A parametrized valence-bond study of the origin of the long, weak N-N bondof asym-N2O3

The results of an all-electron STO-6G valence-bond study of the origin of t he long, weak N-N bond of asym-N2O3 are reported. Attention is given to the valence-bond structures which differ in the locations of eight electrons a mong five overlapping nitrogen and oxygen atomic orbitals, designated as <( pi)over bar>(1), h(2), and <(pi)over bar>(3) for NO2, and h(4) and <(pi)ove r bar>(5) for NO These orbitals accommodate the electrons of the N-N sigma- bond and oxygen lone-pair electrons prior to any delocalization of the latt er electrons. The calculations are parametrized so that UHF/cc-pVQZ estimat es of the atomic spin densities for NO2 and NO are approximately reproduced when their odd electrons are assumed to occupy these overlapping atomic or bitals. The origin of the N-N bond lengthening is thereby shown to be assoc iated with (a) the orientation of the h(4) atomic orbital, and (b) some del ocalization of oxygen <(pi)over bar> electrons into the nitrogen h(2) and h (4) atomic orbitals when the latter orbitals are initially singly occupied. Covalent-ionic resonance (O2N-NO <-> O2N-NO+ <-> O2N+NO-) is calculated to be needed in order to stabilize the N2O3 relative to the NO2 and NO dissoc iation products.