摘要:

AbstractMicrowave heating involves direct absorption of energy by functional groups that bear ionic conductivity or a dipole rotational‐effect, and this energy is then released into the surrounding solution. This absorption of energy causes the functional groups involved to have higher reactivity to other surrounding reactants than when they are simply incubated with the reactants at the same temperature. In other word the enhanced rate of the reaction can be due to the reactant stirred by the molecular dipole rotation and molecules themselves acting as a stirring bar. In contrast to conventional heating, the salient feature of “dipole rotation” constitutes one efficient form of “molecular agitation” or “molecular stirring” many aspects of which can be explore in chemical reactions. We will discuss some of the useful applications of this “molecular agitation” by means of microwave irradiation. Using this unique technology, we have developed: 1) a method to control the cleavage sites of peptide bonds, especially those bonds connected to aspartic acid residues inside the native peptides and proteins, 2) a method to increase coupling efficiency in solid‐phase peptide synthesis using a common microwave oven, 3) a novel procedure that increases the rate of alcalase‐catalyzed reactions using microwave irradiation in peptide‐bond formation with proline as a nucleophile and selective benzoylation of a pyranoside derivative, 4) a procedure to solubilize and hydrolyze retrograded starch, 5) a novel procedure to enhance the rate of saponification in a serum sample for very long ch

ISSN:0009-4536    

DOI:10.1002/jccs.199700028

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractConventionally, natural product chemistry deals with the isolation, characterization, and synthesis of compounds from plants. It a broad sense, natural product chemistry may include all fields of biochemistry. Two projects are presented to illustrate the expanded definition of natural product chemistry. One deals with phosphatidylinositol (PI) and the key enzyme involved in its metabolism, PI‐specific phospholipase C. The other project is on the structure‐function relationship of human tumor suppressor

ISSN:0009-4536    

DOI:10.1002/jccs.199700029

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractMP4/6‐31+G* level calculations are performed to study the reductive bond‐breaking reaction of the C‐X bond in halomethanes, CH3X and CH2X2where X is a fluorine atom or chlorine atom. This type of reaction involves a radical anion, after attaching an extra electron to the halomethane molecule, in which a C‐X bond‐breaking takes place. Products are a radical and a halogen anion. The equilibrium geometry and bond dissociation energy of the C‐X bond thus found are in good agreement with previous theoretical and experimental results. The anomeric effect, electrostatic effect, and radical re‐stabilization effect, are investigated to find their influences on bond length and bond dissociation energy in CH3X and CH2X2. Potential energy curves are calculated for the reductive bond‐cleavage process, and trends in activation energy for various case

ISSN:0009-4536    

DOI:10.1002/jccs.199700030

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractIrradiations of Ni/TiO2catalyst by UV in hydrogen at 77 K produced not only Ni+ions on the catalyst surface, but also Ni3+and Ti3+species in bulk or near the interface between nickel and titania. These photo‐generated species were detected and characterized by low temperature electron paramagnetic resonance (EPR) spectroscopy. Relative spin concentrations of the photogenerated paramagnetic species (Nin+and Ti3+) varied with the nickel content in titania. A high nickel content in the sample resulted in a high peak intensity ratio of Nin+to Ti3+. It was found that the photoinduced self‐redox reaction of Ni2+ions to form Ni+and Ni3+ions has a priority over the photoreduction of Ti4+to Ti3+ions. The characteristic EPR spectrum of the Ni3+(3d7) ions with g1= 2.268, g2= 2.237, and g3= 2.045 indicates that the Ni3+ions are most likely located in the substitutional sites of TiO2, possibly near the surface rutile phase. The Ni+species (3d9) with g4= 2.130 and g1= 2.063 are on the surface of TiO2. Both Ni+and Ni3+ions are quite stable in hydrogen. The Ni3+ions seem to be responsible for anchoring the nickel ions onto titania and stablizing the Ni+species on the surface. The Ni+ions are thus free from oxygen poisoning and still show a high activity toward olefin oligomerizat

