Remarks concerning the d-d spectral transitions in 595-01595-01595-01

The paper interprets d-d spectral transitions of the ceramic superconductor YBa₂Cu₃O_7– within the scope of the crystal field theory. This theory allows the determination of the d-d spectral transitions as functions of chosen charges by using the appropriate choice of charges on ligands and central ions.     

(1-(Dimethylamino)-2-(diphenylphosphino)ethane)(eta(3)-1-arylallyl)palladium Tetrafluoroborates. Preparation, Isomeric Equilibria, and Correlations of NMR Chemical Shifts with Hammett Substituent Constants

13C NMR differences of chemical shifts (delta(X) - delta(H)) of allyl carbon atoms in a series of trans-(1-(diphenylphosphino)-2-(dimethylamino)ethane)(eta(3)-1-arylallyl)palladium tetrafluoroborates, X ranging from NO(2) to OMe, correlate very well with sigma Hammett constants for C-1 and with sigma(+) for C-3, this carbon atom being in a trans relationship with the positively charged nitrogen atom.     

2-Aminopyrimidine directed self-assembly of zinc porphyrins containing bulky 3,5-di-tert-butylphenyl groups

The 2-aminopyrimidin-5-yl ligand is revealed to be a promising candidate for the construction of supramolecular porphyrin arrays with broad absorption bands for efficient light-harvesting. 10-Mono- and 10,20-di(2-aminopyrimidin-5-yl) derivatives of 5,15-bis(3,5-di-tert-butylphenyl)porphyrin have been synthesized in high yield. Their Zn(II) salts show variable concentration and temperature-dependent UV/vis spectra in solution, consistent with supramolecular aggregation. Whereas the FAB mass spectra of the monosubsituted derivative in toluene suggest the formation of a tetramer at high concentrations and low temperatures (estimated association free enthalpy Delta H = 220 +/- 10 kJ/mol), the larger splitting of the Sorret band (ca. 40 nm) in the variable temperature UV/vis spectra of the disubstituted bis(3,5-di-tert-butylphenyl)porphyrin is indicative of yet higher aggregates involving both 2-aminopyrimidin-5-yl groups. The tetrameric nature of the monosubsituted derivative is confirmed by X-ray analysis, which reveals that two of the 2-aminopyrimidin-5-yl groups are encapsulated by the aggregate and consequently are prevented from undergoing hydrogen bonding. NMR studies show there is no exchange of the 2-aminopyrimidin-5-yl groups, so the tetramer is rigid, which is confirmed by molecular modeling calculations. The tetramer formation is governed by pi-pi interactions, metal coordination, and hydrogen bonding. The di(2-aminopyrimidin-5-yl) derivative forms strongly scattering solutions, which upon standing form green flocculate precipitates, reminiscent of shaken suspensions of bacteriochlorophyll c.     

Electron impact mass spectra of 3,5,7-triaryl-and -trialkyl-4H-1,2-diazepines

Electron impact mass spectra of 3,5,7-trisubstituted 4H-1,2-diazepines indicate that aryl substituents lead to N₂ expulsion while alkyl substitutents do not. A common fragmentation pattern is observed and discussed for all alkyldiazepines, most of which are newly reported compounds. Assignments are based on electron impact mass spectra of deuteriated substrates and high resolution mass spectra. A previous interpretation of N₂ expulsion is corrected.     

14-helical folding in a cyclobutane-containing beta-tetrapeptide

The efficient synthesis of tetrapeptide 5 containing, in alternation, cyclobutane and beta-alanine residues is described. NMR experiments both at low temperature in CDCl(3) and at 298 K in DMSO-d(6) solutions show the contribution of a strong hydrogen bond in the folded major conformation of 5. Temperature coefficients and diffusion times point out a hydrogen bond involving the NH proton from the cyclobutane residue 1 whereas NOEs manifest the high rigidity of the central fragment of the molecule and are compatible with a 14-membered macrocycle. Theoretical calculations predict a most stable folded conformation corresponding to a 14-helix stabilized by a hydrogen bond between NH(10) in the first residue and OC(25) in the third residue. This structure remains unaltered during the molecular dynamics simulation at 298 K in chloroform. All these results provide evidence for a 14-helical folding and reveal the ability of cis-2-aminocyclobutane carboxylic acid residues to promote folded conformations when incorporated into beta-peptides.     

Technische Verfahren zur Synthese von Carotinoiden und verwandten Verbindungen aus 6-Oxo-isophoron. III. Ein neues Konzept für die Synthese der enantiomeren Astaxanthine

Technical Procedures for the Synthesis of Carotenoids and Related Compounds from 6-Oxo-isophorone. III. A New Concept for the Synthesis of the Enantiomeric Astaxanthins A new and efficient concept for the total synthesis of (3S, 3'S)- and (3R, 3'R)-astaxanthin ( 1a and 1c , resp.) in high overall yield and up to 99,2% enantiomeric purity is described. Key intermediates are the (S)- and (R)-acetals 10 and 17 , respectively (Scheme 2). These chiral building blocks were synthesized via three different routes: a) functionalization of the enantiomeric 3-hydroxy-6-oxo-isophorons⁴) 2 and 11 , respectively (Scheme 2); b) optical resolution of 3,4-dihydroxy-compound⁴) 19 (Scheme 3), and c) fermentative reductions of 6-oxo-isophorone derivatives (Schemes 4 and 5). - The absolute configurations of the two intermediates 12 and 13 (Scheme 2) have been confirmed by X-ray analysis. - The final steps leading to the enantiomeric astaxanthins are identical with those described for optically inactive astaxanthin [1].     

Photocycloaddition of (Z)-1,2-dichloroethylene to enantiopure 2(5H)-furanones: an efficient strategy for the diastereoselective synthesis of cyclobutane and cyclobutene derivatives

A highly stereoselective and efficient synthetic approach to cyclobutane and cyclobutene derivatives has been developed consisting of a [2+2] photochemical cycloaddition of chiral 2(5H)-furanones to (Z)-1,2-dichloroethylene followed by dihydrodehalogenation or dihaloelimination.     

Integration over quantum permutation groups

We find a combinatorial formula for the Haar measure of quantum permutation groups. This leads to a dynamic formula for laws of diagonal coefficients, explaining the Poisson/free Poisson convergence result for characters.