Substituted 1-hetero-3-aza-1,3-butadienes: Dipole moments and related structural analysis

Dipole moments of selenobenzamide, of eight N'-thio toro or seleno) acylamidines and of two N'-thioacyliminoethylidenetriphenylphosphoranes in carbon tetrachloride, benzene or dioxane solutions were measured or examined. Calculations of molecular moments for the sp (C=S and N=C on the same side) and ap conformations and for the canonical structures implied by conjugation were performed according to a recently proposed method. A comparison between the calculated and the experimental values shows that only strong conjugations with significant statistical percentages p(s)(+/-) of the 1,5 (or 1,6) zwitterionic forms (X(-)-C=N-C=N+ or X(-)-C=N-C=C-P+ with X = 0, S or Se) can explain observed moments. The mu differences between compounds are related to p(s)(+/-) variations which can give information about the molecular structures and their relationship with reactivities.