Splitless on-line coupling of capillary high-performance liquid chromatography,capillary electrochromatography and pressurized capillary electrochromatography with nuclear magnetic resonance spectroscopy

A mixture of unsaturated fatty acid methyl esters was separated with a new splitless capillary set-up. With the employed apparatus configuration different capillary separation techniques such as capillary high-performance liquid chromatography (cHPLC), capillary electrochromatography (CEC) and pressurized capillary electrochromatography (pCEC) could be applied. The detection and identification of the sample compounds were accomplished by hyphenating these capillary separation techniques with nuclear magnetic resonance (NMR) spectroscopy using a novel configuration of the detection capillary set-up. Using modified electrokinetically driven separation techniques, the electric field was applied solely across the separation column. With this improved interface for capillary liquid chromatography-NMR on-line coupling, the stereochemical assignment of the cis and trans configuration of unsaturated fatty acids could be easily accomplished. Finally, the results of cHPLC-NMR, CEC-NMR and pCEC-NMR coupling experiments were compared.Dedicated to Professor Günter Häfelinger on the occasion of his 65th birthday     

In-situ deactivation of packed columns for SFC

Residual adsorptive activity of reversed phase (RP) column packings used in supercritical fluid chromatography (SFC) can be significantly reduced by a dynamic in-situ silanization with diphenyltetramethyldisilazane (DPTMDS). RP-materials thus deactivated were characterized both chromatographically and by solid-phase ²⁹Si NMR.     

Separation of ortho-substituted tetraphenylporphyrins by preparative HPLC

Summary A method for the separation of substituted tetraphenylporphyrins by preparative liquid chromatography is reported. The quality of the columns formed by axial compression of various stationary phases is tested.     

Polymorphie von SrTa2O6

Polymorphism of SrTa₂O₆ Orthorhombic SrTa₂O₆ is a new low temperature modification related to orthorhombic CaTa₂O₆. SrTa₂O₆(orh.) was obtained when the wellknown modification SrTa₂O₆(TTB) which is related to the tetragonal tungsten bronzes was heated in the presence of a transporting agent (chlorine) or a mineralizer (melt of B₂O₃) at temperatures below 1150°C. It could be prepared by the reaction of a 1:1 mixture of Sr(NO₃)₂ or SrCO₃ with Ta₂O₅ in a sealed quartz glass tube as well. SrTa₂O₆(orh.) also occurred as an intermediate phase of the reaction of the corresponding 1:2 mixture at temperatures below 900°C (e. g. 840°C). Indexing of Guinier powder patterns led to the following unit cell: a = 11.006 Å, b = 7.638 Å, c = 5.622 Å. At temperatures above 1220°C SrTa₂O₆(orh.) changes (in air) to SrTa₂O₆(TTB). A reversal of this transition could not be achieved without the presence of a mineralizer or a transporting agent. Ca_xSr_1?xTa₂O₆ solid solutions of the low temperature form could not definitely be established. However, at 1300°C solid TTB solutions of Ca_xSr_1?xTa₂O₆ were formed. For x > 0.05 the TTB unit cells are orthorhombically distorted. For x ≥ 0.85 the x-ray powder patterns of the solid solutions looked like the one of CaTa₂O₆(orh.) and no TTB-structure was observed at 1300°C.     

INVESTIGATION INTO THE SPECTROSCOPY AND PHOTOISOMERIZATION OF A SERIES OF POLY (ETHYLENE GLYCOL) PEPTIDE SCHIFF BASES OF 11-cis RETINAL

Abstract— The 11-cis and all-trans isomers of a series of poly(ethylene glycol)-oligopeptide - Schiff bases as models for rhodopsin were synthesized and studied. Absorption data for certain of the PEG-peptide Schiff bases demonstrated that no intramolecular hydrogen-bonding (or protonation) occurs between the Schiff base and an acidic amino acid residue, as was previously thought. Photoisomerization of the 11-cis protonated and unprotonated Schiff bases were examined using both steady state and laser flash techniques. Also with 355 nm excitation (and additionally 532 nm in one case), an approximate 40% increase in quantum yield of isomerization (φ) occurred for all protonated PEG-peptide Schiff bases compared to the H⁺-n-butylamine counterparts (in methanol). In one case, a > 100% increase in φ was found in dichloromethane. These data show that PEG-oligopeptide Schiff bases are still further improved models for rhodopsin compared to their n-butylamine analogs.     

Proton-14N double resonance study of the structural phase transitions in the perovskite type layer compound (CH3NH3)2CdCl4

Using a proton-nitrogen double resonance technique we have determined the quadrupole coupling of¹⁴N in the room temperature orthorhombic (Cmca), the low temperature tetragonal (P4₂/ncm), and the monoclinic low temperature (P2₁/c) phases of (CH₃NH₃)₂CdCl₄. In all these phases all nitrogens are chemically equivalent demonstrating that the disorder in the orientations and H-bonding arrangements of the CH₃-NH₃ groups in theC m c a andP4₂/ncm phases is indeed dynamic and not static. In the monoclinic phase the¹⁴N quadrupole coupling constant equalse ² qQ/h=880 kHz and the asymmetry parameter is=0.20, wherease ² qQ/h=790 kHz,=0.1 in the tetragonal low temperature phase ande ² qQ/h=726 kHz,=0.21 in the room temperature orthorhombic phase. The observed increase in the¹⁴N quadrupole coupling constant on going from the orthorhombic phase to the tetragonal low temperature phase which is coupled with a simultaneous decrease in the asymmetry parameter can be understood in terms of a partial freezing in of the dynamic disorder in the C-N bond directions whereas the¹⁴N quadrupole coupling tensor in the monoclinic phase is characteristic of a frozen in C-N bond in a deformed lattice, where the N-H — Cl bonds are of different length.     

Bridging Two Worlds: Colloidal versus Epitaxial Quantum Dots

An overview is given of two distinct classes of semiconductor quantum dots, epitaxial and colloidal structures that have been studied intensely for more than 30 years by now, however, without large interconnection between the two involved research communities. The largely parallel and independent evolution of the two structure classes may be partly related to the origin of colloidal systems from chemistry, while epitaxial quantum dots have been addressed mostly by the physics community. These independent evolutions are somewhat surprising because the interest in optics‐related applications is shared by both communities. Here, a short summary of the development of the two structure classes, the present status of activities, and some perspectives for future developments are presented.