VALENCE-BOND STUDIES OF THE BARRIER TO ROTATION AROUND THE N-N BOND OF N2O4

The recent redetermination of the barrier to rotation around the N-N b ond of N2O4 by Koput, Seibert and Winnewisser is used to focus attenti on on the primary valence bond explanation of the ''cis-O-O overlap'' contribution to this barrier. The explanation involves cis-O-O overlap -dependent resonance between covalent (NO2NO2) structures with ''long' ' or formal N-O bonds, and resonance of these structures with the ioni c (NO2-NO2+, NO2+NO2-) structures that are obtained from them via cis- O--->O electron transfer in the planar conformer. The results of some ab initio valence bond calculations for both N2O2 and N2O4 are used to illustrate the phenomenon.