Unpaired Spins in Molecular Physics and Chemistry

The analysis of physical properties connected to the electron spin in terms of spin densities requieres some caution : i) Spin densities cannot be always calculated or discussed in the frame of standard independentparticle model (as the restricted Hartree-Fock theory), neither in the frame of amended spin-unrestricted independent particle models. The former/ neglecting spin-polarization effects, are unable to reproduce possible negative spin densities, where as the latter have serious theoretical drawbacks, due to the relaxation of the self-consistent constraints expressed by the Delbruck-Ripka theorem, ii) Spin densities should not be confused with the so-called'-.spin populations, because the latter are based on an atomic partitioning of the total spin densityinvolving a rather elusive definition of atoms "in situ". 1. UNPAIRED ELECTRONS IN STRUCTURAL CHEMISTRY Although spin was really included in the description of electron only from 1925 owing to requirements of Atomic Spectroscopy, the concept of unpaired electrons or, more exactly, that of atoms with non satisfied valencies in compounds like free radicals, biradicals, etc... goes back much earlier in Chemistry (see, e.g.,[lj) It existence of chemically unlinked radicals, affirmed by Frankland and strongly denied by chemists as Gerhardt, Kekule, Ostwald and many others during the last century, was revived by Gomberg at the outset of the 20th century in his studies on hexaarylethanes : he proved that an equilibrium may take place between some hexaaryl Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982749