Showing papers by "University of Paris published in 1972"

The Electronic Properties of Diradicals

Abstract: A review of the various possible definitions of diradicals leads the authors to describe these systems as having two odd electrons in degenerate or nearly-degenerate molecular orbitals. A study of the wave-function for the two odd electrons shows that its form depends entirely on whether the diradical is homo- or heterosymmetric. Energy schemes are given in these two cases, as well as in the intermediate “non-symmetric” case. The extent of zwitterionic character in diradical states is also investigated. This is followed by a discussion of intersystem crossing between singlet and triplet diradical states via spin-orbit coupling and other mechanisms. The electronic matrix elements for spin-orbit coupling are calculated and evaluated numerically for various model cases. It is then possible to establish general rules for favorable (electronic) intersystem crossing. In 1,3 or 1,4 diradicals its efficiency is estimated to be comparable with that in aromatics. The role of the electron-nuclear hyperfine interaction in mixing singlet and triplet states, particularly in CIDNP, is explained. Finally the question of whether diradicals actually occur as secondary minima on potential energy surfaces is examined. Recent quantum-mechanical calculations, in contradiction to some thermochemical and kinetic evidence, lead to flat singlet surfaces without significant minima.

Groupes réductifs sur un corps local : I. Données radicielles valuées

Abstract: © Publications mathématiques de l’I.H.É.S., 1972, tous droits réservés. L’accès aux archives de la revue « Publications mathématiques de l’I.H.É.S. » (http:// www.ihes.fr/IHES/Publications/Publications.html) implique l’accord avec les conditions générales d’utilisation (http://www.numdam.org/legal.php). Toute utilisation commerciale ou impression systématique est constitutive d’une infraction pénale. Toute copie ou impression de ce fichier doit contenir la présente mention de copyright.

Use of impedance measurements for the determination of the instant rate of metal corrosion

Abstract: The various methods of evaluating corrosion rate and charge transfer resistance have been critically reviewed on the basis of the recent developments on the measurement and interpretation of faradaic impedances. It is demonstrated that the entity which is most accurately correlated with corrosion rate is the transfer resistance, the limit of the faradaic impedance at infinite frequency. In the case of iron, with and without inhibitor (propargylic alcohol), it has been ascertained, under various experimental conditions, that the measurement of this resistance constitutes nowadays the best electrochemical test for corrosion, and allows thea priori calculation of corrosion rate.

Phosphotungstic Acid-Chromic Acid as a Selective Electron-Dense Stain for Plasma Membranes of Plant Cells

Abstract: A mixture consisting of 1% phosphotungstic acid (PTA) in 10% chromic acid (CrO3) selectively stains the plasma membrane of plant cells. Whole tissue or pelleted cell fractions are prepared for electron microscopy using conventional methods including glutaraldehyde fixation and OsO4 postfixation, dehydration in acetone and embedding in Epon. To stain the plasma membrane, thin sections are transferred with a plastic loop to the surface of a 1% aqueous solution of periodic acid for 30 min for destaining. Following transfer through 5 distilled water rinses, the sections are exposed to the PTA-CrO3 mixture for 5 min, rinsed and mounted on grids for viewing with the electron microscope. The selectivity of the stain is retained in homogenates and serves to identify the plant plasma membrane in cell fractions.

The combined effect of curved boundaries and numerical integration in isoparametric finite element methods

Abstract: Consider the model problem where Ω is a bounded open subset of R n with a curved boundary Γ. The finite element method with isoparametric finite elements is applied to this problem, with curved finite elements along the boundary, in connection with a numerical quadrature scheme which is used to compute the coefficients of the resulting linear system. However, except in some very special cases, the interior Ω h of the union of the finite elements is not equal to Ω (although the boundary of Ω h is very close to Γ), and thus, taking also numerical integration into account, we obtain an approximate solution u h in the space H 1 0 (Ω h ). For several types of isoparametric finite elements (of simplicial and quadrilateral type) commonly used by the Engineers, we prove asymptotic error estimates of the form where r = 0, 1 or 2 depending upon the type of finite elements which are used, h is the greatest diameter of the finite elements, and either ũ is an extension of the exact solution u to the set Ω h ⊄ Ω, or ũ = u if Ω ⋐ Ω

