Mesoionic compounds
Molecules with dipolar heterocyclic structures and delocalized charges / From Wikipedia, the free encyclopedia
In chemistry, mesoionic compounds are one in which a heterocyclic structure is dipolar and where both the negative and the positive charges are delocalized.[1] A completely uncharged structure cannot be written and mesoionic compounds cannot be represented satisfactorily by any one mesomeric structure.[1] Mesoionic compounds are a subclass of betaines.[1] Examples are sydnones and sydnone imines (e.g. the stimulant mesocarb), münchnones,[1][2] and mesoionic carbenes.
![Thumb image](http://upload.wikimedia.org/wikipedia/commons/thumb/6/60/Sydnone_structures.png/640px-Sydnone_structures.png)
The formal positive charge is associated with the ring atoms and the formal negative charge is associated either with ring atoms or an exocyclic nitrogen or other atom.[3] These compounds are stable zwitterionic compounds[4] and belong to nonbenzenoid aromatics.[5]