Annulenes aromatic Heilbronner Mobius

Edgar Heilbronner in 1964 worked out that such a twisted system (Mobius system) would be aromatic if it contained 4n conjugated tr-electrons. Curiously, no actual examples were identified until relatively recently, when it was proposed that [12], [16] and [20] annulenes have several higher energy conformations which adopt this mode. In 2003, the first crystal structure of a true stable Mobius [16] annulene was completed, and various heteroannulenes were identified as existing in Mobius form. 

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