Valence-Bond Description of Conjugated Molecules. 3. The Through-Resonance Concept in Para-Substituted Nitrobenzenes

The influence of the mesomeric interaction of a π-donor para substituent on nitrobenzene is studied by means of an analysis, in valence-bond terms, of the ab initio SCF wave functions of p-nitroaniline and p-nitrophenol. It is shown that the six-membered ring undergoes some charge transfer from the π-donor para substituent, but that the NO₂ group does not, contrary to the usual concept of “through resonance” in the valence-bond theory of resonance. A reinterpretation of the available experimental data, concerning the influence of a para substituent on the dipole moments, the negative charges on the outer oxygens, and the carbon-nitrogen bond shortenings in substituted nitrobenzenes, is suggested. Further refinements to estimate the effects of electron correlation lead to the same conclusions.