Direct coulomb and exchange interaction in artificial atoms

We determine contributions from the direct Coulomb and exchange interactions to the total interaction in artificial semiconductor atoms. We tune the relative strengths of the two interactions and measure them as a function of the number of confined electrons. The electrons tend to have parallel spins when they occupy nearly degenerate single-particle states. We use a magnetic field to adjust the single-particle-state degeneracy, and find that the spin configurations in an arbitrary magnetic field are well explained in terms of two-electron singlet and triplet states.     

Evidence for multiphonon giant resonances in electromagnetic fission of 238U

Differential cross sections for electromagnetic fission of 238U projectiles (500 MeV/u) in C, Sn, and Pb targets are measured and analyzed in terms of single- and multiphonon giant resonance excitations as doorway states to fission. A novel experimental method exploits the linear relationship between neutron multiplicity and the primary 238U excitation energy. Multiphonon states contribute up to 20% of the cross section; a component at high excitation energies is indicated that may arise from three-phonon dipole and two-phonon GDR x GQRiv giant resonance excitations.     

Enhanced near-neighbour aggregation of dichloro- and trichloromethane in liquid methane solution from molecular dynamics simulation of radial site–site distribution functions: a simple predictor of solute–solvent compatibility

Molecular dynamics simulations have been performed for a range of equi-site and site–site radial distribution functions for the five-atom halomethane species dichloro-, trichloro-, and tetrachloromethane dissolved in the low-molecular weight hydrocarbons liquefied methane and cyclopropane, with the general aim of using this approach to predict good or bad solvent characteristics. It was found that methane solutions of dichloro- and trichloromethane showed an enhancement of near-neighbour occupancy, the methane solvent seemingly exhibiting a phobic, structure-promoting solvation behaviour towards the two solutes by increasing the number of nearest neighbours above the values that would result from a pure dilution effect caused by the solvent. It was verified that there were no significant regions of solid-like conformations nor remnants of imperfect average homogeneity within the system at the necessarily low temperature (183?K). On the other hand, simulated site–site radial functions with solvents tetrachloromethane and cyclopropane indicate normal solution characteristics towards solutes dichloro- and trichloromethane. The cause of the phobic solvation behaviour of solvent liquid methane towards di- and trichloromethane is not obvious, except that it seemingly involves the presence of hydrogen atoms on the solute species because the site–site centre-of-mass radial distribution functions of tetrachloromethane in liquid methane implied normal solution behaviour.     

Top-down MS for rapid methionine oxidation site assignment in filgrastim

Protein therapeutics have emerged as a major new class of pharmaceuticals. One important shelf-life-limiting factor of biopharmaceuticals is methionine oxidation, and therefore, it is important that analytical methods are able to thoroughly characterize all possible oxidized variants. Here, we present a fast and sensitive method to perform online methionine oxidation site assignment using granulocyte colony-stimulating factor (filgrastim) as a model. The method is based on top-down MS using the all-ion fragmentation mode of the Exactive benchtop mass spectrometer. Conditions that provide information on the intact mass of the protein as well as on fragment ions that allow unambiguous site assignment of methionine oxidation in filgrastim variants as low as 0.12 % of total peak area in a chromatographic time scale were identified. Using this method, we performed methionine oxidation site assignment in H₂O₂-stressed filgrastim and in filgrastim which was stored at intended conditions, respectively. We show that the relative abundance of oxidation species observed in filgrastim stored under intended conditions differs strikingly from the oxidized species observed after H₂O₂ stress. Additionally, we report an oxidized filgrastim variant that has not been previously described in the literature.

Extremal Ternary Self-Dual Codes Constructed from Negacirculant Matrices

In this paper, a construction of ternary self-dual codes based on negacirculant matrices is given. As an application, we construct new extremal ternary self-dual codes of lengths 32, 40, 44, 52 and 56. Our approach regenerates all the known extremal self-dual codes of lengths 36, 48, 52 and 64. New extremal ternary quasi-twisted self-dual codes are also constructed. Supported by an NSERC discovery grant and a RTI grant. Supported by an NSERC discovery grant and a RTI grant. A summer student Chinook Scholarship is greatly appreciated.     

EA-Massenspektren Cyansubstituierter Heterocyclen

The negative ion mass spectra of cyano substituted heterocyclic compounds show specific fragmentation pathways which are of analytical importance for the distinction between isomeric compounds. All the compounds examined fragment directly to give or ions. The ion structures involved were determined by the analysis of the metastable transitions and collisional activation spectra. The fragmentations of tetracyano substituted heterocyclic compounds are explained by rearrangement processes of the molecular anions.     

MASSENSPEKTREN HETEROAROMATISCHER NITRILE

The electron-impact and field ionisation spectra of nineteen di- and tetracyanopyrazines are presented. The use of high resolution mass measurement and metastable analysis allows the proposal of some general fragmentation mechanisms. The 5,6-disubstituted (alkyl or arene) 2,3-dicyanopyrazines examined show, apart from a less important loss of the nitrile and dicyano groups, fragmentation of the heterocyclic system directed by the nitrogen atoms. In contrast, anellation in the 5,6-position, leads only to loss of the nitrile and dicyano groups being observed. These general fragmentation mechanisms are also observed for the tetracyanopyrazines examined. A comparison of di- and tetracyanobenzene with five ring heterocycles containing four cyano groups shows that fragmentation of nitrile groups is greatly dependent upon the aromaticity of the cyclic system.