Ab initio calculations on hydroaromatics: Hydrogen abstraction and dissociation reaction pathways

Three hydrogen abstraction reactions on cyclohexene, cyclohexadiene, and pr opane 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 possible reaction pathways for aromatization processes dur ing oil and gas generation as well as coalification in natural processes. 3 -21G and DZP basis sets were used for the unrestricted Hartree-Fock (UHF), restricted Hartree-Fock (RHF), second-order M phi ller-Plesset perturbation (MP2), and singles and doubles configuration interaction (SDCI) calculatio nal methods. SDCI with size consistency corrections yielded a barrier of 9, 8, and 11 kcal/mol for hydrogen abstraction on cyclohexene, 1,3-cyclohexad iene, and propene, respectively. Near degeneracy causes UHF-based calculati onal methods to predict incorrectly energies for the open shell molecules. Two observations distinguish the transition slate for the selected aromatic molecules from the geometries for saturated hydrocarbons. First the abstra cted hydrogen atom remained closer to its parent C atom in the aromatic mol ecules, and second, their transition state has a lower activation energy ba rrier. Both effects are due to delocalization, which is possible in aromati c systems in the transition state as well as the final products. This study ascertains that abstraction reactions are feasible for aromatization proce sses in kerogen under naturally occurring temperature and pressure conditio ns. Two step reactions contribute to the magnitude of the overall rate-limi ting step of 57 kcal/mol for this reaction pathway. The first one accounts for 49 kcal/mol, and can be attributed to endothermic bond dissociation fol lowing the initial hydrogen abstraction. The second one accounts for 8 kcal /mol and can be attributed to the second hydrogen abstraction reaction.