Nonalternant Aromaticity and Partial Double Bond in Petroleum Molecules Revealed: Theoretical Understanding of Polycyclic Aromatic Hydrocarbons Obtained by Noncontact Atomic Force Microscopy

摘要

Systemic studies are lacking to derive an understanding from the limited number of individually imaged diverse structures from noncontact atomic force microscopy and relate them to the chemistry of a macroscopic ensemble of a heterogeneous petroleum mixture. This initial study intends to fill this gap by studying polycyclic aromatic hydrocarbons (PAHs) and understanding their aromaticity and bonding orders. Both alternant and nonalternant PAHs are present in petroleum molecules, and the significance of nonalternant hydrocarbons in disrupting electron distribution of aromatic hydrocarbons was revealed by quantifying local aromaticity using nucleus-independent chemical shift, complemented by the qualitative prediction with Clar's sextet theory. We found that local aromaticity is maximized, resulting in a new understanding of large aromatic structures in petroleum. In addition, bond order analysis on PAHs from atomic force microscopy imaging revealed that partial double bond character is common in petroleum molecules and could reach a significant degree in certain sites in a structure, with important implications in a wide range of reactivities and properties. Overall, this preliminary study provides means and methodology to connect finite structures from single-molecule imaging with infinite diverse molecules in petroleum.