Aromaticity in pericondensed cyclopenta-fused polycyclic aromatic hydrocarbons determined by density functional theory nucleus-independent chemical shifts and the Y-rule — Implications in oil asphaltene stability

Abstract

The characterization of the stability of the fused aromatic region (FAR) in oil asphaltenes in terms of kinetic and thermodynamic stability is primary. Such an understanding is important if we are to get the optimal use from the heavy fraction of any crude oil. The FAR region is composed of pericondensed cyclopenta-fused polycyclic aromatic hydrocarbon compounds (CPPAHs) with N, S, and O heteroatoms. The Clar model, which states that the most important representation of a PAH is one having the maximum number of disjoint π-sextets, depicted by inscribed circles, and a minimum number of fixed double bonds, captures the essence of the kinetic and thermodynamic stability arguments. This model is readily employed for complex aromatics of the sort to be considered for asphaltenes. In the present research we prove that the aromaticity of CPPAHs can be assessed by using the qualitative easy-to-apply Y-rule. In the literature, it is proven that the Y-rule is applicable to elucidate the aromaticity of benzenoid PAHs and it has been validated for pericondensed benzenoid PAHs but not for pericondensed CPPAHs. Here, we verify that it is applicable for CPPAHs. The applicability of the Y-rule has been theoretically proven by comparing the π-electronic distribution obtained with it with the one obtained from nucleus-independent chemical shift (NICS) calculations at the density functional theory (DFT) level. The importance of doing this is that due to the polydispersity in the composition of the oil asphaltenes, and to understand their aromatic core structure, it is necessary to be able to asses the aromaticity of many cyclopenta-fused PAHs (possibly more than 500), of different sizes (up to 15 rings between hexagons and pentagons), and different spatial rearrangements in a quick but realistic and effective way. To try to do this with NICS will be very time consuming and computationally expensive, especially in the case of big systems.