The consequences of cation-radical fluorination theory—II : Fluorination pathways with less-reactive reagents

The fluorination of benzene with caesium tetrafluorothallate and of pyridine or benzo[b] furan with caesium tetrafluorocobaltate gives sequences of polyfluoroaromatics with vicinal fluorines. Polynuclear aromatics are fluorinated by caesium or potassium tetrafluorocobaltate to give products containing little or no hydrogen, whereas the stronger fluorinating agent cobalt trifluoride gives products which contain much hydrogen. These two previously unexplained features are rationalized by a mechanism which involves 1,2-migrations of fluorine in cyclohexadienyl-type radicals or cations, and is backed up by CNDO/2 calculations. This rationalization implies significantly different rates of atom or ion migrations through the lattices of the different solid fluorinating agents.     

Morphology and rheology of peel of model adhesives

The morphology of surfaces produced during peel experiments was studied to increase the understanding of peel phenomena. Thin films of polystyrenes of narrow molecular-weight distribution and a polydisperse polyester on flexible supports were peeled from rigid substrates at temperatures above their glass transitions with equipment that also permitted quenching and preservation of the peeled surfaces which then could be studied by scanning electron microscopy. The results show many types of cavitation including foams and the coalescence of cavities to form fibers. Cavitation occurred throughout and beyond the region of transition from cohesive to interfacial failure where peel force decreased abruptly; however, cavity growth was abruptly restricted as the failure mode changed. The master curves and shift factors obtained in this work provided strong evidence that the quenching used to preserve the peeled surfaces did not introduce large perturbations in the results. The reason was that failure occurred at or close to the heated substrate.     

Enhancement of long lifetime lines in photoluminescence from Si:In

We report a sample treatment technique which increases by a factor of between 50 and 10,000 the intensity of the long-lived P, Q and R lines previously observed in photoluminescence spectra of Si:In. The intensity of the indium bound exciton line is essentially unaffected. By studying these lines as a function of temperature, excitation density and delay time after pulsed excitation, we conclude that all three transitions are associated with the same isoelectronic binding center; contrary to prior assertions.     

Study of ATLAS sensitivity to FCNC top decays

The ATLAS experiment sensitivity to top quark flavour changing neutral current (FCNC) decays was studied at LHC using tt̄ events. While one of the top quarks is expected to follow the dominant Standard Model decay t→bW, the other decays through a FCNC channel, i.e. t→Zu(c), t→γu(c) or t→gu(c). Different types of analyses, applied to each FCNC decay mode, were compared. The FCNC branching ratio sensitivity (assuming a 5σ signal significance) and 95% confidence level limits on the branching ratios (in the hypothesis of signal absence) were obtained.     

1,4,5,8,9,12-Hexamethyltriphenylene. A molecule with a flipping twist

The synthesis and characterization of 1,4,5,8,9,12-hexamethyltriphenylene (5) is described. In the solid state, X-ray crystallographic studies reveal that compound 5 presents a highly distorted C2 geometry with a 53 degrees end-to-end twist. In solution, variable-temperature 1H NMR studies and molecular modeling present a story of rapid dynamic conformational interconversions between two C2 enantiomers (with a low activation barrier) and a slower C2-D3 interconversion (with a relatively high barrier)--the first time clear evidence of conformational interchange for these hindered triphenylenes has been provided. Further studies have established that 5 is a fluorescent stable blue emitter, and that the compound undergoes an irreversible one-electron electrochemical oxidation. Calculations have predicted this to be a radical cation of C2 geometry with 60 degrees end-to-end twist.