Competing spin interactions and degenerate frustration for discrete molecular species

The concept of spin frustration, introduced by solid state physicists in relation with the spin glass behavior, at the origin was intimately related to the idea of high orbital degeneracy. More recently, the term spin frustration has also been used in molecular magnetism to define isolated systems with mere competing spin interactions. The idea of orbital degeneracy (or quasi-degeneracy) has unfortunately disappeared. The case of discrete molecular species for which competing spin interactions lead to an orbitally degenerate ground state, with at least one magnetic component, seems to us to be specifically important, and we suggest to define such a situation as being degenerate frustrated. When this is so, an instability of the spin populations with respect to a weak structural deformation may be observed. Several typical situations are investigated, and the occurrence of degenerate frustration in isolated open-shell molecules is discussed.