Caging of molecules by asphaltenes. A model for free radical preservation in crude oils

Using a hybrid computational strategy, based an semiempirical quantum chemi cal (ZINDO) and molecular mechanics estimations of interaction energies, as well as experimental data in the literature, a caging model for free radic als in crude oil is proposed,In this model, the free radical is efficiently shielded by asphaltenes, protecting these reactive species from hydrogen t ransfer and other reactions and preserving them through geological times in the crude. The relevance of this model to important properties of asphalte nes, such as solubility and aggregation, is discussed. The complex asphalte nes were modeled as polycyclic aromatic hydrocarbons (PAH). Our calculation s showed that the intermolecular interaction energy for the R...T vdW compl ex, consisting of molecular fragments R and T, was more negative when one s ubsystem (R or T) was a free radical, suggesting that these radicals could easily be involved in aggregation. Support for these ideas was also found i n the relatively large stabilization energy calculated for a paramagnetic v anadyl-porphyrin-pyrene complex, consistent with the known behavior of meta llic petroporphyrins found in asphaltenes. Possible implications and genera lization of this model for related chemical systems are also analyzed.