摘要:

AbstractA classical organic redox reaction is the conversion of an aromatic compound from its reduced form R to a semioxidized state S, and further to a totally oxidized (quinoid) species T. Complex redox systems are present in a reaction of quinoid and aromatic forms derived from different parent compounds. In this case, the exchange of one or two electrons in one step between T and R can be distinguished experimentally. These reactions were investigated in a stopped‐flow apparatus with optical detection. In the system N,N,N′,N′‐tetramethyl‐para‐phenylene‐diamine/N,N‐dimethyl‐para‐phenylene‐diamine the rate constants for all elementary reactions have been determined. Protonated, and unprotonated R were found to be reactive. In this system, as well as the others considered, no direct transfer of two electrons between T1and R2takes place. The essential steps of the mechanism have to be seen in a mixed symproportionation yielding S1and S2, and a subsequent fast oxidation of S2by surplus starting material T1. As a generalization, a tangent criterion has been derived to distinguish between one‐ and two‐electron steps in

ISSN:0005-9021    

DOI:10.1002/bbpc.198800110

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1988

数据来源: WILEY

摘要:

AbstractA simple equation for the line shape of Raman (or infrared) spectra is provided for systems subject to fast chemical exchange with average life‐times between about 3 · 10−13and 10−11s. The equation is compared with the corresponding one for NMR line broa

ISSN:0005-9021    

DOI:10.1002/bbpc.198800111

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1988

数据来源: WILEY

摘要:

AbstractThe 90 MHz1H NMR spectra of15N,15N′‐dipentadeuterophenylformamidine (DPFA), a nitrogen analogue of formic acid, dissolved in tetrahydrofuran‐d8(THF) have been measured as a function of concentration, deuterium fraction in the1H‐15N sites, and of temperature. The spectra show characteristic changes, from which thermodynamic and kinetic information on hindered rotation, hydrogen bond association and proton exchange of DPFA in THF are obtained by NMR lineshape analysis. DPFA forms two conformers A and B in THF, to which s‐trans and s‐cis structures have been assigned. At low concentrations both DPFA conformers are located in a hydrogen bond to the solvent molecules. However, as a result of the different structure, only A is able to selfassociate to any observable extent. This effect leads to concentration dependent A/B populations. Higher selfassociates or mixed AB associates are not observed. Whereas B is not able to exchange protons, A is subject to a very fast proton transfer. By measuring proton lifetimes as a function of concentration and of the deuterium fraction in the labile proton sites, it was established that two protons are transferred in every exchange process. Thermodynamic data of the association process obtained by the analysis of the chemical shifts, of the A/B populations and the proton lifetimes as a function of concentration agreed very well. These results are evidence that A forms only selfassociated hydrogen bonded dimers with a cyclic structure in which the double proton transfer takes place. The observation of a kinetic HH/HD isotope effect of 20 at 178 K establishes this transfer as the rate limiting step of the overall proton exchange. Rate constants of the double proton transfer in the cyclic dimer were obtained as a function of temperature from which an activation energy of about 19 kJ mol−1was obtained. In addition, the rates of interconversion between the two conformers were determined as a function of temperature. Details of the reaction mechanism and differences to the related carboxylic acids are discussed. Thus, it is shown that dynamic NMR spectroscopy can be a useful tool for elucidating elementary steps of complex reac

ISSN:0005-9021    

DOI:10.1002/bbpc.198800112

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1988

数据来源: WILEY

摘要:

AbstractThe elementary deactivation processes of CHF (Ã, υ′2) by H2(X1Σ+g), O2(X3Σ−g), and CO(X1Σ+) were studied in a quasistatic photolysis chamber. Determination of time and spectrally resolved laser‐induced fluorescence of CHF (Ã1A″, υ′2) in single vibronic excited levels (SVL) (0 ≦ υ′2≦ 5) was carried out in the gas phase at room temperature. CHF [X̃1A′ (0,0,0)] was generated in the fluorescence cell by multiphoton dissociation (MPD) of CH2F2.The overall CHF (Ã, υ′2)‐depletion rate constants:A significant increase of the rate constants for higher vibronic states was observed. The contribution of vibration deactivation to the overall depletion of the initially excited state was found to be small (≦ 10%), with approximately the same importance of collision‐induced vibrational quantum change of Δυ′2= 1 and Δυ′2= 2 respect

ISSN:0005-9021    

DOI:10.1002/bbpc.198800113

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1988

数据来源: WILEY

摘要:

AbstractMeasurements of vibrational relaxation rates of HBr (v= 1) in collisions with n‐H2, para‐enriched H2, HD and D2are presented for temperatures ranging from room temperature down to 140 K. In all cases strong deviations of the V‐R, T rate from Landau‐Teller behaviour are observed. Neither the temperature dependence nor the mass dependence of the data can be explained by Landau‐Teller or Schwartz‐Slawsky‐He

