An analysis of the polarizabilities in some conjugated molecules

Atom-atom, atom-bond and bond-bond polarizabilities are calculated with a previously used LCAO approximation. Some properties of naphtalene, assumed as general for alternant hydrocarbons, seem to depend on the approximation employed. Atom-atom polarizabilities in azines are related to other theoretical and experimental quantities. In all the molecules studied, the part played by formal bonds is highly stressed.

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Zusammenfassung Atom-Atom-, Atom-Bindungs- und Bindungs-Bindungs-Polarisierbarkeiten werden mit einem schon früher benutzten LCAO-Verfahren berechnet. Einige Eigenschaften von Naphthalin, die für alternierende Kohlenwasserstoffe als charakteristisch angesehen werden, scheinen von der benutzten Näherung abzuhängen. Atom-Atom-Polarisierbarkeiten in Azinen werden auf andere theoretische und experimentelle Größen zurückgeführt. In allen betrachteten Molekülen spielen formale Bindungen eine bedeutende Rolle.

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Résumé Calcul des polarisabilités atome-atome, atome-liaison et liaison-liaison dans une approximation LCAO utilisée auparavant. Certaines propriétés du naphtalène, supposées générales pour les hydrocarbures alternants, semblent dépendre de l'approximation employée. Les polarisabilités atome-atome des azines sont reliées à d'autres grandeurs théoriques et expérimentales. Dans toutes les molécules étudiées, le rôle joué par les liaisons formelles est mis en relief.

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With a fellowship from the Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina.

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With a fellowship from the Centro Latino Amerioano de Física.

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The authors gratefully acknowledge the helpful discussions with Dr. J. Brieux and Lic. H. C. González, who has besides collaborated in part of the calculations.     

Transesterifications mediated by t-BuNH2

A mild protocol for transesterification of simple esters is described. The method is based on the use of t-BuNH₂/ROH (R = Me, Et, i-Pr, t-Bu) with or without LiBr. The scope of the procedure was explored for aliphatic and aromatic esters. The protocol is particularly useful when going from higher to lower hindered esters and harsh reaction conditions are needed for the reversal process. A rationalization of the mechanism is presented. The scope and limitation of this transformation are also described.     

Syntheses of the tedanolides and myriaporones

This review covers the synthetic contributions leading to the syntheses of the polyketides tedanolide, 13-deoxytedanolide and the structurally related myriaporones. Fragment syntheses that lead to valuable insight into the subtle synthetic problems of the tedanolides are also presented.     

A laser-wire system for the international linear collider

A new laser-wire has been installed in the extraction line of the ATF at KEK. It aims at demonstrating that laser-wires can be used to measure micrometre scale beam size. In parallel, studies have been made to specify a laser suitable for the ILC laser-wires.      

Specific architectures for optical parametric oscillators

Optical parametric oscillators are coherent light sources showing properties that are not encountered with usual laser sources. This article deals with optical architectures that take advantage of the specific properties of the three-wave mixing parametric interaction. We show that optical cavities can be specifically designed to increase either the optical conversion of the parametric process or to reduce dramatically the emitted line width. To cite this article: A. Berrou et al., C. R. Physique 8 (2007).     

Optical performance of carbon-nanotube electron sources

The figure of merit for the electron optical performance of carbon-nanotube (CNT) electron sources is presented. This figure is given by the relation between the reduced brightness and the energy spread in the region of stable emission. It is shown experimentally that a CNT electron source exhibits a highly stable emission process that follows the Fowler-Nordheim theory for field emission, fixing the relationship among the energy spread, the current, and the radius. The performance of the CNT emitter under realistic operating conditions is compared with state-of-the-art electron point sources. It is demonstrated that the reduced brightness is a function of the tunneling parameter, a measure of the energy spread at low temperatures, only, independent of the geometry of the emitter.     

The glucosinolate-myrosinase system. New insights into enzyme-substrate interactions by use of simplified inhibitors

Myrosinase, a thioglucoside glucohydrolase, is the only enzyme able to hydrolyse glucosinolates, a unique family of molecules bearing an anomeric O-sulfated thiohydroximate function. Non-hydrolysable myrosinase inhibitors have been devised and studied for their biological interaction. Diverse modifications of the O-sulfate moiety did not result in a significant inhibitory effect, whereas replacing the D-glucopyrano residue by its carba-analogue allowed inhibition to take place. X-Ray experiments carried out after soaking allowed for the first time inclusion of a non-hydrolysable inhibitor inside the enzymatic pocket. Structural tuning of the aglycon part in its pocket is being used as a guide for the development of simplified and more potent inhibitors.     

Definition of a multicenter bond index

The tensor character of the first-order density matrix leads to the definition of an MO multicenter bond index for closed-shell systems. It is here applied to three-center bonds. Satisfactory results are obtained for compounds involving secondary bonds, strong and normal hydrogen bonds; the index for the peptide bond is found to be similar to that of strong hydrogen bonds.On leave of absence from Inst. de Física, UFBA, 40000 Salvador, BA, Brasil.     

Weathering Phenomena on Naturally Weathered Potash-Lime-Silica-Glass with Medieval Composition Studied by Secondary Electron Microscopy and Energy Dispersive Microanalysis

 Samples of two model glasses with chemical compositions similar to medieval stained glass were exposed to the natural environment at 23 test sites for a period of 6 months, 1 and 2 years within an exposure programme of the “Working Group of Effects on Materials, Including Historic and Cultural Monuments” of the Economic Commission for Europe of the United Nations. During the exposure the environmental data were measured and collected at each test site in order to enable a correlation between the pollutant load in the ambient atmosphere and the weathering phenomena of the glass samples. After the exposure, which was performed in a sheltered as well as in an unsheltered mode for gaining information about the influence of dry and wet deposition of air pollutants, the glass samples were analysed in the scanning electron microscope with energy dispersive microanalysis (SEM/EDX). Model glass M1 – a potassium rich glass – was covered to a high amount with crystalline weathering products (above all syngenite), whereas glass M3 – a glass with a higher Ca and Si content – turned out to be more stable against weathering. This glass was covered with gypsum and arcanite crystals but in total to a less extent than glass M1. Received March 30, 1998. Revision February 8, 2000.