Effects of low hydrogen partial pressures on propylene polymerization by Cp*TiMe2(μ-Me)B(C6F5)3

Polymerization of propylene by Cp*TiMe₂(μ-Me)B(C₆F₅)₃ in the presence of increasing partial pressures of H₂ results in ever decreasing polymer molecular weights, which is consistent with the hydrogenolytic chain transfer processes involving metal–polymer bonds in many heterogeneous and homogeneous systems. However, catalytic activities are not significantly increased as the extent of hydrogenolysis increases, unlike metallocene catalyst systems in which the H₂ reacts primarily with dormant catalytic sites containing propylene 2,1-insertion products. It was shown previously that monocyclopentadienyl systems do not become seriously deactivated following 2,1 insertions, and thus hydrogenolysis does not result in enhanced activities. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4386–4389, 1999     

Measurement of the caesium 6S1/2?8P1/2 transition frequency

The caesium 6S _1/2? 8P _1/2,3/2 transitions at 387.6 nm and 388.8 nm have been observed with a linewidth of <5 MHz by laser spectroscopy. The absolute frequency of the 6S _1/2?8P _1/2 transition was determined to an uncertainty of ±1 MHz using the calibrated rubidium 5S _1/2?5D _5/2 two-photon transition and accurate interferometry. The potential of this caesium transition as a useful secondary frequency standard in blue–UV region is discussed. Received: 7 February 2000 / Revised version: 24 March 2000 / Published online: 24 July 2000     

Encapsulating Knowledge: The Direct Reading Spectrometer

The direct reading emission spectrometer was developed during the1940s. By substituting photo-multiplier tubes and electronics forphotographic film spectrograms, the interpretation of special lineswith a densitometer was avoided. Instead, the instrument providedthe desired information concerning percentage concentration ofelements of interest directly on a dial. Such instruments `de-skill' the job of making such measurements. They do this by encapsulatingin the instrument the skills previously employed by the analyst,by `skilling' the instrument. This paper presents a history of thedevelopment of the Dow Chemical/Baird Associates direct reader. Thishistory is used to argue for a materialist conception of knowledge.The instrument is a material form of knowledge, knowledge of aspectsof spectroscopy, analytical spectrochemistry, electronics, instrumentdesign and construction, and metal production industry economics.     

Investigations of olefin hydrogenation catalysts. The major species present in solutions containing rhodium(I) complexes of chelating diphosphines

¹H and ³¹P NMR spectroscopy are used to determine the nature of the species present in catalytically active solutions prepared by treating [RhCl(C₂H₄)₂]₂ with diphosphines and [Rh(norbornadiene)diphosphine]BF₄ with hydrogen (diphosphine = 1,3-bis(diphenylphosphino)propane (dppp) and isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane (diop)).     

Charge density distributions and sigma bond inductive effects in hydrocarbons and hydrocarbon ions

Atomic orbital charge densities have been calculated for the electron systems of a number of hydrocarbon molecules, by an electronegativity equalization method. These electron distributions have been used to analyze the relative inductive effects of alkyl groups, the transmission of electronic effects through localized bonds, the inductive stabilization of simple hydrocarbon ions, and the dependence of the ¹H and ¹³C chemical shifts on the corresponding partial charges of the atoms. The charge densities calculated by electronegativity equalization were compared to those predicted by various molecular orbital methods.

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Zusammenfassung Mit Hilfe einer Methode, die auf der Angleichung von Elektronegativitäten beruht (electronegativity equalization), sind atomare Orbitalladungsdichten für -Elektronensysteme einer Reihe von Kohlenwasserstoffmolekülen berechnet worden. Die Ladungsdichten werden mit den Voraussagen verschiedener MO-Methoden verglichen.Diese Elektronenverteilungen werden zur Analyse der relativen induktiven Effekte von Alkylgruppen, der Weiterleitung elektronischer Effekte durch lokalisierte Bindungen, der induktiven Stabilisierung einfacher Kohlenwasserstoffionen und der Abhängigkeit der chemischen Verschiebung von ¹H und ¹³C von den partiellen Ladungen der Atome benutzt.

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Résumé Les densités de charge dans les orbitales atomiques ont été calculées pour les systèmes d'électrons d'un certain nombre de molécules d'hydrocarbure à l'aide d'une méthode d'égalisation des électronégativités. Ces distributions électroniques ont été utilisées pour analyser les effects inductifs relatifs des groupements alkyles, la transmission des effets électroniques à travers les liaisons localisées, la stabilisation par effet inductif des ions hydrocarbures simples, et la relation entre les déplacements chimiques de ¹H et ¹³C et les charges atomiques partielles correspondantes. Les densités de charges calculées ici ont été comparées à celles obtenues par différentes méthodes d'orbitales moléculaires.

     

Errata

The complexes [η⁶-C₆H₆RuCl(H₂NCHRCO₂)] (R = H,Me) are prepared by treating [η⁶-C₆H₆RuCl₂]₂ with the corresponding amino acid anion in methanol. Although the coordinated chloride is readily substituted by water in both cases, giving the species [η⁶-C₆H₆Ru(H₂O)(H₂NCHRCO₂)]⁺, ¹H and ¹³CNNMR spectroscopy readily distinguish between the diastereomers of the alanato complex (R = Me). Thus epimerization at the ruthenium is a relatively slow process.     

Palladium-catalyzed cross-coupling reactions of 2-indolyldimethylsilanols with substituted aryl halides

[reaction: see text] A mild and general cross-coupling reaction of 2-indolylsilanols has been developed. The experimental variables that lead to successful coupling are (1) the use of sodium tert-butoxide as the activator, (2) the use of copper(I) iodide in stoichiometric quantities, and (3) the use of Pd2(dba)3.CHCl3 as the catalyst. Under these conditions N-(Boc)-2-indolyldimethylsilanol reacts with a variety of aromatic iodides to afford the coupling products in good yield (70-84%).