Hydrogenic systems,frequency standards and fundamental constants

Hydrogenic (two-body) systems are the only atomic systems for which uncertainties in calculations of the energy levels approach the current state of the art in frequency measurement. This article discusses progress in the theory and measurement of transition frequencies in hydrogenic systems. These studies have relevance to the determination of fundamental constants and the testing of physical theories, especially quantum electrodynamics. A set of high accuracy calculable frequency standards could also be realized by using hydrogenic systems.     

New high-energy luminescence bands from Co2+ in ZnSe

Three sharp absorption features in the energy range 2.36–2.55 eV have been detected in the transmission spectrum of Co-diffused ZnSe, and a number of luminescence transitions originating from the lowest of these states at 2.361 eV have been observed. Photoluminescence excitation spectra prove that these are high energy excited states of the Co²⁺_Zn impurity, a conclusion confirmed by comparison of measured and predicted luminescence energies. This represents the first identification of luminescence branching from a higher excited state of a transition metal ion in any semiconductor. The sharp, weakly phonon-coupled transitions involve either intra-impurity excitation or transitions from the impurity to localised states split off from a minimum in the conduction band. The implications of these observations for the mechanism of host-impurity energy transfer and for the nature of the excited state wavefunctions are discussed.     

Crystal structure of the potent bisquinoline antimalarial agent (±)-trans-N 1,N 2-bis(7-chloroquinolin-4-yl) cyclohexane-1,2-diamine dimethanesulfonate salt hydrate in relation to its biological properties

The neighboring 4-aminoquinoline substituents of the bisquinoline (±)-trans-N ¹,N ²-bis(7-chloroquinolin-4-yl)cyclohexane-1,2-diamine dimethanesulfonate salt crystallized in the diequatorial position which is in agreement with quantum chemical calculations. The bisquinoline salt crystallized in the monoclinic C2/c space group with a = 19.328(4) Å, b = 15.618(3) Å, c = 20.382(4) Å, = 98.84(3)°. The two quinoline nitrogen atoms are protonated by the salt as predicted by calculated electrostatic surface potentials. The –5 kcal/mol isopotential surface of the bisquinoline resembles that of chloroquine, which may explain the potent inhibition of hematin polymerization by both the bisquinoline and chloroquine. The smaller HOMO/LUMO gap of the bisquinoline is consistent with its phototoxicity. The overall conformation, the ability of all nitrogen atoms to participate in hydrogen bonding, and the electrostatic interaction of the quinoline rings in the crystal packing may all aid in the determination of the binding pharmacophore of the bisquinoline.     

Direct sample fraction deposition using electrospray in narrow-bore size-exclusion chromatography/matrix-assisted laser desorption/ionization time-of-flight mass spectrometry for polymer characterization

A direct sample fraction deposition method was developed for off-line size-exclusion chromatography (SEC)/matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry. By using electrospray, the SEC eluent, together with a suitable matrix solution added coaxially, was directly deposited on the MALDI plate. Owing to the formation of very small droplets in electrospray, solvent evaporation is much faster. The fractionation volume in narrow-bore SEC, which can directly be collected in one MALDI spot, can easily be optimized in the range of a few microlitres. In addition, fairly homogeneous sample spots were obtained. The possible influence of composition variation of the SEC effluent on the analytical results using direct fraction deposition was investigated; no substantial effects were observed. The applicability of the method was demonstrated by characterizing a broad poly(methyl methacrylate) sample. Copyright 2000 John Wiley & Sons, Ltd.     

Synthesis of tetrasubstituted ozonides by the Griesbaum coozonolysis reaction: diastereoselectivity and functional group transformations by post-ozonolysis reactions

The diastereoselectivity of the Griesbaum coozonolysis reaction with O-methyl 2-adamantanone oxime and 4-substituted cyclohexanones reveals that the major tetrasubstituted ozonide isomers possess cis configurations, suggesting a preferred axial attack of the carbonyl oxide on the cyclohexanone dipolarophiles. It is evident that these tetrasubstituted ozonides are quite stable to triphenylphosphine, borohydrides, hydrazine, alkyllithiums, Grignard reagents, mercaptides, and aqueous KOH as illustrated by the synthesis of amine, alcohol, acid, ester, ether, sulfide, sulfone, and heterocycle-functionalized ozonides by a wide range of post-ozonolysis transformations.