posted on 2020-08-27, 23:29authored byMasoumeh Mousavi, Elham H. Fini
In recent years,
the dominant organizing role of non-covalent π-stacking
interactions in the association of asphaltenes and porphyrins was
criticized and replaced with cooperative forces that are mostly covalent
in nature. Here, we show the significant contribution of non-covalent
forces in stabilizing the π-stacking of asphaltenes and porphyrins.
To understand the binding chemistry of metalloporphyrin–asphaltene,
the interaction of nickel octaethylporphyrin with a series of model
fragments for asphaltene was studied in two different pathways: axial
coordination and π-stacking. Nickel octaethylporphyrin was specifically
studied because a main fraction of vanadium and nickel metals in petroleum
residues are chelated with porphyrins, and the refining processes
in petroleum industries are affected by the significant detrimental
impact of these metal compounds. The results of the extended transition
state–natural orbital of chemical valence (ETS-NOCV) technique
provide strong evidence that the bonding interaction in the π-stacking
configuration is much preferred to the axial coordination. Energy
decomposition analysis verifies the significant contribution of non-covalent
forces in stabilizing the π-stacking of asphaltene–porphyrin,
showing that there are other forces driving the formation of asphaltene–porphyrin
stacks. Indeed, a non-negligible portion of these stabilizing forces
is contributed by strong orbital mixing interactions through charge
transfer between active centers; this contribution is mostly overlooked
in π-stacking interactions. This matter includes the π-stacking
interactions of asphaltene–asphaltene. Isosurfaces of deformation
density (Δρ) provide better insights into the π-stacking
preference. NOCV deformation densities are delocalized over the entire
complex in the π-stacking conformer, leading to the multi-centric
charge transfer zone; Δρ isosurfaces of axial coordination
are mostly localized on the limited centers involved in chemical bonding.