Beiträge zur Chemie der Pyrrolpigmente, 72. Mitt.

A strategy for the synthesis of bilatrienes-abc bridged between rings A and C has been developed and tested in case of a bibenzylethane bridging fragment. This strategy was then applied to the synthesis of a 2,3-dihydrobilatriene-abc bridged by a (S)-lysine moiety from ring A to ring C (positions 3 and 12) by means of peptide bonds between the two amino groups of lysine and the acetic and propionic acid side chains of the chromophoric unit. The two diastereomers resulting from the two chiral centers of the lysine residue and at position 3 of the chromophore were separated by chromatography.

     

Non‐Linear Signal Detection Improvement by Radiation Damping in Single‐Pulse NMR Spectra

When NMR lines overlap and at least one of them is affected by radiation damping, the resonance line shapes of all lines are no longer Lorentzian. We report the appearance of narrow signal distortions, which resemble hole‐burnt spectra. This new experimental phenomenon facilitates the detection of tiny signals hidden below the main resonance. Theoretical analysis based on modified Maxwell–Bloch equations shows that the presence of strong transverse magnetization creates a feedback through the coil, which influences the magnetization of all spins with overlapping resonance lines. In the time domain this leads to cross‐precession terms between magnetization densities, which ultimately cause non‐linear behavior. Numerical simulations corroborate this interpretation.     

Deposition of amorphous chromium layers

Deposition of amorphous chromium-carbon layers is possible from a trivalent chromium electrolyte containing 0.6?mol/l formic acid. The formation of chromium/formic acid complexes inhibits the aging of the electrolyte and the oligomerization. Deposition of a 15–20?μm thick chromium film is possible with DC condition. Thicker films can be deposited with pulse plating using reversed pulse sequences and pulse lengths in the millisecond region. The optimized conditions concerning pulse current densities and pulse times were determined and the composition and morphology of the films investigated.     

Structure and Bonding Nature of the Strained Lewis Acid 3-Methyl-1-boraadamantane: A Case Study Employing a New Data-Analysis Procedure in Gas Electron Diffraction

Base-free 3-methyl-1-boraadamantane was synthesized by starting from its known THF adduct, transforming it to a butylate-complex with n-butyllithium, cleaving the cage with acetyl chloride to give 3-n-butyl-5-methyl-7-methylene-3-borabicyclo[3.3.1]nonane and closing the cage again by reacting the latter with dicyclohexylborane. The identity of 3-methyl-1-boraadamantane was proven by (1) H, (11) B and (13) C?NMR spectroscopy and elemental analysis. The experimental equilibrium structure of the free 3-methyl-1-boraadamantane molecules has been determined at 100?°C by using gas-phase electron diffraction. For this structure determination, an improved method for data analysis has been introduced and tested: the structural refinement versus gas-phase electron diffraction data (in terms of Cartesian coordinates) with a set of quantum-chemically derived regularization constraints for the complete structure under optimization of a regularization constant, which maximizes the contribution of experimental data while retaining a stable refinement. The detailed analysis of parameter errors shows that the new approach allows obtaining more reliable results. The most important structural parameters are: r(e) (B-C)(av) =1.556(5)??, ${\angle }$(e) (C-B-C)(av) =116.5(2)°. The configuration of the boron atom is pyramidal with ${\sum \angle }$(C-B-C)=349.4(4)°. The nature of bonding was analyzed further by applying the natural bond orbital (NBO) and atoms in molecules (AIM) approaches. The experimentally observed shortening of the B?C bonds and elongation of the adjacent C?C bonds can be explained by the σ(C-C)→p(B) hyperconjugation model. Both NBO and AIM analyses predict that the B?C bonds are significantly bent in the direction out of the cage.     

Electrospray mass spectrometry of lanthanide β-diketone complexes. II-Ligand-exchange processes involving acetate ions and Ln(tfc)3 complexes

Electrospray ionization on Ln(tfc)₃ complexes (Ln = Eu, Yb; tfc = D -3-trifluoroacetylcamphorate) was performed with samples dissolved in methanol–water containing acetic acid. The spectra were obtained with a four-sector mass spectrometer. The mass spectra exhibit ions containing tfc ligand in addition to ions with acetato ligand. Electro-spray and tandem mass spectra are presented.     

Korrosion von Messing und Bronze durch Ammoniumhalogenide

Corrosion of Brass and Bronze by Ammonium Halides The intermetallic phases brass (Cu/Zn) and bronze (Cu/Sn) are corroded by ammonium fluoride and chloride, NH₄F and NH₄Cl, through selective oxidation of the less noble component zinc and tin, respectively. Copper is recrystallized as cube‐like or tabular single crystals under the respective influence of fluoride and chloride. Zinc and tin are incorporated in complex compounds of which (NH₄)ZnF₃, (NH₄)₂ZnF₄, Zn(NH₃)₂Cl₂ and (NH₄)₃SnF₇ were detected by X‐ray powder diffraction.     

Ethyl methyl 1,4‐dihydro‐4‐(3‐nitrophenyl)‐2,6‐bis(1‐piperidylmethyl)­pyridine‐3,5‐dicarboxylate

In the title compound, C₂₈H₃₈N₄O₆, the 4‐aryl substituent occupies a pseudo‐axial position approximately orthogonal to the plane of the di­hydro­pyridine ring [88.1 (3)°]. The di­hydro­pyridine ring adopts a flattened boat conformation. The H atom on the pyridine N atom is involved in a bifurcated intramolecular hydrogen bond, the acceptors being the N atoms of the two piperidyl­methyl groups [N?N 2.629 (4) and 2.695 (4) Å].     

Regiospecific nucleophile-induced fragmentation of α,α-dichloro-β-iminocarbonyl compounds

The reaction of α,α-dichloro-β-(N-alkyl or N-aryl)iminocarbonyl compounds with nucleophiles resulted in a regiospecific fragmentation to afford α,α-dichloroketimines.