Hückel's rule
Antiaromaticity is a characteristic of a cyclic molecule with a π electron system that has higher energy due to the presence of 4n delocalised (π or lone pair) electrons in it. Unlike aromatic compounds, which follow Hückel's rule ([4"n"+2] π electrons) and are highly stable, antiaromatic compounds are highly unstable and highly reactive. To avoid the instability of antiaromaticity, molecules may change shape, becoming non-planar and therefore breaking some of the π interactions. In contrast to the diamagnetic ring current present in aromatic compounds, antiaromatic compounds have a paramagnetic ring current, which can be observed by NMR spectroscopy.