Computational Study of the Effect of Dispersion Interactions on the Thermochemistry of Aggregation of Fused Polycyclic Aromatic Hydrocarbons as Model Asphaltene Compounds in Solution

Density functional theory (DFT), Moller-Plesset second-order perturbation theory (MP2), and semiempirical methods are employed for the geometry optimization and thermochemistry analysis of pi-pi stacked di-, tri-, tetra-, and pentamer aggregates of the fused polycyclic aromatic hydrocarbons (PAHs) naphthalene, anthracene, phenanthrene, tetracene, pyrene, and coronene as well as benzene. These aggregates (stabilized by dispersion interactions) are highly relevant to the intermolecular aggregation of asphaltenes, major components of heavy petroleum. The strength of pi-pi stacking interaction is evaluated with respect to the pi-stacking distance and thermochemistry results, such as aggregation enthalpies, entropies, and Gibbs free energies (Delta G(298)). For both pi-stacking interplanar distances and thermochemistry, the omega B97X-D functional with an augmented damped R-6 dispersion correction term and MP2 are in the closest agreement with the highly accurate spin-component scaled MP2 (SCS-MP2) method that we selected as a reference. The Delta G(298) values indicate that the aggregation of coronene is spontaneous at 298 K and the formation of pyrene dimers occurs spontaneously at temperature lower than 250 K. Aggregates of smaller PAHs would be stable at even lower temperature. These findings are supported by X-ray crystallographic determination results showing that among the PAHs studied only coronene forms continuous stacked aggregates in single crystals, pyrene forms dimers, and smaller PAHs do not form pi-pi stacked aggregates. Thermochemistry analysis results show that PAHs containing more than four fused benzene rings would spontaneously form aggregates at 298 K. Also, round-shaped PAHs, such as phenanthrene and pyrene, form more stable aggregates than linear PAHs, such as anthracene and tetracene, due to decreased entropic penalty. These results are intended to help guide the synthesis of model asphaltene compounds for spectroscopic studies so as to help understand the aggregation behavior of heavy petroleum.