Thermal Aromatization of Selected TLC-Type Aliphatic Chemically Bonded Stationary Phases,as Monitored by Use of HPLC-DAD

JPC – Journal of Planar Chromatography – Modern TLC - In our earlier papers we provided indirect experimental evidence strongly in support of our long-established hypothesis that...     

Photophysics of Soret-excited tetrapyrroles in solution. I. Metalloporphyrins: MgTPP, ZnTPP, and CdTPP

The photophysical behavior of three Soret-excited diamagnetic meso-substituted tetraphenylmetalloporphyrins, MgTPP, ZnTPP, and CdTPP, have been examined in a wide variety of solvents using both steady-state and femtosecond fluorescence upconversion methods. The S 2 population of MgTPP decays to S 1 on the time scale of a few picoseconds with unit S 2-S 1 internal conversion efficiency, and the decay rates conform to the weak coupling case of radiationless transition theory. The energy gap law parameters characterizing the coupling of the S 2 and S 1 states of MgTPP have been obtained. The most important accepting vibrational modes in the S 1 state are multiple in-plane C-C and C-N stretches in the 1200-1500 cm (-1) range. Net S 2-S 1 decay is the dominant decay path for ZnTPP and CdTPP as well, but the process occurs at rates that exceed (in the case of CdTPP, they vastly exceed) those predicted by weak interstate coupling. Alternate mechanisms for the radiationless decay of the S 2 states of ZnTPP and CdTPP have been explored. Large spin-orbit coupling constants and the presence of multiple, near-equiergic triplet states suggest that S 2-T n intersystem crossing might occur at rates competitive with internal conversion. However, the measured efficiencies of S 2-S 1 internal conversion show that, at most, only a few percent of the S 2 population of ZnTPP and no more than about 30% of the S 2 population of CdTPP can decay by a "dark" path such as intersystem crossing.     

Mathematical models of solute retention in gas chromatography as sources of thermodynamic data. Part IV. Aliphatic alcohols as the test analytes

This study is the fourth consecutive part belonging to the cycle devoted to an alternative approach to deriving certain thermodynamic magnitudes. The previous three papers were dedicated, respectively, to ketones, aldehydes, and alkylbenzenes. In our present study (similar to the previous ones) the following working procedure is adopted. With the aid of capillary gas chromatography, the retention times are obtained for a wide variety of the aliphatic alcohols. The analyses are carried out isothermally on stationary phases of different polarity and at five different measuring temperatures. These data constitute an experimental basis for further processing with the aid of the specially devised mathematical equations. The fitting parameters of these equations, due to their physicochemical meaning, enable determination of certain thermodynamic data. Nine equations used in this study are the relationships coupling the selected retention data [relative retention (r), non-reduced relative retention (rG), the retention factor (k), or the Kováts retention index (I)] and a variety of the physical magnitudes [the boiling point of the analyte (T(B)), its molar volume (Vm), or its molar refraction (Rm)]. These relationships are tested with respect to their performance to predict the molar enthalpy of vaporization (deltaHvap) of the analytes of interest (i.e., of aliphatic alcohols). Evaluation of the equations' performance is carried out through a comparison of the numerical values generated from this approach with those originating from the other methods, and a very good agreement was found between these two series of the data. The best molar enthalpy vaporization values (deltaHvap) are obtained from the retention data originating from the most polar of the three investigated stationary phases (i.e., DB-Wax). Models V and VIII proved the best performing ones among the nine models tested in this study.     

A Rapid and Sensitive Strip‐Based Quick Test for Nerve Agents Tabun,Sarin, and Soman Using BODIPY‐Modified Silica Materials

Test strips that in combination with a portable fluorescence reader or digital camera can rapidly and selectively detect chemical warfare agents (CWAs) such as Tabun (GA), Sarin (GB), and Soman (GD) and their simulants in the gas phase have been developed. The strips contain spots of a hybrid indicator material consisting of a fluorescent BODIPY indicator covalently anchored into the channels of mesoporous SBA silica microparticles. The fluorescence quenching response allows the sensitive detection of CWAs in the μg m^?3 range in a few seconds.     

Chromatographic determination of the solubility of low soluble substances—A practical possibility

Using selected examples we demonstrate the determination of solubility of low soluble substances with the help of planar chromatography, which is a new, simple and rapid approach to the problem, to a certain extent challenging the established methods. Our approach is a practical consequence of the thermodynamic view of the chromatographic processes, involving either the adsorption or partition mechanism of separation.

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Chromatographische Bestimmung der Löslichkeit von schwerlöslichen Substanzen — eine praktische Möglichkeit

Zusammenfassung An den gewählten Beispielen wird die Bestimmung der Löslichkeit schwerlöslicher Substanzen mit Hilfe der planaren Dünnschichtchromatographie demonstriert. Dieses Verfahren stellt eine neue, einfache und schnelle Methode dar, die in einem gewissen Maß in Konkurrenz zu den älteren und anerkannten Methoden tritt. Die Methode ist die praktische Konsequenz der thermodynamischen Betrachtung des chromatographischen Prozesses in der Adsorptions- und Verteilungsdünnschichtchromatographie.

     

Circular dichroism and conformational dynamics of cephams and their carba and oxa analogues

The biological activity of bicyclic beta-lactam antibiotics depends strongly on the absolute configuration of the bridgehead carbon atom. Frelek and co-workers proposed an empirical helicity rule relating the configuration of the bridgehead carbon atom to the sign of the 220 nm band in the electronic circular dichroism (CD) spectrum of beta-lactams. Here we use synthetic organic chemistry, CD spectroscopy, and time-dependent density functional theory (TDDFT) to investigate the validity of this structure-property relationship for eight model compounds. For conformationally flexible beta-lactams, substantial thermal effects are found which must be included in calculations. To this end, we combine TDDFT calculations of CD with full quantum-mechanical Born-Oppenheimer molecular dynamics (MD) simulations for the first time. The CD spectra are sampled with ground-state density functional trajectories of up to 60 ps. The MD simulations show a surprisingly high sensitivity of the CD to the molecular conformation. On the other hand, the relation between CD and thermally averaged structural parameters is much less complex. While the helicity rule does not seem to hold for individual conformers, it is confirmed by the calculations for seven out of eight systems studied if thermally averaged CD spectra and structures are considered. Since thermal effects on CD can be larger than typical inherent inaccuracies of TDDFT, our results emphasize the need for a systematic treatment of conformational dynamics in CD calculations even for moderately flexible systems. Temperature-dependent CD measurements are very useful for this purpose. Our results also suggest that CD spectroscopy may be used as a sensitive probe of conformational dynamics if combined with electronic structure calculations.     

Characteristics of the oxidation process in the oleic acid-propionic acid system by means of dielectric constant measurements

The influence of the hydrogen-bond association on the course of autoxidation of oleic acid in the binary system oleic acid-propionic acid was discussed. The association was characterized with the help of dielectric constant measurements.     

Spectroscopic and dielectric examination of the oleyl alcohol-oleic acid system

The comparison of association was made for the oleyl alcohol-oleic acid systems composed in several molar ratios. The ir spectroscopy and dielectric constant measurements were applied as analytical techniques. It was stated that oleyl alcohol and oleic acid interact in a number of molar ratios giving the hydrogen-bonded associates. In the range of temparatures 30–60 °C the most stable hydrogen-bonded associate is formed in a ratio of alcohol to acid of 9:1.