HOMO-LUMO gap as an index of molecular size and structure for polycyclic aromatic hydrocarbons (PAHs) and asphaltenes: A theoretical study. I

Theoretical calculations are presented for the effect on the HOMO-LUMO gap due to the successive addition of aromatic rings and their different distributions, isomers, for polycyclic aromatic hydrocarbons (PAHs). The study is based on ZINDO/S calculations. PAHs with 1-14 fused aromatic rings (FAR) are considered. The results of these calculations are addressed to a currently existing controversy regarding the number of FAR in asphaltene structures. Asphaltenes are considered as polycyclic aromatic compounds similar to PAHs but containing heteroatoms and alkyl side chains. The theoretical results are compared with fluorescence emission (FE) experimental data. It is found that the asphaltene experimental FE range does not necessarily correspond to different chromophores with different number of FAR but may be different isomers with the same number of FAR. Also, the effect of the presence of alkyl chains and heteroatoms in the asphaltene structures on the HOMO-LUMO gap is almost negligible. We conclude that the FAR region in asphaltenes has PAH chromophores with 5-10 fused rings. The 100% compactness (circular) PAH structures, beyond 10 fused rings, and the 0% compactness (linear or zigzag) PAH structures are not possible for asphaltenes. Relationships between the HOMO-LUMO gap and structural parameters for PAH chromophores in asphaltenes were found. The effect of the number of FAR and Clar sextets, the compactness, and longest dimension on the HOMO-LUMO gap of PAHs is evaluated.