Dephasing calculations of de Haas-van Alphen scattering

A review and assessment of existing dephasing calculations is given.     

Spin-trapping of ligated boryl radicals

Amine- and phosphine-boryl radicals add to 2-methyl-2-nitrosopropane, 2,4,6-tri-t-butylnitrosobenzene, and phenyl-N-t-butylnitrone to give nitroxide spin-adducts and the application of these spin-trapping reactions as mechanistic probes is described.     

New cesium titanate layer structures

A phase study of the Cs₂OTiO₂ system in the composition range 75–100 mole% TiO₂ and the temperature range 850–1200°C revealed the existence of two new cesium titanates, with compositions Cs₂Ti₅O₁₁ and Cs₂Ti₆O₁₃. The former compound undergoes a reversible hydration reaction below 200°C to form Cs₂Ti₅O₁₁ · (1 + x)H₂O, 0.5 < x < 1. The structures of the three phases have been determined. They are based on corrugated layers of edge-shared octahedra, with cesium ions (and H₂O) packing between the layers. In Cs₂Ti₆O₁₃, the layers are continuous in two dimensions, whereas in Cs₂Ti₅O₁₁ and Cs₂Ti₅O₁₁ · (1 + x)H₂O, the layers are periodically stepped to give 5-octahedra wide, corner-linked ribbons.     

The effect of ionicity and hybridization on the calculation of spectroscopic coupling parameters and dipole moments

Calculations have been carried out in order to calculate thes-character and ionicity of the carbon-halogen bonds in eight halomethanes from experimental values of (1)¹³C-H couplings, (2) nuclear quadrupole couplings, and (3) dipole moments. Two sets of results were obtained based on the use, first, of (1) and (2) and, second, on the use of (1) and (3). The two sets of results show a reasonable degree of consistency, under the assumptions and methods involved, and tend to support the need for considering both hybridization and charge transfer in accounting for substituent effects on the properties studied.

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Zusammenfassung Es wurden Rechnungen zur Bestimmung dess- und des Ionencharakters von Kohlenstoff-Halogen-Bindungen in acht Halogenmethanen aus den experimentellen Daten von (1)¹³C-H-Koppelungen, (2) Kernquadrupolkoppelungen und (3) Dipolmomenten durchgeführt. Zwei Sätze von Ergebnissen wurden erhalten, die auf der Benutzung von (1) und (2) bzw. (1) und (3) basieren. Die Ergebnisse der beiden Rechnungen sind im Rahmen der Annahmen und Methoden konsistent und ergeben die Notwendigkeit, sowohl Hybridisierung als auch Ladungsübergang bei der Betrachtung von Substitutionseffekten auf die zu untersuchenden Eigenschaften zu berücksichtigen.

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Résumé On a calculé le caractères et l'ionicité des liaisons carbone-halogène dans huit halogénométhanes à partir des valeurs expérimentales (1) des couplages¹³C-H, (2) des constantes de couplage quadrupolaire nudéaire (3), des moments dipolaires. Deux ensembles de résultats ont été obtenus à partir de (1) et (2) et de (1) et (3). Ces deux ensembles de résultats montrent un degré raisonnable de consistance, étant données les hypothèses et les méthodes utilisées, et sont en faveur d'une intervention simultanée de l'hybridation et du transfert de charge pour rendre compte des effets de substituants sur les propriétés étudiées.

     

A combined LEED/RHEED Auger study of oxygen adsorption on the W(112) surface

LEED, RHEED and Auger spectroscopy have been used to study the adsorption of oxygen on to a clean and carbon contaminated (112) face of tungsten. At room temperature all the features reported previously were observed together with a p(1 × 4) surface structure which appeared at an exposure of about 1. 4L just before the formation of the p(1 × 2). Previously a p(1 × 4) structure has been reported only after heating to 2000K. RHEED showed this p(1 × 4) structure clearly; using LEED, the structure was difficult to distinguish. This appears to confirm suspicions that in some situations involving gas adsorption, RHEED has a greater sensitivity than LEED. Possibly most of these situations involve, as does the present p(1 × 4) structure, monolayer islands where the differing coherence widths of the RHEED and LEED beams account for the differing sensitivities. Carbon on the (112) surface also appears to exist as thin islands, either of the previously reported c(6 × 4) structure, or in smaller amounts, on a surface showing (1 × 1) symmetry. Removal of all carbon by heat treatment alone was found to be impossible in a reasonable time and heating in oxygen was necessary. Oxygen adsorption on a carbon contaminated surface did not give rise to any new structures but rather a reduction in the visibility/formation of the clean surface/oxygen structures.     

Striking contrasts in stereoselectivities of spiro-claisen rearrangements of pyranosides versus carbocycles.

Structurally equivalent derivatives of pyranosides and cyclohexanes undergo the spiro-Claisen rearrangement with different stereochemical results. A rationalization for the course observed with the pyranosides is suggested, which invokes interaction of the oxygen lone pair with a (developing) electron deficient centre at the spiro carbon.     

The kinetics of the decarboxylation of malonic acid in normal alkanols

Rate constants and activation parameters are reported for the decarboxylation of malonic acid in seven normal alkanols (butanol-l to decanol-l inclusive). It is found that the enthalpy of activation of the reaction is a linear function of the number of carbon atoms in the hydrocarbon chain of tthe solvent, expressed by the equation: ΔH^≠ = –600n + 30,000, where n is thenumber of carbon atoms in the chain. Also an equation is developed relatingthe rate constant for the decarboxylation of malonic acid in normal alkanols to n (the number of carbon atoms in the chain): log K = 10.854283 – 0.3212674n + (131.136876n – 6556.5438)/T + log T. With the aid of this equation rate constants may be calulated for the decarboxylationof malonic acid in any alcohol at any temperature which agree with experimental values to within the limit of error of the experiments. A comparison of the data obtained in the present research for the decarboxylation of malonic acid in normal alkanols with previously reported data for the reaction in amines indicates that for reaction taking place in alcohols the transition state probably contains two molecules of solvent but only one for the reaction in amines.     

Rotational structure in the infrared spectra of carbon dioxide and nitrous oxide dimers

High-resolution infrared predissociation spectra have been measured for dilute mixtures of CO₂ and N₂O in helium. Rotational fine structure is clearly resolved for both (CO₂)₂ and (N₂O)₂, the linewidths being instrument-limited. This establishes that predissociation lifetimes are longer than approximately 50 ns.