Half-Pancake Bonding in Asphaltenes
Benjamin G. Janesko, Edward N. Brothers
Abstract
Aggregation of asphaltenes in petroleum crude oil is a well-known and ongoing problem for petroleum production. Asphaltenes aggregate even in situations where polycyclic aromatic hydrocarbon (PAH) model compounds do not, which implies that there is more to the phenomenon than just π-stacking. Half-pancake bonds involving stable radical PAH have been hypothesized to contribute to asphaltene aggregation (Zhang, Y.; Energy Fuels 2020, 34 (8), 9094−9107). We report density functional theory calculations on half-pancake bonding in asphaltene model compounds. Pancake and half-pancake bonds can be viewed as multi-center generalizations of (common) two-electron covalent bonds and (rare) odd-electron bonds, respectively. In the absence of other effects, we find negligible half-pancake bonding between radical PAH and neutral nonradical PAH. Introducing heteroatoms can produce modestly strong donor-acceptor half-pancake-type interactions. Our results suggest that half-pancake bonding alone is too weak and too rare to drive asphaltene aggregation. However, interactions between stable organic radicals, heteroatoms, and polar groups may contribute to aggregation.