7, n = 1; 8, n = 2; 9, n = 3), the corresponding mononuclear zirconocene (C5H5)(C9H7)ZrCl2, 10, and the pentamethylene-bridged dinuclear zirconocene [(CH2)5(C5H4)2][(C9H7)-ZrCl2]2, 13. From the polymerization studies using these catalysts it was found that (i) activities of the polysiloxane dinuclear zirconocenes 7-9 wre lower than that of the corresponding mononuclear zirconocene 10, (ii) molecular weights of polyethylenes produced by the dinuclear metallocenes are greater than that of polyethylene produced by the mononuclear metallocene, (iii) the complex 9 holding the longest bridging ligand exhibited the highest activity but produced a polymer having the smallest molecular weight among the polysiloxane-bridged dinuclear zirconocenes, and (iv) the pentamethylene-bridged dinuclear metallocene 13 showed higher activity than the complexes 7-9 and the mononuclear zirconocene 10. The formation of the lowest molecular weight of polyethylene by 9 was attributed to the influence of electron withdrawal caused by the Lewis acid-base interaction between the acidic aluminum of the cocatalyst and the basic oxygen at the polysiloxane linkage as well as the lack of a steric problem. An increase in steric congestion around the metal center led to not only a decrease in catalytic activity due to preventing facile monomer access to the active site but also an increase in the molecular weight of polyethylenes due to supressing β-H elimination. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 3717-3728, 1997
    Additional Material: 4 Ill.
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  • 6
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The base-promoted nitrile-forming elimination reactions of YCH=CHCβH=NOCH=CHZ (Y=OCH3, H or Cl and Z=H or NO2) were studied by the AM1 MO theoretical method with Cl- as a base. The reaction is found to proceed by an E1cB-like E2 mechanism in which Cβ - H bond cleavage is more advanced than N - O bond breaking. The syn-elimination has a more E1cB-like transition state (TS) than the anti elimination, which is attributed to the structurally favourable nN - ó * (Cβ - H) charge-transfer interaction. An electron-withdrawing Y substituent lowers the activation barrier by stabilizing negative charge developed on Cβ in the TS. An electron-withdrawing substituent in the leaving group (Z = NO2) tends to enhance the anti relative to the syn elimination process by depressing the δ*(N - O) level, which in turn makes the nć - δ*(N - O) interaction more effective. The YCH=CH -  and  - CβH=N fragments are perpendicular in the TS, which is stabilized by delocalization of negative charge developed on the Cβ atom.
    Additional Material: 3 Ill.
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  • 7
    ISSN: 0006-3592
    Keywords: Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Data reported here and previously indicate that when dextrin is hydrolyzed in the presence of immobilized glucoamylase, use of a larger average molecular weight substrate leads to lower overall rates of hydrolysis, while the maltose concentration during the bulk of the reaction and the maximum glucose concentration are lower than when the soluble form of the enzyme is employed under the same conditions. Computer simulation of the system demonstrated that all three observations were caused by pore diffusion limitation: the first by slow diffusion of substrate, the second by slow diffusion of intermediates, and the third by slow diffusion of glucose. Follow-up experiments with glucoamylase immobilized to particles of different sizes confirmed this finding, as results with the smallest beads were identical to those with soluble glucoamylase.
    Additional Material: 8 Ill.
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  • 8
    ISSN: 1022-1336
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Two kinds of the trisiloxane-bridged dinuclear titanium metallocene-type complexes were synthesized. They are monocyclopentadienyl and biscyclopentadienyl dinuclear titanocenes with trisiloxane bridges. The above complexes with modified methylaluminoxane (MMAO) cocatalyst initiate the polymerizations of ethylene and styrene to produce high-density polyethylene and syndiotactic polystyrene. In addition, a copolymer of ethylene and styrene was obtained.
    Additional Material: 1 Ill.
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  • 9
    ISSN: 0887-6266
    Keywords: unsaturated polyester ; free radical polymerization ; curing kinetics ; induction time ; kinetics model ; Physics ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Curing kinetics of unsaturated polyester resin system exhibiting apparent induction periods was investigated by modeling free radical initiation and propagation processes. The isothermal curing induction time as well as the maximum-rate time provided the same activation energy in the Arrhenius relation, and therefore, the isothermal curing master curve was constructed by using the reduced time method. Two model elementary rate equations for radical and monomer were proposed to describe the free radical polymerization of unsaturated polyester resin systems. The power law was adopted to express the conversion dependence function of the initiation efficiency and the monomer reaction rate. Demonstrating the capability of the developed model, the agreement between experimental and predicted data was excellent in both isothermal and dynamic-heating conditions, even with the same model parameters in different thermal conditions. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2447-2456, 1997
    Additional Material: 8 Ill.
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  • 10
    ISSN: 1022-1336
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The disiloxane-bridged zirconocene complexes, tetramethyldisiloxanediylbis (cyclopentadienyl)zirconium dichloride and tetramethyldisiloxanediylbis(cyclopentadienyl) dimethylzirconium initiate the homopolymerization of ethene as well as the copolymerization of ethene and α-olefin with a modified methylaluminoxane as cocatalyst. The catalyst systems give resonable activity but the molecular weight of polyethene decreases drastically with increasing polymerization temperature.
    Additional Material: 2 Ill.
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