Bond energy essential single

Azulene does have an appreciable dipole moment (0.8 The essentially single-bond nature of the shared bond indicates, however, that the conjugation is principally around the periphery of the molecule. Several MO calculations have been applied to azulene. At the MNDO and STO-3G levels, structures with considerable bond alternation are found as the minimum-energy structures. Calculations which include electron correlation effects give a delocalized n system as the minimum-energy structure. ... 

A classic example is formaldehyde (I) which in its ground state is a planar molecule with no unpaired electrons (singlet state), a double CO bond, and a dipole moment of 2.3 D. In its lowest excited state, this molecule has 76 kcal/mole more energy than the ground state, it is not planar but pyramidal, and it has two unpaired electrons (triplet state), an essentially single CO bond, and a dipole moment of 1.3 D. Obviously, the two states exhibit completely diflFerent reactivity. 

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