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.

Starting from norpseudopelletierine, syntheses of two monoradical nitroxides and of a biradical nitroxide with a diazaadamantane skeleton are described. Esr and nmr measurements on monoradicals are reported and interpreted. In the solid state, the pure N,N'-dioxy-2,6-diazaadamantane biradical is diamagnetic. Its esr spectrum in glassy solution is typical of an axially symmetric triplet: D = 250 G. It shows resolved hyperfine structures from both nitrogen nuclei. Isotropic and anisotropic hfsc's have been determined : ax = 20 G, ASz* = Axyy = 13 G, Ah.*' = 34 G. Although poorly resolved due to the large dipolar interaction, a hyperfine structure is observed in dilute solution. Singlet-triplet splitting is measured: ~J1 = 170°K = 0.3 kca1,'mol = 110 cm-l = 3 X lo6 MHz. The biradical is found to be a ground state triplet. This may be attributed to its D2d symmetry.t is well known that stable nitroxide biradicals' afford

No abstract

No abstract