Configuration mixing involving σ and π orbitals

The effect (on the energy of the different states) of including doubly excited configurations in a - plus -configuration interaction treatment, is studied within the CNDO/2 framework. For moderately large molecules the problem of the choice of the type ( or ) of configurations taken into account is examined. When possible, comparison is made with similar non empirical calculations.

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Zusammenfassung Im Rahmen der CNDO/2 Methode wird der Einfluß (auf die Energien verschiedener Zustände) des Einschlusses doppelt angeregter Konfigurationen in einer - und -Konfigurationswechsel-wirkungs-Behandlung studiert. Für nicht allzu große Moleküle wird das Problem der Wahl der Art ( oder ) der berücksichtigten Konfigurationen untersucht. Soweit möglich, werden die Resultate mit denen ähnlicher nichtempirischer Rechnungen verglichen.

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Résumé L'effet (sur l'énergie des différents états) de l'introduction de configurations diexcitées dans une interaction de configuration plus , est étudié dans le cadre de la méthode CNDO/2. Pour des molécules de taille moyenne le problème du choix du type ou des configurations est examiné. Les résultats obtenus sont comparés aux calculs non-empirique similaires disponibles.

     

Energy flow and multiplicity measurement in ultra-relativistic nucleus-nucleus collisions

Experimental results from the HELIOS set-up (High Energy Lepton and Ion Spectrometer, in operation at the CERN Super Proton Synchrotron to study high-energy nucleus-nucleus collisions) are presented and discussed. The transverse energy differential cross section can be parametrized by a geometrical model. The energy density reached in these collisions is much larger than that inside the nucleon and is in the region of the critical value 2·5 GeV/fm³ obtained from lattice QCD calculations for the deconfinement transition. The transverse momentum spectra of photons agree in shape and absolute cross section with the photon spectra expected from hadronic sources.Presented at the IIIrd International Symposium High Energy Experiments And Methods (HEXAM '89), Bechyn, Czechoslovakia, June 25–30, 1989.I would like to thank the organizers of the HEXAM'89 Symposium in Bechyn for giving me the opportunity to participate in this very pleasant and stimulating conference and the hospitality they have extended. This work was supported in part by the National Sciences and Engineering Research Council of Canada (NSERC).     

Enantiomeric analysis of baclofen analogs by capillary zone electrophoresis, using highly sulfated cyclodextrins: Inclusion ionization constant pKa determination

Using cyclodextrin-capillary zone electrophoresis (CD-CZE), baseline separation of baclofen phaclofen, saclofen, and hydroxy-saclofen, potent gamma-aminobutyric acid(B) (GABA(B)) agonist or antagonists was achieved. A method for the enantioresolution of those analogs of GABA was developed using anionic cyclodextrins (highly sulfated CD or highly S-CD) as chiral selectors and capillaries dynamically coated with polyethylene oxide (PEO). With charged CDs we observed good resolutions due to the large electrophoretic mobility of these chiral selectors opposite to the mobility of the solutes. The highly S-alpha-CD and S-beta-CD were found to be complementary and the most effective complexing agent, allowing good enantiomeric resolution in short runtimes. The complete resolution was obtained using 25 mM phosphate buffer at pH 2.5 containing 3% w/v of highly S-alpha-CD or S-beta-CD at 25 degrees C with an applied field of 0.30 kV/cm. The apparent binding constants of the inclusion complexes were evaluated and the migration order was determined. A comparison was possible to investigate the importance of the anionic group of the molecules in the separations. The pK(a) values were determined for all four compounds in order to explain relative electrophoretic migration of the solutes.     

Versatile reactions of a para-bromophenylacetylide iron(II) derivative and X-ray structure of the fluoro analogue. Synthesis of new redox-active organoiron(II) synthons

