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leads to the conclusion that HeF2 is highly unlikely to be bound. This prediction based on this qualitative argument has been stated several times in recent theoretical treatments,6-8 yet, not unexpectedly, this simple argument is stated with considerably less conviction than would be expressed by a chemist who had no knowledge of xenon chemistry. In fact, a detailed and well-reasoned communication has been published claiming the likely existence of a stable HeF2.9 It is for these reasons that the qualitative chemical arguments and their manifestation in terms of simple Hiickel-like model theories6-8•10 are subject to the "strong inference" criticism of Platt11 and are inadequate to answer decisively the question of the existence of HeF2. It is also necessary that the electronic wave function for the HeF2 system be complete enough to provide a description of the free helium atom at the same level of accuracy as that needed to produce a smooth change from a free xenon atom at infinite separation to the observed stable binding. This has been accomplished in our wave function by carrying the ionic state and in-out configuration interaction into the free helium atom as infinite separation is approached.

The use of various binitroxides, with both radical sites of similar reactivity, as mediator in the controlled radical polymerization of styrene was examined and the rate constants of the reactions involved were determined at 130 °C. Typical features of a controlled radical polymerization were observed in the early stage of the polymerization, leading to the formation of two-arm macromolecules containing the binitroxide at the core. At higher conversion, however, continuous decomposition reaction resulted in a break of the two-arm macromolecules into a dead chain having an unsaturation end group and a living chain capped by a modified binitroxide. The latter had the free nitroxide site inactivated by conversion into hydroxylamine. The extent of this side reaction was much larger than in classical nitroxide-mediated controlled radical polymerization of styrene. This feature was assigned to the structure of the growing chains. As they contain the binitroxide at the core, the activation reaction produces a propagating chain and a nitroxide to which another polymeric chain remains attached by the second alkoxyamine bond. Thus, deactivation of the propagating radical by nitroxide is a bimolecular process between two macromolecular species. This unique situation had no significant effect on the rate constant of the alkoxyamine homolytic dissociation, but more importantly decreased the rate constant of recombination. The latter was 30 times lower than that determined for the recombination of TEMPO with polystyryl radical at 130 °C. The slow recombination was assigned to steric hindrance and resulted in an unusually high concentration of nitroxide in the polymerization medium, together with a large concentration of propagating radicals. A consequence of the high concentration of both radicals was the enhanced rate of hydrogen transfer from the active species to the persistent radical, leading to alkoxyamine bond breaking and, hence, to arm separation.