Die elektronischen Eigenschaften von Diradikalen

Abstract: Eine Ubersicht uber die Definitionen von Diradikalen fuhrt die Autoren dazu, diese als Systeme mit zwei einsamen Elektronen in entarteten oder nahezu entarteten Molekulorbitalen zu beschreiben. Die Form der Wellenfunktion fur die beiden einsamen Elektronen hangt davon ab, ob das Diradikal homo- oder heterosymmetrisch ist. Sowohl fur diese beiden Falle als auch fur den dazwischenliegenden „nicht-symmetrischen” Fall werden Energieschemata angegeben. Das Ausmas an zwitterionischem Charakter in Diradikalzustanden wird ebenfalls untersucht. Nach einer Diskussion der Spinumkehr (intersystem crossing) zwischen Singulett- und Triplettdiradikalzustanden durch Spin-Bahn-Kopplung und andere Mechanismen werden die Elektronenmatrixelemente fur die Spin-Bahn-Kopplung berechnet und numerisch fur Modellfalle ausgewertet. Das ermoglicht die Aufstellung allgemeiner Regeln fur gunstige (elektronische) Spinumkehr. Bei 1,3- oder 1,4-Diradikalen durfte sie so wirksam wie bei Aromaten sein. Die Rolle der Elektron-Kern-Hyperfeinwechselwirkung beim Mischen von Singulett- und Triplettzustanden, besonders im CIDNP, wird ebenso wie die Frage besprochen, ob Diradikale tatsachlich als Nebenminima auf Potentialenergiehyperflachen auftreten. Neuere quantenmechanische Berechnungen fuhren zu flachen Singuletthyperflachen ohne signifikante Minima.

Chromosomal location of mutations affecting sucrose metabolism in Bacillus subtilis Marburg.

Abstract: Mutations affecting sucrose metabolism have been mapped by PBS1 transduction on the Bacillus subtilis chromosome in seven loci sacA, sacB, sacQ, sacR, sacS, sacT and sacU. sacA and sacB are presumed to be the structural genes of a sucrase and a levansucrase respectively. sacR, sacS and sacT correspond to groups of mutations leading to constitutive synthesis of sucrase or both sucrase and levansucrase. In sacQ, sacS and sacU are located either mutations increasing the level of synthesis of levansucrase specifically (sacQ h , sacS h , sacU h ) or mutations abolishing specifically the synthesis of levansucrase (sacU − ). sacA, sacS and sacT map to the left of purA16. sacQ is located to the left of thr5, sacB and sacR between cysB3 and hisA1 and sacU between uvr1 and gtaB.

Noctilucent Clouds in Daytime: Circumpolar Particulate Layers Near the Summer Mesopause

Abstract: Observations with a horizon scanning airglow photometer on OGO-6 have revealed the presence of a dense scattering layer near 80 km over the geographic pole during the local summer. The layer is detected on all satellite passes above 80° latitude beginning 15 days before the solstice. The optical depth of the layer increases by more than a factor of 50 between 70° and 85°. It is suggested that noctilucent clouds are weak sporadic manifestations of these persistent polar layers.

Acetylcholine compartments in stimulated electric organ of Torpedo marmorata

Abstract: Resume— L'acetylcholine ‘libre’ et ‘liee’ a ete mesuree avant et apres stimulation de l'organe electrique de la Torpille in vivo ou in vitro, sur des tranches de tissu incubees dans une solution physiologique. Des potentiels de plaque miniatures peuvent etre enregistres. Des deharges d'une amplitude atteignant 30 V ont ete obtenues apres stimulation du tissu. La reponse electrique decroit rapidement lors de stimulations repetitives. L'acetylcholine ‘libre’ est celle qui est hydrolysee en cours d'homogeneisation par les esterases du tissu, l'acetyl-choline ‘liee’ ne l'est pas. Dans l'homogenat, cette derniere est associee aux vesicules synaptiques. L'acetylcholine ‘libre’ est mesuree par la difference entre l'acetylcholine totale et l'acetylcholine ‘liee’. Une baisse d'acetylcholine ‘libre’ est mise en evidence lorsque la reponse electrophysiologique est epuisee par la stimulation. La chute d'acetylcholine ‘libre’ est, comme l'amplitude de la deharge, dependante du rapport Ca2+/Mg2+. Dans d'autres experiences une diminution des deux compartiments a ete notee a la fin de la periode de stimulation. Cependant nous avons aussi observe successivement la chute de l'acetylcholine ‘libre’ puis celle de l'acetylcholine ‘liee’ lorsque la stimulation est poursuivie. En presence d'inhibiteurs d'esterase on peut recueillir de l'acetylcholine liberee par stimulation dans la solution physiologique. Une preincubation des tissus avec de la [14C]choline permet de marquer les compartiments d'acetylcholine. La radioactivite speifique varie dans le compartiment ‘libre’ apres une serie de stimulations, alors qu'elle reste stable dans le compartiment ‘lie’. Ces resultats indiquent que l'acetylcholine ‘libre’ represente un compartiment immediatement disponible, libere quand on stimule le tissu. Abstract— Changes in ‘free’ and ‘bound’ acetylcholine before and after stimulation have been investigated in vivo and in slices of electric organ of Torpedo marmorata incubated or superfused with physiological saline solutions. Spontaneous miniature end-plate potentials could be recorded and on electrical stimulation discharges of up to 30 V could be elicited. The electrical response fell off rapidly on repetitive stimulation. ‘Bound’ acetylcholine is that which relhains after the tissue has been homogenized since any ‘free’ acetylcholine is hydrolysed by the esterases when the tissue is disrupted. ‘Free’ acetylcholine can therefore be determined as the difference between the total acetylcholine found when the tissue is extracted with trichloroacetic acid and that which remains when the tissue is homogenized. Most of the ‘bound’ acetylcholine is present in synaptic vesicles. Stimulation of the tissue until the electrical response had fallen was accompanied by a drop in the level of ‘free’ acetylcholine. Lowered calcium and increased magnesium concentrations in the medium caused a decrease in the electrical response to stimulation and a decrease in the fall of ‘free’ acetylcholine. In other experiments, a decrease of both compartments was noticed at the end of the stimulation period. However the drop in ‘bound’ acetylcholine could also be elicited after the ‘free’ had fallen, by continuing the stimulation. When anticholinesterases were put in the medium, acetylcholine released on stimulation could be collected. On pre-incubation of the slice with [14C]choline, the acetylcholine stores became labelled. The specific radioactivity of the ‘free’ acetylcholine fluctuated on serial stimulations, whereas the specific radioactivity of the ‘bound’ acetylcholine remained stable under these experimental conditions. It is concluded that the ‘free’ compartment of acetylcholine is the most immediately available for release on stimulation.

