Knowledge Bank

The $\pi$-binding energies of twelve aromatic hydrocarbons are calculated by the H""{u}ckel, Pople and SPO methods. These results are combined with experimental heats of formation and assumed CH bond energies to obtain estimates of the bond energy of a CC $\sigma -bond$ in an aromatic ring; the values obtained for the twelve compounds agree very closely, implying that a good estimate of the heat of formation of such hydrocarbons can be obtained by adding the $\sigma -bond$ energies to the calculated $\pi -energy$. This approach is used to estimate resonance energies of a series of cyclic polyenes $(CH)n$. The resonance energies of the cyclic polyenes are found to alternate strongly, regardless of the choice of methods used; the values are uniformly larger for rings with $4n+2\pi -electrons$ than for rings with $4n+2\pi -electrons$. In the Pople and SPO cases, bond alternation is predicted for all the rings with 4n carbon atoms as well as for those with 4n+2 carbon atoms starting at $n=4$.