Potential energy curve for singlet-oxygen quenching reaction by vitamin E

Abstract

The singlet-oxygen (¹O₂) quenching-reaction by vitamin E (α-tocopherol) was studied by using density functional theory, and the potential energy curve was calculated. On the basis of the computational results, the following mechanism was suggested for the quenching reaction. In the initial stage of the quenching reaction, ¹O₂ and α-tocopherol approach each other, a partial electron transfer takes place from α-tocopherol to ¹O₂, and an exciplex between the two molecules is formed. Further approach raises the potential energy of the exciplex and leads to a crossing point between the potential energy curves of the lowest singlet and triplet states. Then, physical quenching from ¹O₂ to ³O₂ is caused by α-tocopherol. Even more approach of ¹O₂ and α-tocopherol yields a pseudo-stable 6, 8a-endoperoxide of α-tocopherol after passing through a transition state. The 6, 8a-endoperoxide could be transformed to the primary product (8a-hydroxyperoxy-α-tocopherone) of a chemical reaction between ¹O₂ and α-tocopherol. The geometries at the potential energy minima and the transition state in the lowest singlet state were optimized, and the geometry at the potential-energy curve crossing-point between the lowest singlet and triplet states was obtained besides stabilization and activation energies included in the potential energy curve. The geometry of the primary product of the chemical reaction was also optimized.