Three-electron bonds and a valence-bond study of the rotation barrier for N₂O₄

Abstract

From molecular orbital studies, a number of previous workers have concluded that a most significant contribution to the barrier to rotation around the NN bond of N₂O₄ arises from the overlap of pairs of atomic orbitals located on the cis oxygen atoms of the planar conformation. A valence-bond study of this overlap contribution is reported. It is calculated that the 'long OO bond' formed by overlap of singly occupied 2pπ-orbitals in the valence-bond structure (3) for the planar conformation has negligible strength, and that insufficient stabilization is obtained when this structure participates in resonance with ionic structures of the type (5a).

Diagram

The primary overlap stabilization for the planar conformation is calculated to arise from resonance between structures of the types (2a) and (7). The O. :O ↔ 0: .O resonance that pertains here is equivalent to the formation of a Pauling 'three-electron bond' O...O between the two atoms. Therefore, the development of this type of bond in the planar conformation can be associated with the cis OO overlap contribution to the barrier to rotation around the NN bond. Another type of Pauling three-electron bond resonance, namely (4) ↔ (6), is calculated to produce a smaller but not insignificant stabilization of the planar conformation.