An Elementary Molecular Orbital Description of NO₂ Dimerization to N₂O₄

Abstract

Aspects of the 2NO₂ + N₂O₂ dimerization process are examined by constructing a limited MO-CI wavefunction for the ten electrons that are primarily responsible for the NN σ-bond properties in valence-bond descriptions of the dimer. With essentially INDO-type assumptions, this CI wavefunction has been parametrized so that it reproduces almost exactly the experimental values for two ionization potentials of the dimer, namely those which have been associated with ionizations from the two molecular orbitals that correlate with the odd-electron molecular orbital of NO₂. The wavefunction is expressed as a linear combination of configurations constructed from NO₂ molecular orbitals. The primary contribution to this linear combination is calculated to involve two NO₂ monomers in their ground states, with their delocalized odd-electrons spin-paired in a Heitler-London sense. Little contribution to the linear combination is calculated to arise from either ionic NO₂+NO₂- or excited NO₂NO₂ configurations.

For this parametrized wavefunction, the configuration interaction stabilizes the S = 0 ground state relative to a low-lying S = 1 excited configuration.