摘要:

AbstractSolid state11B (and in one case10B) NMR spectra of four cyclic boron‐chalcogen compounds have been measured at various frequencies and analysed in terms of nuclear quadrupole and chemical shift interactions. Application of the Townes‐Dailey theory to the quadrupole results, corrected for ionic contributions to the electric field gradient and to the bonding, yields detailed information about the importance of μ‐bonding in the respective materials. The occupation number of the 2pzorbitals of the sp2‐hybridized boron atoms appear to be particularly large for systems containing chalcogen‐chalcogen bridges. From both the chemical point of view and the spectroscopic result, among the six‐ and five‐membered rings, diiodotriselenadiborolane is distinguished by an except

ISSN:0005-9021    

DOI:10.1002/bbpc.19910950401

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

年代:1991

数据来源: WILEY

摘要:

AbstractThe ortho‐para conversion of hydrogen adsorbed in chabazite has been observed by recording the neutron transmission of the sample after gradual adsorption of hydrogen in steps of one molecule per unit cell at temperatures between 40 and 70 K. The neutron data were fitted by a model assuming that the o‐H2(s= 1) state decays exponentially with a time constantK= 0.5% per minute at a porefilling ofn= 1 (molecule/unitcell). With increasing adsorption the value found for the o‐p‐H2conversion rate is smaller:K= 0.2% per minute

ISSN:0005-9021    

DOI:10.1002/bbpc.19910950402

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

年代:1991

数据来源: WILEY

摘要:

AbstractThe fermionic second order reduced density matrix with complex dilation and thermalization describes generally quantum correlation effects in condensed matter. Connection with recent experiments on high‐Tccopper‐oxide superconductors is made. The following experimental results are explained from first theoretical principles: (1) Quantum correlation contributions to the energy gap in agreement with e.g. precise infrared experiments. (2) The universal relation betweenTcandns/m* (carrier density over effective mass) [Uemura et al., Phys. Rev. Lett.62, 2317 (1989)]. (3) The saturation ofTcwith increasing carrier density. (4) The considerable absorption “in” the energy gap. A prediction concerning the sudden breakdown of superconductivity at a specific (high) carrier density follows from the general theory. All the above results are independent of the specific (and still unknown) Cooper pair formation me

ISSN:0005-9021    

DOI:10.1002/bbpc.19910950403

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

年代:1991

数据来源: WILEY

摘要:

AbstractA structural analysis of microclusters is carried out on 36 high‐symmetry structures using classical and quantum mechanical minimization methods. For the classical calculations, the total energy is optimized utilizing two‐body potential functions first and then the effects of three‐body forces to these structures are analyzed. The same set of high‐symmetry clusters made of Carbon atoms are studied by a semi‐empirical quantum mechanical method (AM1) and correlations between two approaches are studied. – At large three‐body intensity parameter Z*, both AM1 and static results show that linear conformations are preferred to two‐dimensional structures. Three‐dimensional structures seem to be the least stable ones. But at small Z*, these results are reversed. Overall there is a good agreement between both set of results for Z* = 0.8, suggesting that a qualitative analysis of Carbon clusters can be done with simple potential functions provided that properly weighted three‐body interaction

ISSN:0005-9021    

DOI:10.1002/bbpc.19910950404

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

年代:1991

数据来源: WILEY

摘要:

AbstractThe NaCl – SnCl2and NaBr – SnBr2systems were investigated under equilibrium conditions in the temperature ranges between 713 to 963 K and 653 to 893 K, respectively, by using Knudsen effusion mass spectrometry with a two‐compartment Knudsen cell. The gaseous species NaX, (NaX)2, SnX2, and NaSnX3(X = Cl, Br) are present in the vapour of the two systems. The enthalpy and entropy changes of the dissociation reactionsNaSnCl3(g) ⇄ NaCl(g) + SnCl2(g) (1)andNaSnBr3(g) ⇄ NaBr(g) + SnBr2(g) (2)resulted asΔHo298(Eq. (1)) = 197 ± 6 kJ mol−1,ΔSo298(Eq. (1)) = 153 ± 7 J mol−1K−1,ΔHo298,(Eq. (2)) = 190 ± 6 kJ mol−1, andΔHo298(Eq. (2)) = 160 ± 7 J mol−1K−1.The enthalpy and entropy changes of the reactionsSnX2(g) + (NaX)2(g) ⇄ NaSnX3(g) NaX(g), X = Cl, Br, (3)are additionally evaluated as well as equilibrium constants for the reactions Eqs. (1) to (3). The