The synthesis of the new (η²-dppe)(η⁵-C₅Me₅)Fe---CC---1,3-(C₆H₄X) (m-2a/2b; X=F/Br) and (η²-dppe)(η⁵-C₅Me₅)Fe---CC---1,4-(C₆H₄I) (2c) complexes, as well as the solid-state structure of the known (η²-dppe)(η⁵-C₅Me₅)Fe---CC---1,4-(C₆H₄F) (2a) complex are described. The catalytic coupling reactions of the bromo complexes with various alkynes were next investigated. Starting from the known (η²-dppe)(η⁵-C₅Me₅)Fe---CC---1,4-(C₆H₄Br) complex (2b), the synthesis of the (η²-dppe)(η⁵-C₅Me₅)Fe---CC---1,4-(C₆H₄)---CC---H complex (6d) and of the corresponding silyl-protected precursors (η²-dppe)(η⁵-C₅Me₅)Fe---CC---1,4-(C₆H₄)CC---SiR₃ (6b/6c; R=ⁱPr/Me) are reported. By use of lithium---bromine exchange reactions on 2b, the silyl- (7a; E=Si; R=Me) and tin- (7b–7d; E=Sn; R=Me, Bu, Ph) substituted analogues (η²-dppe)(η⁵-C₅Me₅)Fe---CC---1,4-(C₆H₄)ER₃ are also isolated. The spectroscopic and electrochemical characterisations of all these new Fe(II)/Fe(III) redox-active building blocks are presented and the electronic substituent parameters for the “(η²-dppe)(η⁵-C₅Me₅)Fe---CC” group are determined by means of ¹⁹F-NMR.     

Spektrophotometrische Bestimmung von Nickel und Kobalt nebeneinander mit N,N'-Bis-(o-aminobenzyliden)-?thylendiamin

Zusammenfassung N,N'-Bis-(o-ammobenzyliden)-äthylendiamin (ABAD) bildet mit Ni²⁺ und Co³⁺ stabile Chelate, wobei schon durch Luftsauerstoff eine Oxydation von Co^IIzu Co^III-ABAD erfolgt. Ni-ABAD kann quantitativ mit Benzol extrahiert und spektrophotometrisch bei 486 nm bestimmt werden. Co^III-ABAD wird in der wäßrigen Schicht bei 430 nm bestimmt. Die Grenzkonzentrationen betragen 0,5 g Ni/ml und 0,6 g Co/ml in Konzentrationsverhältnissen von 1001 bis 1100. Die Methode ist ebenfalls zur quantitativen Bestimmung von Co neben Cu geeignet.

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Summary N,N'-Bis-(o-aminobenzylidene)-ethylenediamine (ABAD) quantitatively reacts with Ni²⁺ and Co³⁺ to form stable chelates. Co^II-ABAD is easily oxidised by air to Co^III-ABAD. Ni-ABAD can be extracted with benzene and determined spectrophotometrically at 486 nm, Co^III-ABAD is determined in the water-layer at 430 nm. The limit of detection is 0.5 g Ni/ml and 0.6 g Co/ml by the ratio from 1001 to 1100. The method is also suitable for separation and determination of Co in the presence of Cu.

     

Kinetic and thermodynamic acidities of substituted 1-benzyl-1-methoxy-2-nitroethylenes. Strong reduction of the transition state imbalance compared to other nitroalkanes

Acidity constants of six substituted 1-benzyl-1-methoxy-2-nitroethylenes (2-Z with Z = m-NO(2), m-CF(3), m-Cl, H, p-Me, p-MeO) and their respective nitronic acids were determined in 50% DMSO-50% water (v/v) at 20 degrees C. Kinetic data were obtained on the reversible deprotonation of all six 2-Z by OH(-) and piperidine and on the reversible deprotonation of 2-NO(2)() by piperazine, 1-(2-hydroxyethyl)piperazine, and morpholine in the same solvent. These data allowed a determination of the Br?nsted coefficients alpha (dependence on acidity of 2-Z) and beta (dependence on amine basicity). The fact that alpha > beta indicates the presence of a transition state imbalance which, however, is much smaller than that for the deprotonation of arylnitromethanes. The reasons for this reduction in the imbalance and their relevance to a recent study of the deprotonation of Fischer carbene complexes are discussed.     

