The Persistent Radical Effect: From Mechanistic Curiosity to Synthetic Tool

The Fischer–Ingold persistent radical effect (PRE) provides a simple conceptual framework to rationalize reactivities in systems involving two radicals with very different self‐reaction rate constants; in a limiting, but rather common case, one of these radicals is persistent at room temperature. In these cases, the cross‐coupling product is strongly favored. This contribution summarizes some of the work carried out at the University of Ottawa, where the PRE plays a key role in determining product distributions. Much of this work was inspired by the seminal contributions of Hanns Fischer.     

Further EPR studies of molecular motions in polymer/plasticizer mixtures

Molecular motions in mixtures of the side chain polymer—poly(vinyl acetate) and dibuthyl phthalate were studied as a functionof polymer concentration and temperature using the technique of paramagnetic resonance (EPR). When the small spherical probe tempol (TPL) was used, we were able to approximate the observed EPR spectrum with a simulation using a single rotational correlation time τ. The peviously developed Grest–Cohen all-temperature model matched the Arrhenius polts. The EPR spectra from a cigar-shaped cholestane (COL) probe could not be adequately matched by single τ simulation when the polymer was at temperatures somewhat above the glass to rubber transition temperature (T_g). Points corresponding to these temperatures were left of the Arrhenius plot and a discontinuity was observed where the gap in the data occurred. As the concentration of plasticizer was increased, we found that the discontinuity became less steep, but the τ at which the gap occurs was always ≈ 10^?8. The spectra observed at the temperature region of the gap were approximately 50–50 composites of experimental spectra observed at ± K. In both the TPL and COL cases, there was evidence of the existence of multiple correlation times. Preliminary studies of other polymers, both with and without side chains, also indicated the existence of the gap when COL is used as the probe. © 1993 John Wiley & Sons, Inc.     

The use of electrostatic probes for plasma diagnostics—A review

The use of electrostatic, or Langmuir, probes for plasma diagnostics is reviewed. The emphasis is on experimental implementation and current techniques, and particular attention is paid to sources of error in theoretical interpretation as well as to experimental problems that can occur in complex, reactive plasmas.     

Synthesis of novel Tröger's bases analogues. The first ones fluorescent by excited state intramolecular proton transfer (ESIPT)

Three new Tröger's bases, fluorescent by an ESIPT mechanism and with large Stokes shift has been synthesized from 2-(4^′-amine-2^′-hydroxyphenyl)benzazoles using urotropine and trifluoroacetic acid.     

Orientation analysis of uniaxially drawn polystyrene films doped with fluorescent molecules by fluorescence polarization,UV-and IR-dichroism and birefringence

Atactic polystyrene (PS) is doped in solution with various fluorescent molecules and cast to films. Rectangular samples, cut from these films, are uniaxially drawn at 100 °C and 110 °C. The orientation of the fluorescent probes is characterized by UV-dichroism and fluorescence polarization, that of the polymer segments by IR-dichroism and birefringence. It is observed that the fluorescent molecules depending on their structure tend to orientate more or less perpendicular to the drawing direction. This can be understood by assuming that the benzene rings of the fluorescent probes try to associate to those of the polymer. As consistent values of the angle between the 2850 cm^–1 transition moment and the chain axis and of the intrinsic birefringence result 70° and –0.10 respectively.Dedicated to Prof. Dr. -Ing. R. Bonart with congratulations on his 60th birthday     

Detection by ion-pairing probes in reversed-phase liquid chromatography

Summary Compounds without own detector response can be detected and quantified by a UV-absorbing or fluorescent, ion-pairing probe in reversed-phase liquid-solid systems. Charged as well as uncharged samples give response by affecting the distribution of the probe between the phases. The probe is usually a hydrophobic organic ion. It is included in the mobile phase, which also contains a counter ion and an ion of the same charge as the probe, both fairly hydrophilic. The choice of properties and concentrations of the hydrophilic ions is essential for the response since too low and too high distribution of the mobile phase ions to the adsorbent will make the system insensitive. It is with UV-absorbing probes possible to get a response corresponding to a molar absorptivity of more than 3000 for ionic samples. Fluorescent probes have given 3–5 times higher sensitivity. Uncharged compounds give as a rule lower response. A combination of two UV-absorbing probes with different charge gives a considerable improvement of the response for samples with low retention. A simplified theoretical model for the detector response is suggested on the basis of detailed studies of the concentration changes in the eluted mobile phase.