Theoretical enthalpies of formation and O-H bond dissociation enthalpy of an α-tocopherol model and its free radical

Using DFT computations (B3LYP and BHandHLYP functionals) with isodesmic reactions as working chemical reactions, and extended basis sets with diffuse functions, the standard enthalpies of formation of an alpha-tocopherol model (where the aliphatic chain and the neighbour methyl group have been changed to hydrogen atoms) and its free radical alpha-tocopheroxy were theoretically estimated for the first time: -79.4 +/- 2.0. and -54.9 +/- 2.0 kcal mol(-1), respectively. These enthalpies of formation correspond to the O-H bond dissociation enthalpy of BDE(O-H) = 76.6 +/- 2.0 kcal mol(-1), in excellent agreement with the gas-phase experimental value for natural a-tocopherol, which lends confidence to the method and model used. (C) 2004 Elsevier B.V. All rights reserved.