Tropolone resonance structures

Some compounds have partial aromatic or antiaromatic character due to the presence of a minor aromatic or antiaromatic resonance structure. Tropolone (cy-cloheptadienone) is much more stable than one would expect from a highly unsaturated ketone because its C-O resonance structure is aromatic. On the other hand, cyclopentadienone is extremely unstable because its C-O resonance structure is antiaromatic. 

Solution (a) Tropolone belongs to the class of nonben-zenoid aromatic compounds. It is not adequately represented by the structural formula given in the problem. A significant number of other resonance structures can be written for tropolone, which are given in (b). Tropolone is a hybrid of all its resonance structures and is believed to look like the following ... 

The many valence bond structures we can draw for tropolone indicate that a great deal of resonance exists in this molecule. This resonance is responsible for the thermodynamic stability of tropolone, which is revealed in the lower-than-expected heat of combustion of tropolone. The 20 kcal energy of stabilization is better known as conjugation energy. 

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