NONEMPIRICAL VALENCE BOND STUDIES OF THE ORIGIN OF THE ROTATION BARRIER FOR N2O4

The origin of the barrier to rotation around the N-N bond of N2O4 is examined by means of nonempirical valence bond calculations with 34 valence shell electrons. Particular attention is focussed on the electrons that are considered to be primarily associated with this phenomenon, namely, two N-N σ-bonding electrons and four sets of oxygen lone-pair electrons, whose orbitals overlap appreciably with those of the N-N σ bond. In agreement with the conclusion obtained from previous molecular orbital studies it is calculated that overlap between atomic orbitals located on pairs of cis oxygen atoms in the planar conformer is primarily responsible for the stabilization of this conformer relative to the perpendicular conformer. The "cis O-O overlap" stabilization only manifests itself when there is some delocalization of the oxygen lone-pair electrons into the atomic orbitals that form the N-N σ bond and is mostly associated with covalent-ionic resonance, i.e., NO2-NO2 ⇔ NO2+ NO2- ⇔ NO2- NO2+ with an important involvement by covalent and ionic Lewis structures of types A and B. It is also calculated that N-N π bonding in the planar conformer is too weak to account for the existence of an appreciable rotation barrier. (Chemical Equation Presented) © 1990 American Chemical Society.