24 research outputs found
Surface Charging of Zinc Oxide During XPS Examination
Surface charging of zinc oxide during XPS examination has
been studied. The magnitude of the charge is 0.4-0.6 eV at room
temperature and decreases at higher temperature to almost zero
(nominal 0.1 eV at 300 Ā°C) . Reproducible BE values can only be
achieved after correction for this charging. Possible mechanisms are
briefly discussed
Regiospecific Synthesis and X-ray Structural Study of Trifluoromethyl Substituted Dibenzosemibullvalene
Preparation of the novel regiospecifically trifluoromethyl substituted dibenzobarrelene 5 and dibenzosemibullvalene 6 is described. Unequivocal proof for the stereostructure of 6 was obtained by its X-ray crystallographic analysis. Geometrical data from X-ray structural analysis showed that the ester carbonyl groups adopt unfavorable conformations for conjugative interactions with respect to the cyclopropane ring involved in the skeleton of 6
Diastereoisomerization and X-ray Crystal Structure of meso-di-(2H-Chromene-2-yl) Ether
It was shown by X-ray crystal structure analysis that di-(2Hchromene-
2-yl)-ether (1) exists in the meso-form, i.e. as (2S, 2\u27R)-1. The Xray
structural data indicate, also, that geometries of the two 2H-1-benzopyran parts, of which the skeleton of 1 consists, are not identical, that is the molecule is pseudosymmetrical. The meso-l undergoes thermally induced electrocyclic ring opening and reclosure. Thus, meso-1 equilibrates with enantiomers (2S, 2\u27S)-1 and (2R, 2\u27R)-1, as conf\u27irmed by their lH_ NMR spectra after preparative separation of stereoisomers using liquid chromatography on triacetylcellulose
(1S,2S)-(+)- and (1R,2R)-(ā)-1,2-Diphenylcyclopropane from their racemic mixture by liquid chromatography on triacetylcellulose
The title compounds were isolated with 86 and 100% enantiomeric purity, respectively, and characterised by their rotations and c.d.-spectra
Molecular-mechanics calculations of the geometry and racemization energies of an N-aryl-2(1H)-quinolone and N-aryl-4-pyridones
The geometries and energies for ground and possible transition states of the quinolone (1) and pyridones (2)ā(4) were calculated by the molecular-mechanics method. The calculated energy differences between ground and the lowest transition state are in good correlation (r = 0.994) with the corresponding experimental racemization energies for interconversion of enantiomers (P) rlhar (M) in (1) ā (4). However, the calculated potential energy differences do not correspond to measured differences in Gibbs energies. The fact was tentatively attributed to neglection of the entropy contribution to Gibbs energies
Enantiomers and Barriers to Racemization of Sterically Hindered AT-Aryl- and N-Heteroarylpyrroles
The N-aryl- and N-heteroaryl-2,5-dimethylpyrrole-3-carbaldehydes 2ā8 have been synthesized by condensation of hexane-2,5-dione with the appropriate amine and subsequent Vilsmeier-Haack formylation of the pyrrole ring. Diastereomeric association complexes of the racemic pyrrole 8 were studied by pmr chemical shifts and the splittings induced by (+)-1-(9-anthryl)-2,2,2-trifluoroethanol. Enrichment of the enantiomers M and P of 2 and 6 was achieved by liquid chromatography on triacethylcellulose. The barrier to partial rotation about the C-N bond in 6 was determined by thermal racemization and its lower limits in 2 and 8 were estimated