ISSN:0005-9021    

DOI:10.1002/bbpc.19910950405

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

年代:1991

数据来源: WILEY

摘要:

AbstractThe electronic energy transfer from rhodamine B to several acceptor dyes in cellulose nitrate solid solution and on the surface of membrane filters of the same polymer has been investigated at room temperature. In polymer matrix the Förster theory has been confirmed and critical distancesRohave been calculated from the theoretical equation and have also been determined by energy transfer experiments measuring the fluorescence decay time of the donor. Applying new fractal concepts to energy transfer in porous membranes, the fractal dimension of the polymer surface has been determined under several conditions. The values agree with others obtained by adsorption experiments

ISSN:0005-9021    

DOI:10.1002/bbpc.19910950406

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

年代:1991

数据来源: WILEY

摘要:

AbstractWater‐in‐oil microemulsions, stabilized by the nonionic surfactant Igepal CO‐520, show a temperature dependent structure change which can be monitored by electric conductivity. We show that the transition which involves the percolation of the aqueous phase can also be induced by high electric fields. A system is investigated which contains only a few volume percent of the aqueous phase (0.1 M KCl) in a mixture of n‐ and c‐hexane but still exhibits a highly cooperative transition from the conducting state at lower temperature to the almost nonconducting state at higher temperature. Electric fields, applied up to ca. 10 kV cm−1, shift the transition curves to the high temperature side, i.e. restore a structure with continuous water channels. The percolation point exhibits a quadratic field strength dependence. The dynamics of the transition in the micro‐ to millisecond range shows characteristic differences below and above the percolation point. The transient behavior and the electric field strength dependence of the conductivity change are discussed with respect to a theory [9] developed for ionic microemulsions. – A special field pulse method is described which allows measurement of current relaxation and electri

ISSN:0005-9021    

DOI:10.1002/bbpc.19910950407

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

年代:1991

数据来源: WILEY

摘要:

AbstractThe synthesis of two new asymmetrical diacetylene (DA) derivatives is described: 1‐(4‐fluorobenzenesulfonyloxy)‐6‐(4‐trifluoromethylbenzenesulfonyloxy)‐2,4‐hexadiyne (FBS/TFMBS) and 1‐(2,4‐dinitrophenoxy)‐6‐(4‐methyl‐2‐nitrophenoxy)‐2,4‐hexadiyne (DNP/MNP). Results are presented on crystal growth, crystal structure analysis and dielectric properties. Both DAs polymerize thermally in the solid state and have polar (“ferroelectric”) individual chains. The different chains in the unit cell are arranged centrosymmetrically in FBS/TFMBS, but

ISSN:0005-9021    

DOI:10.1002/bbpc.19910950408

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

年代:1991

数据来源: WILEY

摘要:

AbstractWater dissociation effects in ion transport through anion exchange membranes with adhered cation exchange surface films are studied on the basis of a theory previously proposed for bipolar membranes. The effects that the adhered surface film exerts on the limiting current and on the electric current carried by the ions coming from water dissociation are found to be of particular relevance for the fouling phenomena observed in ion exchange membranes. Finally, some limitations of the theory concerning the use of the Onsager's theory of the second Wien effect to describe the electric‐field‐enhanced water dissociation are discus

ISSN:0005-9021    

DOI:10.1002/bbpc.19910950409

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

年代:1991

数据来源: WILEY

摘要:

AbstractThis paper presents structural, optical, and electronic properties of Si coated with a thin epitaxial layer of n‐type BP, which serves as protective optical window. n‐Si/n‐BP, as well as p‐Si/n‐BP heterostructures have been grown by CVD and investigated. p‐Si/n‐BP heterojunction photo‐electrodes produce large and stable photocurrents up to 15 mA/cm2when applied in liquid junction solar cells. The origin of this photocurrent is discussed. Impedance spectroscopy is used to obtain Mott‐Schottky plots from which the flatband potential of crystalline boron phosphide is determined. The band structure of the heterojunction is shown to agree with the optical features. Finally, the distribution of the potential drop in the hetero‐stru

ISSN:0005-9021    

DOI:10.1002/bbpc.19910950410

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

年代:1991

数据来源: WILEY