Configuration interaction, ionization potentials and valence-bond aspects of the rotation barrier for dinitrogen tetroxide

Abstract

The results of recent He _II studies of the N₂O₄ photoelectron spectrum are used to obtain an empirical estimate for a cis O-O core resonance integral, β°oo'. This integral is associated with the overlap between pairs of cis oxygen in-plane (2pπ) orbitals (which are doubly occupied in valence-bond structures of types (A) and (B)), and is therefore relevant for an analysis of the cis O-O overlap contribution to the barrier to rotation around the NN bond of N₂O₄. Limited SCF-CI calculations for the planar and skew conformations give a rotation barrier of 7.2 kJ mol^-1 (cf. expt., 8.4-12.6 kJmol^-1) when the NN σ-bond order is 0.525 for the planar conformation. The CI expression for the rotation barrier is expressed as a sum of cis O-O overlap, Coulomb and exchange terms, of which the cis O-O overlap contribution has the largest magnitude. The sensitivity of the magnitude of the rotation barrier to variation in the value of the NN σ-bond order is also demonstrated. A valence-bond analysis of the cr wavefunction provides further support for an earlier conclusion that resonance between covalent (NO₂NO₂) and ionic (NO₂⁺ NO₂^- and NO₂^- NO₂⁺) structures provides the valence-bond explanation for the origin of the cis O-O overlap contribution to the rotation barrier. An unexpected result obtained from this analysis is that, although resonance between covalent and ionic structures of types (A) and (B) alone is not cis O-O overlap dependent, this type of resonance does provide a contribution to the cis O-O overlap stabilization of the planar conformation when it is coupled to the other types of covalent-ionic resonance, which arise when the oxygen 2pσ-electrons of (A) and (B) delocalize. Each of the latter types of resonance alone is cis O-O overlap dependent.