ISSN:0005-9021    

DOI:10.1002/bbpc.198800114

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1988

数据来源: WILEY

摘要:

AbstractA method is presented for predicting gas‐ and liquid‐phase association constants from vapor pressure data using the homomorph concept. The gas‐phase dimerization parameters for alcohols, phenols, amines and carboxylic acids are compared with those obtained fromp‐V‐T, thermal conductivity, heat capacity and spectroscopic data. The predicted association enthalpies are in agreement with the results of dilute solution calorimetric measurements. A generalization of the method in terms of topological parameters characterizing molecular structure is proposed. The correlations of association constants with topological parameters allow to reproduce second virial coefficients or vapor pressures on the basis of the structural formula of an associating

ISSN:0005-9021    

DOI:10.1002/bbpc.198800115

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1988

数据来源: WILEY

摘要:

AbstractSteady‐flow viscosities of colloidal spheres, i.e., colloidal silica (8–45 nm in diameter) and monodisperse polystyrene latices (diam. 85 to 780 nm), are measured in deionized suspensions and in the presence of a small amount of NaCl. The plots of shear stress (S) of the crystal‐like structure of the colloids against shear rate (q) show the following features: (1) at very small values ofq(smaller than 0.1 s−1),Sincreases very sharply with increasingq. TheS – qcurve is S‐shaped, and the rigidity (G) of the colloidal crystal is evaluated from the plots ofSagainstqt/2 (shear strain), wheretis time. TheG‐values are between 20 and 3000 Pa, which are consistent with those reported hitherto, and increase as the sphere's concentration increases. (2) The sharp increase inSceases above the criticalqvalue, from which the yield stress of the crystal‐like structure is evaluated to be between 0.1 to 2 Pa. (3) The log (viscosity, η) vs. logqplots shows non‐Newtonian flows: η increases asqdecreases and then η reachs a constant value. Furthermore, log η begins to increase linearly as logqdecreases with a slope of −1, which supports the crystal‐like

ISSN:0005-9021    

DOI:10.1002/bbpc.198800116

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1988

数据来源: WILEY

摘要:

AbstractThe1H‐NMR spin lattice relaxation times,T1, were observed for ethylammonium nitrate, (CH3CH2NH3)NO3, and propylammoniumnitrate, (CH3CH2CH2NH3)NO3, and for their partially deuterated analogs, (CH3CH2ND3)NO3, and CH3(CH2)2ND3NO3, respectively. For each salt two solid phases were found, one of them being metastable and existing below ca. 200 K and ca. 210 K for the ethylammonium nitrate and propylammonium nitrate, respectively. The results of theT1measurements are consistent with the assumption that CH3and NH3group reorientations about their C–C and C–N bond axes take place in the stable and in the metastable phase of both salts. The motional parameters of the cations in each phase were determined. The asymmetry in the curvesT1=f(T) observed in the metastable phases of (CH3(CH2)2NH3)NO3and (CH3(CH2)2ND3)NO3suggests a distribution of correlation times in the cationic motions within these p

ISSN:0005-9021    

DOI:10.1002/bbpc.198800117

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1988

数据来源: WILEY

摘要:

AbstractSurface compositions of Fe/Mn‐oxide Fischer‐Tropsch catalysts were determined by AES. Bulk compositions of the samples varied from pure iron oxide to pure manganese oxide. A method for quantification of Auger spectra was developed. The iron content in the surfaces of the samples after calcination and reduction is larger than in the bulk. The surface composition after FT synthesis depends on the temperature of reduction before synthesis. The results of surface analysis of the calcined and reduced samples are compared with those of adsorption measurements described elsewh

ISSN:0005-9021    

DOI:10.1002/bbpc.198800118

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1988

数据来源: WILEY

摘要:

AbstractThe adsorption of water and isopropanol on a pure microcrystalline rutile has been studied by means of conventional adsorption isotherm and1H magnetic resonance intensity measurements. Evidence is obtained that irreversible adsorption of both water and isopropanol is caused by bonding to the coordinatively unsaturated Ti4+ions of the (110) crystallographic planes which contribute by about 85% to the surface area exposed. On these planes water is displaced by isopropanol such that the amount of alcohol adsorbed is not influenced by preadsorbed water. Reversible isopropanol adsorption is attributed to bonding onto Ti4+sites of the remaining planes exposed, most likely (100) and (101). It is suggested that isopropanol adsorption can be used to determine the specific surface area of microcrystalline rutile as well as the fraction of it which consists of the (110) crystallographic planes.

ISSN:0005-9021    

DOI:10.1002/bbpc.198800119

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1988

数据来源: WILEY