Complexation en solution aqueuse des ions magnesium(II), manganese(II), nickel(II), cuivre(II) et plomb(II) avec la s-carboxymethyl-l-cysteine: Complex formation of magnesium(II), manganese(II), nickel(II), copper(II) and lead(II) with S-carboxymethyl-L-cysteine in aqueous solution

Complex formation of magnesium(II), manganese(II), nickel(II), copper(II) and lead(II) with S-carboxymethyl-L-cysteine in aqueous solution.The complex formation between Mg(II), Mn(II), Ni(II). Cu(II), Pb(II) ions and S-carboxy-methyl-l-cysteine (H₂A) has been studied by measurement of pH at 25°C and constant ionic strength (1 M NaClO₄). Although no interaction occurs with Mg(II), this work provides evidence for a variety of complexes: MnA; CuHA⁺; CuA; CuA₂^2-; NiHA⁺; NiA; NiA₂^2-; PbHA⁺; PbA et PbA(OH)^-. The overall formation constants of all these species are computed and refined. The results allow the determination of the distribution of the complexes as a function of pH; some structural features of the metal complexes in solution are indicated.     

Molecular simulation of the homogeneous crystal nucleation of carbon dioxide

We report on a molecular simulation study of the homogeneous nucleation of CO2 in the supercooled liquid at low pressure (P = 5 MPa) and for degrees of supercooling ranging from 32% to 60%. In all cases, regardless of the degree of supercooling, the structure of the crystal nuclei is that of the Pa3 phase, the thermodynamically stable phase. For the more moderate degree of supercooling of 32%, the nucleation is an activated process and requires a method to sample states of high free energy. In this work, we apply a series of bias potentials, which promote the ordering of the centers of mass of the molecules and allow us to gradually grow crystal nuclei. The reliability of the results so obtained is assessed by studying the evolution of the nuclei in the absence of any bias potential, and by determining their probability of growth. We estimate that the size of the critical nucleus, for which the probability of growth is 0.5, is approximately 240 molecules. Throughout the nucleation process, the crystal nuclei clearly exhibit a Pa3 structure, in apparent contradiction with Ostwald's rule of stages. The other polymorphs have a much larger free energy. This makes their formation highly unlikely and accounts for the fact that the nucleation of CO2 proceeds directly in the stable Pa3 structure.     

Structure de la gouregine,alcaloïde orginal apparente aux cularines

The structure of guoregine, a new isoquinoline alkaloid from $\text{G}$$\text{u}$$\text{a}$$\text{t}$$\text{t}$$\text{e}$$\text{r}$$\text{i}$$\text{a}$$\text{o}$$\text{u}$$\text{r}$$\text{e}$$\text{g}$$\text{o}$$\text{u}$, Annonaceae, has been deduced by spectral analysis and confirmed by an X-ray structure determination. It is the first member of a new class of cularine-related alkaloids (α-$\text{g}$$\text{e}$$\text{m}$-dimetyltetradehydrocularines).     

Etude quantitative de la distinction entre une liaison de covalence et une liaison de coordination: les molecules de borazane et d'aminoborane

Résumé On propose un exemple quantitatif de la distinction entre une liaison de covalence et une liaison de coordination; cette différenciation, fondée sur les propriétés de la molécule une fois formée, semble très nette pour les énergies de liaison, les populations de recouvrement et les charges des groupes; en particulier, un critère récent, proposé par deux des auteurs, est remarquablement vérifié. Enfin, on essaie de préciser la signification des symboles chimiques traditionnels.

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Quantitative studies of the difference between a covalence and a coordination bond: The molecules of amine-borane and aminoborane

An illustration of the distinction between these two types of chemical bonds is proposed, which is based on the properties of the molecule once built. Bond energies, overlap populations and group charges vary considerably from one compound to the other; more, a recent criterion given by two of the authors seems to be particularly suitable for describing the character of the bond. Lastly, one tries to give an explicit signification to the usual chemical symbols.

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Zusammenfassung Es wird ein quantitatives Beispiel der Unterscheidung zwischen einer kovalenten und einer koordinativen Bindung angegeben, die auf den Eigenschaften des vorliegenden Moleküls beruht. Sie erscheint sehr günstig für die Bindungsenergien, die Überlagerungspopulationen und die Gruppenladungen. Insbesondere wird ein Kriterium, das kürzlich von zwei der Autoren angegeben wurde, gut verifiziert. Schließlich wird versucht, die üblichen chemischen Symbole zu präzisieren.

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Les auteurs tiennent à remercier très vivement M. M. Berthier et Millié d'avoir mis à leur disposition le programme de localisation indispensable à ce travail, ainsi que Mademoiselle Le Guen et le C.I.R.C.E. pour la réalisation des calculs sur l'ordinateur IBM 360-75.