ISSN:0009-4536    

DOI:10.1002/jccs.199700031

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractA main purpose of this paper is to present a density matrix treatment of the magnetic quenching of molecular luminescence. Unlike the previous theories, which only consider the magnetic effect on the nonradiative rate constant, the present theory describes the time‐evolution of the system under the action of a magnetic field. Our theory can treat both the steady state and transient molecular luminescence affected by a magnetic fiel

ISSN:0009-4536    

DOI:10.1002/jccs.199700032

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe syntheses and the electronic properties of oligo(2,7‐biphenylenylene‐(E)‐vinylene)s are reported. According to our spectral analysis, a limit of 2.5 ev for the peak band‐gap of poly(2,7‐biphenylenylene‐(E)‐vinylene)s is estimated. In addition, we discovered that oligo(2,7‐biphenylenylene‐(E)‐vinylene)s are amphoteric and can be either oxidized or reduce

ISSN:0009-4536    

DOI:10.1002/jccs.199700033

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractA series of fourteen anilide derivatives of ibuprofen were resolved on six chiral stationary phases (CSPs) derived fromN‐arylcarbamoyl derivatives of (S)‐phenylglycine. Excellent chiral resolutions were achieved on these CSPs. The ionic‐type CSPs showed better chiral recognition abilities than the corresponding covalent‐type CSPs, and the CSP bearing two chiral centers has better performance than the CSPs bearing only one chiral center. The highest separation factor was achieved using the ionic‐type CSP bearing two chiral centers for the resolution of the 3,5‐dinitroanilide derivative of ibuprofen. This result is better than those reported in literature for the resolution of ibuprofen on the CSPs derived from amino acids, According to the chromatographic behaviors, the hydrogen bonding interaction, the π‐π interactions provided by the phenyl groups in CSPs bearing one chiral center, and the phenylethylcarbamoyl moiety in CSPs bearing two chiral centers dominate the chi

ISSN:0009-4536    

DOI:10.1002/jccs.199700034

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractA chemically modified electrode (CME) containing salicylaldehyde thiosemicarbazone (TSCsal) was evaluated for the ability to preconcentrate copper(II) prior to quantification by voltammetry. The CME has been used for the very sensitive and selective analysis of trace amounts of copper(II). A detection limit of 0.1 ppb was obtained by applying anodic stripping voltammetry with a flow system. The parameters that affect the sensitivity and possible interference by other ions or chelating agents have been examined in detail. The CME exhibits high stability and the response could be reproduced for four preconcentration‐determination‐renewal cycles [10ppbCu(II)] with a 2.87% relative standard deviation. The proposed method has been applied to the determination of copper(II) in tap water, drinking water, and NASS‐3 standard reference sea water samples. The results gave satisfactory recov

ISSN:0009-4536    

DOI:10.1002/jccs.199700035

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe15N NMR spectra of the complexes Os3(CO)10(μ2‐CONHPri)(μ2‐CNHR) (1a, R = Pr; 1b, R = CH2Ph) and Os3(CO)9(NH2Pri)(μ2‐CONHPri)(μ2‐CNHR) (2a, R = Pr; 2b, R = CH2Ph) are studied by using the1H detected (inverse)1H‐15N correlated spectroscopy. The15N chemical shifts and the1H‐15N coupling constants fall in characteristic regions for each of the coordinated amine, aminocarbyne, and carboxamido ligands and these values are related to their bonding types. The NMR data are discussed in terms of the influence of the paramagnetic term which is the major factor determining the chemical shifts. A comparison is made to understand the15N chemical‐shift differences between the coordinated nitrogen‐containing ligands and the corresponding

ISSN:0009-4536    

DOI:10.1002/jccs.199700036

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe mechanism for the photochemical reactions ofo‐methyl‐benzaldehyde (1),o‐methyl‐acetophenone (2) ando‐methyl‐benzophenone (3) in the presence of α‐phenyl‐N‐tert‐butylnitrone (PBN) to the formation of stable nitroxyl radicals4–6is studied.The nitroxyl radical product6can also be obtained by the thermolysis of benzocyclobutenol with PBN. Thus, the radical products were derived from a novel and regioselective 4+2 cycloaddition of the photogenerated dienol

ISSN:0009-4536    

DOI:10.1002/jccs.199700037

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY