Theoretical study on the heat of formation of some polycyclic aromatic hydrocarbons

This paper reports the results on how structural details govern the heat of formation (ΔHf) of the catacondensed benzenoid hydrocarbons, a wide subclass of polycyclic aromatic hydrocarbons. To accomplish this goal, the influence of the molecular size (expressed through the number of six-membered rings, NR), number of bays (NB), number of coves (NC), number of fjords (NF), and molecular branching (expressed through the number of B3-type rings, NB3), as well as the position of the latter four structural features in a molecule were inspected. To obtain the ΔHf values, the semiempirical PM7 method was applied, and it showed good agreement with the existing experimental results. These structural properties affect ΔHf of these compounds differently. The strongest effect on ΔHf has NR, and it was found that ΔHf increases with increasing NR. ΔHf also increases with increasing NB3. However, when the molecular branching is moved across the molecule, it was found that ΔHf decreases, as it moves to the middle of a molecule. ΔHf of the examined compounds decreases with increasing NB, NC, and NF, and as the bay, cove, and fjord move to the middle of a molecule. When NB, NC, and NF take large values, this decrease slows down.