E.S.R. Spectra and hyperfine structure of symmetric trinuclear clusters

The E.S.R. Spectrum hyperfine structure of trigonal trinuclear exchange clusters with half-integral spins is presented. In the case of antiferromagnetic exchange coupling, the ground state with the total spin S = ½ is four-fold degenerate. The Heisenberg—Dirac-Van Vleck model is shown to be incorrect for the problem of hyperfine interactions. The theory of E.S.R. hyperfine structure presented here arises from the group-theoretical analysis of exchange multiplets. By means of irreducible tensor methods the hyperfine interaction effective hamiltonian is constructed. The angular and field dependence of the hyperfine structure is investigated.