| Abstract
| - Three hydrogen abstraction reactions on cyclohexene, cyclohexadiene, and propene involving hydrogen radicals,and five bond dissociation reactions on cyclohexene, cyclohexene-3-yl, 1,3-cyclohexadiene, 1,3-cyclohexadiene-5-yl, and propene were studied using ab initio quantum chemical methods. The aim was to indicate possiblereaction pathways for aromatization processes during oil and gas generation as well as coalification in naturalprocesses. 3-21G and DZP basis sets were used for the unrestricted Hartree−Fock (UHF), restricted Hartree−Fock (RHF), second-order Møller−Plesset perturbation (MP2), and singles and doubles configuration interaction(SDCI) calculational methods. SDCI with size consistency corrections yielded a barrier of 9, 8, and 11 kcal/mol for hydrogen abstraction on cyclohexene, 1,3-cyclohexadiene, and propene, respectively. Near degeneracycauses UHF-based calculational methods to predict incorrectly energies for the open shell molecules. Twoobservations distinguish the transition state for the selected aromatic molecules from the geometries for saturatedhydrocarbons. First the abstracted hydrogen atom remained closer to its parent C atom in the aromatic molecules,and second, their transition state has a lower activation energy barrier. Both effects are due to delocalization,which is possible in aromatic systems in the transition state as well as the final products. This study ascertainsthat abstraction reactions are feasible for aromatization processes in kerogen under naturally occurringtemperature and pressure conditions. Two step reactions contribute to the magnitude of the overall rate-limiting step of 57 kcal/mol for this reaction pathway. The first one accounts for 49 kcal/mol, and can beattributed to endothermic bond dissociation following the initial hydrogen abstraction. The second one accountsfor 8 kcal/mol and can be attributed to the second hydrogen abstraction reaction.
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