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) an d 1,2,5-oxadiazole-2-oxides (furoxans) were measured in benzene soluti on. The dipole moment in furazan is oriented with the negative end tow ards oxygen in agreement with common opinion but contrary to the liter ature. Comparison of the two classes of compounds revealed a strong el ectron shift in furoxans, from the exocyclic oxygen back into the hete rocyclic 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 charac terized as electron-overcrowded, particularly in the vicinity of the n itrogen atom N(2).