Low energy barriers of H-atom abstraction from phenols

The energy barriers governing the hydrogen atom transfer between phenols related to Vitamin E and methylperoxyl radical are determined using the B3LYP/6-31G(d,p) method. For phenol, o,o-dimethylphenol, and o,o,m-trimethylphenol, they are equal to 6.0, 4.2, and 3.5 kcal/mol, respectively. While in both reactants and products the H-bond is nearly coplanar with the aromatic ring, it becomes essentially twisted out of the ring in the transition state structures. This implies that the transition states in such reactions are likely located at the avoided crossing of the lower-lying electronic states of the H-bonded complex, correlating with the ground π and first excited σ states of the incipient phenoxyl radical.