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 (Delta H-f) 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, N-R), number of bays (N-B), number of coves (N-C), number of fjords (N-F), and molecular branching (expressed through the number of B3-type rings, N-B3), as well as the position of the latter four structural features in a molecule were inspected. To obtain the Delta H-f values, the semiempirical PM7 method was applied, and it showed good agreement with the existing experimental results. These structural properties affect Delta H-f of these compounds differently. The strongest effect on Delta H-f has N-R, and it was found that Delta H-f increases with increasing N-R. Delta H-f also increases with increasing N-B3. However, when the molecular branching is moved across the molecule, it was found that Delta H-f decreases, as it moves to the middle of a molecule. Delta H-f of the examined compounds decreases with increasing N-B, N-C, and N-F, and as the bay, cove, and fjord move to the middle of a molecule. When N-B, N-C, and N-F take large values, this decrease slows down.