MESOMERIC DIPOLE-MOMENTS .13. DIPOLE-MOMENTS AND ELECTRON-DISTRIBUTION OF FUROXANS AND FURAZANS

Dipole moments of a set of substituted 1,2,5-oxadiazoles (furazans) and 1,2,5-oxadiazole-2-oxides (furoxans) were measured in benzene solution. The dipole moment in furazan is oriented with the negative end towards oxygen in agreement with common opinion but contrary to the literature. Comparison of the two classes of compounds revealed a strong electron shift in furoxans, from the exocyclic oxygen back into the heterocyclic system, corresponding to a mesomeric moment of approximately 3D. However, the electron redistribution cannot be described in terms of simple mesomeric structures. The molecule of furoxan is well characterized as electron-overcrowded, particularly in the vicinity of the nitrogen atom N(2).