Theoretical studies of effective exchange interactions between molecular oxygens Possibility of Haldane systems

Abstract

Ab initio post-Hartree-Fock calculations have been carried out for the lowest-spin (LS) and highest-spin (HS) states in several conformations of molecular oxygen dimers. The LS-HS energy gap can be embodied by effective exchange interactions ( J ab) in the Heisenberg hamiltonian when the O 2-O 2 intermolecular distance ( R) exceeds 3.2 Å. The J ab-values decrease exponentially with the increase of R, in accord with experiments. For all orientations examined the effective exchange interactions are found to be antiferromagnetic. The calculated results are in good agreement with the experimental ones in the a phase of the O 2 crystal, which is found to be a collinear antiferromagnet on experimental grounds. The implications of the calculated results are also discussed in relation to a theoretical possibility of Haldane systems of one-dimensional O 2 arrays embedded in host molecules.