Half-Pancake Bonding in Asphaltenes

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.