Dipole moments and electronic structure of 2,6‐bis(dialkylamino) derivatives of 4 H ‐pyran‐4‐one(thione) and 4 H ‐thiopyran‐4‐one(thione)

Dipole moments of a series of 2,6‐bis(dimethylamino) and 2,6‐bis(pyrrolidino) derivatives of 4 H ‐pyran‐4‐one (2), 4 H ‐thiopyran‐4‐thione (3), 4 H ‐pyran‐4‐thione (4) and 4 H ‐thiopyran‐4‐thione (5) were determined in benzene and dioxane solutions at 25°C. High values of the dipole moments (6·44–8·44 D) and mesomeric moments originated from delocalization of the nitrogen lone pairs into the π‐system of the heterocyclic moieties (1·11–3·08 D) indicate a strong polarization of the molecules 2–5 due to a tendency to stabilize pentamethinium and pyrylium (thiopyrylium) ionic structures. The MNDO‐calculated electronic distribution in 2–5 compared with that for the parent 4 H ‐pyran(thiopyran)‐4‐one(thione)s is in line with this conclusion.