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Keywords:

  • Pauli principle;
  • Bond theory

Abstract

The starting point of this contribution is a discussion of the concept of exchange interactions and its relation to chemical bonding and valence. Then, a class of phenomena characterized by weak chemical bonding (≲1eV) in solids and molecules is analyzed in more detail, with “superexchange” in insulating solids with paramagnetic 3d-cations serving as prototype. A model of “effective electrons” is developed for weak bonding on the basis of exchange perturbation theory, taking full account of the Pauli principle. The model is applied to: (i) magnetic interactions in solids (interaction energy 10−2 to 10−4 eV); (ii) stability of noble-gas halides (binding energy ≈ 1eV); and (iii) rotational barriers in simple molecules (barrier heights of several 0.1 eV).