Low energy barriers of H-atom abstraction from phenols Oksana Tishchenko, Eugene S. Kryachko * ,1 , Minh Tho Nguyen Department of Chemistry, University of Leuven, Celestijnenlaan 200 F, B-3001 Leuven, Belgium Received 7 November 2001; revised 2 January 2002; accepted 2 January 2002 Abstract 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 p and first excited s states of the incipient phenoxyl radical. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Phenols; Hydrogen bonding; Antioxidants; Hydrogen atom transfer; Potential energy surface 1. Introduction Many phenols are natural or human-made anti- oxidants due to their ability to ‘neutralize’ free radicals by donating the hydroxylic hydrogen atom. Owing to their extremely important role in biological and commercial systems, extensive experimental and theoretical studies of these species have been conducted in the past [1–12]. However, a clear understanding of the mechanism of the H-atom abstraction from phenols is still hindered due to an insufficient knowledge of the reaction potential energy surfaces (PES). In order to have a deeper appreciation of the remarkable aptitude of phenols as antioxidants, we have undertaken a thorough study of the ground state PES describing the H-atom abstrac- tion from phenols structurally related to Vitamin E by the simplest peroxyl radical, namely methylperoxyl OOCH 3 . 2. Computational framework In the present work, density functional theory (B3LYP) calculations were performed in conjunction with the 6-31G(d,p) basis set as implemented in the GAUSSIAN 98 suite of programs [6,7]. The geometries of the H-bonded complexes were fully optimized and harmonic vibrational frequencies were subsequently calculated in order to characterize the stationary points, to determine the zero-point energies (ZPE) and to analyze characteristic frequency shifts. Throughout this paper, the energy comparison is made in terms of enthalpies at 298.15 K. A well-known problem in 0022-2860/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S0022-2860(02)00225-9 Journal of Molecular Structure 615 (2002) 247–250 www.elsevier.com/locate/molstruc 1 On leave from Bogoliubov Institute for Theoretical Physics, Kiev, Ukraine 03143. * Corresponding author. Tel.: þ32-16-32-73-84; fax: þ 32-16- 32-79-92. E-mail address: eugene@bohr.quantchem.kuleuven.ac.be (E.S. Kryachko).