Substituted 1-Hetero-3-aza-1,3-butadienes: Dipole Moments and Related Structural Analysis

Dipole moments of selenobenzamide, of eight N′-thio (oxo 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 ps(±) of the 1,5 (or 1,6) zwitterionic forms (X−CN−CN+ or X−CNCCP+ with X  O, S or Se) can explain observed moments. The μ differences between compounds are related to ps(±) variations which can give information about the molecular structures and their relationship with reactivities.