Some unexpected interferences in flames

A series of interferences has been observed in emission and absorption for rare earth elements in the nitrous oxide-acetylene flame and for some first row transition elements with air and acetylene. Signal changes, mostly suppressions, are observed at low (ppm) concomitant levels with the degree of interference consistently showing an abrupt slope change at mole ratios of concomitant to analyte element of between 1-1 and 2-1. The effects are observed for high boiling, oxygen containing mineral acids and for many organic compounds which contain oxygen and which decompose instead of vaporizing when heated. Some non-volatile, nitrogen containing compounds also give suppressions but here the behavior is less consistent. The effects are very sensitive to flame richness and often can be overcome by proper fuel to oxidant ratios. Also, the effects are much more severe when only analyte and concomitant are present in contrast to solutions containing other salts, i.e. a sample matrix. Though we cannot yet offer a complete explanation for the suppressions, definite stoichiometry is indicated in reactions involving the solid phase after desolvation and prior to vaporization.     

Rotational spectra and hyperfine interactions of the mono 17O substituted ozones

The rotational spectra of ¹⁷O¹⁶O¹⁶O and ¹⁶O¹⁷O¹⁶O have been observed. The hyperfine structure has been analyzed to give the ¹⁷O quadrupole coupling and spin-rotation tensors for both species. Preliminary values of the molecular parameters are given.     

Insulating ferromagnetism in La4Ba2Cu2O10: an Ab initio wannier function analysis

Microscopic mechanisms of the puzzling insulating ferromagnetism of half-filled La4Ba2Cu2O10 are elucidated with energy-resolved Wannier states. The dominant magnetic coupling, revealed through evaluated parameters (t, U, and J), turns out to be the intersite direct exchange, a currently ignored mechanism that overwhelms the antiferromagnetic superexchange. By contrast, the isostructural Nd4Ba2Cu2O10 develops the observed antiferromagnetic order via its characteristics of a 1D chain. Surprisingly, the in-plane order of both cases is not controlled by coupling between nearest neighbors. An intriguing pressure-induced ferromagnetic to antiferromagnetic transition is predicted.     

Hyperfine heat capacity of metallic europium and erbium

The specific heats of the Rare Earth metals Eu and Er have been measured in the temperature range 0.03 to 0.8K. The measurements yield magnetic hyperfine parameters of 13.2 mK, 5.85 mK, and 42.3 mK for ¹⁵¹Eu, ¹⁵³Eu, and ¹⁶⁷Er, respectively, corresponding to magnetic hyperfine fields of 0.26 and 7.2 MOe for Eu and Er. The nuclear quadrupole coupling constants were found to be ?0.06 mK, ?0.16 mK, and ?2.7 mK for ¹⁵¹Eu, ¹⁵³Eu and ¹⁶⁷Er.     

Hollow-core dendrimers revisited

In revisiting the original argument of de Gennes and Hervet [J. Phys. (Paris) 44, L351 (1983)]] leading to "hollow-core" dendrimers, we show that a self-consistent application of their model leads to the "filled-core" model first elucidated by Lescanec and Muthukumar [Macromolecules 23, 2280 (1990)]]. The monomer density falls off parabolically from the center of the dendrimer, and the tips are distributed throughout the molecule.     

Origin of spontaneous electric dipoles in homonuclear niobium clusters

Surprisingly large, spontaneous electric dipole moments recently observed in homonuclear niobium clusters below 100 K are explained using first-principles electronic structure calculations. The calculated moments for Nb(n) (n < or =15) generally agree with the experimental data. A strong correlation is found between the geometrical asymmetry of the cluster and electric dipole: its magnitude is proportional to the spread in the principal moments of inertia and its direction aligns with the axis of the largest principal moment. Charge deformation densities reveal directional, partially covalent bonds that stabilize structural asymmetry. Classical simulations of the deflection of a cluster in a molecular beam reveal that the electronic dipole may persist at higher temperatures, but is masked by the rotational dynamics of the cluster.     

Optimizing the size and configuration of combinatorial libraries

This paper addresses a major issue in library design, namely how to efficiently optimize the library size (number of products) and configuration (number of reagents at each position) simultaneously with other properties such as diversity, cost, and drug-like physicochemical property profiles. These objectives are often in competition, for example, minimizing the number of reactants while simultaneously maximizing diversity, and thus present difficulties for traditional optimization methods such as genetic algorithms and simulated annealing. Here, a multiobjective genetic algorithm (MOGA) is used to vary library size and configuration simultaneously with other library properties. The result is a family of solutions that explores the tradeoffs in the objectives. This is achieved without the need to assign relative weights to the objectives. The user is then able to make an informed choice on an appropriate compromise solution. The method has been applied to two different virtual libraries: a two-component aminothiazole library and a four-component benzodiazepine library.