Ground States of Conjugated Molecules. I. Semiempirical SCF MO Treatment and Its Application to Aromatic Hydrocarbons

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Abstract
The π‐bonding energies and resonance energies for a series of monocyclic polymethines (CH) n , and several benzenoid and nonbenzenoid hydrocarbons are calculated by the Hückel, Pople, and SPO methods. Special attention is paid to the formulation and definition of such energy terms. For simplicity equal bond lengths are assumed in each case; in the case of the polymethines, the planar configurations of regular polygons are taken. The calculated π‐bonding energies of 11 benzenoid hydrocarbons are combined with experimental heats of formation and an assumed carbon—hydrogen bond energy to obtain estimates of the bond energy of a carbon—carbon σ bond in an aromatic ring. The values obtained for the eleven compounds agree very closely, implying that a good estimate of the heat of formation of such hydrocarbons can be obtained by adding the σ‐bond energies to the calculated π energy. This approach is used to estimate resonance energies of the aromatic hydrocarbons. The resonance energies of the polymethines are found to alternate strongly, regardless of the choice of methods used; the values are uniformly larger for rings with 4n+2 π electrons than for rings with 4n π electrons. In the Pople and SPO cases, bond alternation is predicted for all the rings with 4ncarbon atoms as well as for those with 4n+2 carbon atoms starting at n=4. For certain nonbenzenoid hydrocarbons the calculated resonance energies are only in fair agreement with experimental findings. For most, agreement is satisfactory.

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