ABINITIO HEATS OF FORMATION OF MEDIUM-SIZED HYDROCARBONS .13. STUDIES OF BENZO[E]PYRENE, BENZO[GHI]PERYLENE, CORONENE, AND CIRCUMCORONENE

The geometries and energies of the title compounds are reported at the STO-3G and 6-31G* SCF levels, except for circumcoronene, where only the STO-3G level was employed. Group equivalents and homodesmic reactions are used to obtain heats of formation from ab initio energies. At the 6-31G* SCF level, the DELTA-H(f)-degrees values for the first three compounds are similar, ranging from 66.3 to 72.2 kcal/mol. Evidence of a small additional aromatic stability for coronene is found. The ab initio enthalpy changes for homodesmic reactions containing similar structural parameters are shown to be proportional to the natural logarithm of a quantity we term the Kekule ratio: the product of reactant and product Kekule numbers, each raised to a power equal to its stoichiometric coefficient.