Systems with Seven Conjugated Atoms

Seven-membered conjugated systems having a hetero atom, heteropin (7), consist of 177-azepine (7, X = NH), oxepin (7, X = O), and thiepin (7, X = S). Since these divalent hetero atoms are isoelectronic with the ethylenic linkage, heteropins might be considered to be 8tt electron heteroannulenes which are not aromatic but antiaro-... 

Huckel realized that his molecular orbital analysis of conjugated systems could be extended beyond the realm of neutral hydrocarbons. He pointed out that cycloheptatrienyl cation contained a tt system with a closed-shell electron configuration similar to that of benzene (Figure 11.13). Cycloheptatrienyl cation has a set of seven tt molecular orbitals. Three of these are bonding and contain the six tt electrons of the cation. These six tt electrons are delocalized over seven carbon atoms, each of which contributes one 2p orbital to a planar, monocyclic, completely conjugated tt system. Therefore, cycloheptatrienyl cation should be aromatic. It should be appreciably more stable than expected on the basis of any Lewis structure written for it. 

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