d
\((d)\) Polycyclic systems : Huckel's \((4n + 2)\) rule applies to monocyclic systems. The  rule, however, is not necessarily successful for p'olycyclic conjugated systems the. \(( 4n + 2)\) rule should be applied to the peripheral (conjugated) \(\pi\) -  electrons. The five-membered ring has five and the seven-membered ring has  seven \(\pi\) -electrons (two \(1^{st}-\)electrons are common to both rings). If one \(\pi\) - electron is transferred from the seven - to the five - ring, each ring will now have a  closed shell of six \(\pi\) - electrons (\(cf\). naphthalene). In this condition, the  molecule will have a dipolar structure, and this has been shown to be the case from  dipole - moment measurements. Azulene also behaves chemically as an aromatic  compound.