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

Using a hybrid computational strategy, based on semiempirical quantum chemical (ZINDO) and molecular mechanics estimations of interaction energies, as well as experimental data in the literature, a caging model for free radicals in crude oil is proposed. In this model, the free radical is efficiently shielded by asphaltenes, protecting these reactive species from hydrogen transfer and other reactions and preserving them through geological times in the crude. The relevance of this model to important properties of asphaltenes, such as solubility and aggregation, is discussed. The complex asphaltenes were modeled as polycyclic aromatic hydrocarbons (PAH). Our calculations showed that the intermolecular interaction energy for the R⋯T vdW complex, consisting of molecular fragments R and T, was more negative when one subsystem (R or T) was a free radical, suggesting that these radicals could easily be involved in aggregation. Support for these ideas was also found in the relatively large stabilization energy calculated for a paramagnetic vanadyl-porphyrin-pyrene complex, consistent with the known behavior of metallic petroporphyrins found in asphaltenes. Possible implications and generalization of this model for related chemical systems are also analyzed.