Electronic structure calculations of vitamin E analogues: A model for calculated geometries, hyperfine coupling constants, reaction enthalpies (ΔHr) and relative bond dissociation enthalpies (ΔBDE)

Electronic structure calculations have been carried out on 18 α-tocopherol analogues at B3LYP/6-31G(d)//AM1 level. At this level of theory hyperfine coupling constants (HFCC) show good agreement with experimental values. The spin distribution of the peroxyl radical is probed by the Mulliken population analysis and populations based on the theory of atoms in molecules (AIM). The reaction enthalpies for H-atom abstraction between the tocopherol analogues and the peroxyl radical are also computed. There is an excellent correlation with the experimental rate constant and the reaction enthalpy. The calculated bond dissociation enthalpies (BDE) show good agreement with experiment. Introduction of the strong electron withdrawing CN substituent on the heterocyclic ring of the model tocopherols raises the BDE by 2.1 kcal/mol. This result indicates that electron withdrawing groups attached to the heterocyclic ring cannot enhance antioxidant activity.