Ab Initio Calculations on Hydroaromatics: Hydrogen Abstraction and Dissociation Reaction Pathways

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 possible reaction pathways for aromatization processes during 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 Moller−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 degeneracy causes UHF-based calculational methods to predict incorrectly energies for the open shell molecules. Two observations distinguish the transition state...