A critical survey on the mesomeric moments of nitrogen-oxide compounds

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A comparison of the experimental dipole moments of the N-oxides of pyridine, pyrazine, benzylidenemethylamine, benzylideneaniline, benzonitrile, ( Z)- and ( E)-diphenyldiazenes with the corresponding un-oxided nitrogen compounds enabled us to obtain information about the electronic structure of the quoted N-oxides. Assuming these compounds are resonance hybrids between the valence structures A = (A o ⇆ P) with a dative N → O bond and D with an O = N + double bond, whilst P visualizes the π-inductive effect of the N +(O)nitrogen atom on the adjacent π-electron system, the relative contribution from the D structure was determined in most cases from the molecular mesomeric moment, defined as m rarr ∗ = μ rarr exp. − M( A) , and (with a lesser accuracy) from the molecular moment (as a vector) and the values calculated for the A and D structures. Attempts were made to rationalize the mesomeric moments of the N, N-dimethylamino and nitro groups in the 4-substituted pyridines and pyridine N-oxides, as well as the interaction moments of these two groups with the CN or NN bond in the 4-substituted benzylidenemethylamines, benzylideneanilines and ( E)-diphenyldiazenes, and (if any) in the corresponding N-oxides. In addition arguments are given supporting the existence of a mesomeric effect in nitrobenzene.