Equations aux derivees partielles stochastiques non lineaires

Abstract: Le but de cet article est d’etudier les equations d’evolution non lineaires monotones a entrees stochastiques du type suivant: $$\frac{{dy}}{{dt}}\left( {t; } \right) + A\left( t \right)y\left( {; } \right) = g \left( {t; } \right) + \frac{{df}}{{dt}}\left( {t; } \right)$$ ouA(t) est une famille d’operateurs monotones d’un espace de BanachV dans son dualV′. Il s’agit d’une generalisation a des equations aux derivees partielles des travaux de Ito sur les equations differentielles stochastiques.

Molecular electrostatic potentials: Comparison of ab initio and CNDO results

Abstract: The possibilities of utilization of CNDO wave functions for computing molecular electrostatic potentials are studied by comparison with ab initio results for H2O and H2CO.

Left-Right Asymmetry in the Scattering of Electrons by Magnetic Impurities, and a Hall Effect

Abstract: Left-right asymmetry is expected in the scattering of electrons by magnetic impurities in metals; we present a simple calculation of the asymmetry in a virtual-bound-state model. When a magnetic field lines up the impurity moments, this left-right asymmetry should generate an extraordinary Hall effect proportional to the impurity concentration. This effect appears to occur in many magnetic alloys (e.g., ${\mathrm{Ni}}^{\ensuremath{-}}$, ${\mathrm{Cu}}^{\ensuremath{-}}$, and ${\mathrm{Au}}^{\ensuremath{-}}$ based alloys).

Membrane potential and slow inward current dependence of frog cardiac mechanical activity.

Abstract: Both contractile response and ionic currents are recorded during voltage clamp experiments on frog atrial trabecles. In Ringer solution, tension elicited by depolarizing steps of different duration and amplitude may be considered as composed of two elements. One depends on the slow inward current; the estimated variation in [Ca++]i can account for the tension elicited. At its level, the competition between Ca and Na ions is manifest. The other component, depending on the membrane potential, exists even in absence of external Ca ions. Such a component of mechanical response suggests an intracellular source of activator-calcium and these ions can be displaced by membrane potential. Moreover, both this component and the absence of threshold in the tension development agree with the fact that the membrane potential controls the diastolic tension. These two components share in the contraction elicited by action potential.

Aspects of the Localizability of Electrons in Atoms and Molecules: Loge Theory and Related Methods

Abstract: Publisher Summary This chapter presents the aspects of the localizability of electrons in atoms and molecules. The focus is on the loge theory. The origin of the concept is analyzed in the first followed by mathematical definitions. Various methods are proposed to obtain an approximation of the best decomposition in loges, starting from wave functions in which the localizability of electrons is not introduced a priori. Relations with localized orbitals are discussed. On the contrary, it is shown that the loge theory can be used to build new kinds of wave functions or to improve approximate wave functions. Relations between loge functions and group functions are analyzed. The chapter also presents various applications of the loge theory for stationary states. It is shown that this theory leads to a criterion that makes it possible to distinguish from a mathematical viewpoint between covalent and dative bonds. Furthermore, a theoretical justification of empirical molecular additivity is given. A very simple procedure to establish general relations between isomerization energies is derived. The chapter also deals with the possibility of extending the localization concept to electronic excitation. It is shown that under certain conditions the excitation energy may be localized in loges. More generally, excited-state properties may be expressed as contributions from loges.