Exchange Interaction Between Degenerate π Orbitals and Orbital Valence

Abstract

Magnetic interaction between two π orbitals in D_∞h symmetry is discussed using an effective Hamiltonian set up mainly from symmetry consideration. The resonance effect between orbital structures π_a+π_b− and π_a−π_b+ is found most important in determining the ground state (³Σ_g⁻) of the system. The obtained energy levels give a good description of the ground and lower excited states of the O₂ molecule, so that the magnetism of O₂ can be explained by the valence bond theory only with correct care of the orbital valence. It is also shown that the same resonance effect makes ¹Σ⁺ state of the π–π³ system (e.g., CO molecule) most stable. The effective Hamiltonian for a p-p system with D_∞h symmetry is derived in a similar way. With t₂-p isomorphism, the results reduce to the restricted form given recently by Drillon and Georges when several relations are assumed to hold among the parameters.