Dynamic NMR study of the flexibility of 2-amino-3-aroyl-4,6-diaryl-pyrylium salts

Dynamic NMR investigations of a number of 2-amino-3-aroyl-4,6-diaryl-pyrylium salts were carried out. The barrier to rotation of the partial C, N double bond was determined and proved to be in the range of 62 to 63 kJ/mol. Quantum chemical calculations of bond orders and electron densities of the different atoms in the molecules show the distinct double bond character of the exocyclic C, N bond. This is in agreement with the relatively high barrier to rotation. By quantum chemical ab initio 3-21G calculations, the dynamic behaviour of this kind of compounds was simulated; two pairs (image and mirror image) of ground state conformations, in coincidence with the experiment, were obtained. Received: 10 May 1996 / Revised: 1 July 1996 / Accepted: 4 July 1996     

Synthesis and conformational analysis of naphth[1′,2′:5,6][1,3]oxazino[3,2-c][1,3]benzoxazine and naphth[1′,2′:5,6][1,3]oxazino[3,4-c][1,3]benzoxazine derivatives

A new functional group, the hydroxy group, was inserted into a Betti base by reaction with salicylaldehyde, and the naphthoxazine derivatives thus obtained were converted by ring-closure reactions with formaldehyde, acetaldehyde, propionaldehyde or phosgene to the corresponding naphth[1′2′:5,6][1,3]oxazino[3,2-c][1,3]benzoxazine derivatives. Further, the conformational analysis of these polycyclic compounds by NMR spectroscopy and an accompanying molecular modelling are reported; especially, both quantitative anisotropic ring current effects of the aromatic moieties in these compounds and steric substituent effects were employed to determine the stereochemistry of the naphthoxazinobenzoxazine derivatives.     

Electronic influences on 3J(C,H) coupling constants via -S-, -S(O)- and -SO2-: their determination, calculation and comparison of detection methods

3J(C,H) coupling constants via a sulfur atom in two series of compounds, both including a sulfide, a sulfoxide and a sulfone, were detected experimentally and calculated by quantum mechanical methods. In the first series (1-3) the coupling between a hydrogen, bonded to an sp3 carbon, and an sp2 carbon is treated; the second series (4-6) deals with the coupling between a hydrogen, bonded to an sp3 carbon, and an sp3 carbon. Different pulse sequences (broadband HMBC, SelJres, 1D HSQMBC, J-HMBC-2, selective J-resolved long-range experiment and IMPEACH-MBC) proved to be useful in determining the long-range 3J(C,H) coupling constants. However, the dynamic behaviour of two of the compounds (4 and 6) led to weighted averages of the two coupling constants expected (concerning equatorial and axial positions of the corresponding hydrogens). DFT calculations proved to be useful to calculate not only the 3J(C,H) coupling constants but also the different contributions of FC, PSO, DSO and SD terms; the calculation of the Fermi contact term (FC) was found to be sufficient for the correct estimation of 3J(C,H) coupling constants.     

Untersuchung dünner Isolatorschichten in Sandwich-Anordnungen mit dem Elektronenspiegel-Mikroskop

Characteristic image structures observed by investigations of Au–SiO_x–Ge sandwich systems using mirror electron microscopy are described. These image structures are caused by inhomogeneities in the conductivity of the insulating layer. An estimation of the dimension and the power of these inhomogeneities by the evaluation of the experimental results is possible.     

Comparative characterization of boron-implanted silicon after pulse laser annealing along single traces by SEM (SE,EBIC) and TEM studies

(111) oriented n-type silicon wafers which were implanted with 2 × 10¹⁵ B⁺/cm² at 77 K and at an energy of 60 keV are laser annealed (Nd:YAG pulse laser) in air for mean laser powers between P = 1.8 W and P = 4.0 W. The comparative application of scanning electron microscopy (SE, EBIC) and high voltage transmission electron microscopy (HVTEM, 1000 keV) allows an estimation of the annealing quality in separated lines without overlapping. Values specifically ascertained by SEM in the SE contrast mode for the annealing threshold power P _th, the power range for optimal annealing ΔP ₀ and for the geometric width of the monocrystalline resolidification perpendicular to the direction of the specimen movement (and related to the spot diameter) at different laser pulse powers P agree completely with the results found in EBIC. At an optimal laser pulse power of P ₀ = 3.2 W HVTEM enables monocrystalline epitaxially resolidified areas free of defects to be identified in the same geometric width as that ascertained by SE and EBIC.     

Structure investigations of the Ge-GeO2 system by high-resolution electron microscopy

The crystal structure of thermally oxidized Ge was investigated by high-resolution electron microscopy (HREM), mainly the interface Ge/oxide. Under special conditions the reaction Ge + O₂ → GeO₂ which takes place at (111) surface planes leads to suitable thin crystal regions. The GeO₂ occurs normally as amorphous films on the crystal surface. Furthermore, hexagonal GeO₂ can grow at the interface Ge/oxide by a topotaxial reaction; the orientation relation between these two lattices was ascertained. Intensive electron irradiation was used to initiate and to observe structure changes in boundary regions.     

Analysis of solid state reactions at film/substrate interfaces by electron microscopy

Heterogeneous thin film solid state reactions are a matter of topical interest, under both fundamental and applied aspects. The forming phases are in the nanometer size range; the structure and morphology of the involved solid/solid interfaces exerts a strong influence on the reaction kinetics and on the phase formation sequence. Thus electron microscope methods are essential, if the characteristics of film/substrate reactions are to be investigated. Results from a number of electron microscope methods, among them high-resolution and in situ TEM of cross sections, bright- and dark-field diffraction contrast TEM, energy-dispersive X-ray microanalysis and selected area electron diffraction have been used during investigations of interfacial reactions in three thin film systems: TiN/MgO, NiSi₂/Si and TiO₂/MgO. In addition to the indentification and characterization of the new phases formed, valuable information has been obtained on reaction mechanisms, interfacial kinetic reaction barriers and on the influence of stress on the phase formation.