Beschreibung:
<jats:title>Abstract</jats:title><jats:p>Dipole moments of a series of 2,6‐bis(dimethylamino) and 2,6‐bis(pyrrolidino) derivatives of 4<jats:italic>H</jats:italic>‐pyran‐4‐one (2), 4<jats:italic>H</jats:italic>‐thiopyran‐4‐thione (3), 4<jats:italic>H</jats:italic>‐pyran‐4‐thione (4) and 4<jats:italic>H</jats:italic>‐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<jats:italic>H</jats:italic>‐pyran(thiopyran)‐4‐one(thione)s is in line with this conclusion.</jats:p>