Theoretical study of deuterium kinetic isotope effect in peroxidation of phenol and toluene

Reaction mechanisms of hydrogen abstraction from phenol and toluene by the hydroperoxyl radical are probed by theoretical calculations of deuterium kinetic isotope effect (KIE). In experiment the given free-radical reactions have nearly equal reaction heats and rates differing by 6 orders of magnitudes, yet demonstrate high H/D KIEs. The mechanism of phenol-peroxyl reaction is described by the proton-coupled electron transfer (PCET), while the toluene-peroxyl reaction follows the non-polar H-atom transfer (HAT). In present work, the H/D KIEs are assessed for several isotopomers of phenol and toluene using the DFT B3LYP/6-311+G(2d,2p) calculations and the post-processing Bigeleisen treatment with one-dimensional tunnel corrections. Differing patterns of bending vibrations are noted for the PCET and HAT TSs considered. The computed KIEs are 10.7 and 17.0 (at 65 °C) for the phenol and toluene reactions, respectively, that agrees with the available experimental results. The corresponding semi-classical contributions are 4.5 and 5.1, whereas the tunnel correction computed for unsymmetrical Eckart function yields the factors of 2.4 and 3.3 for phenol and toluene, respectively. The advantage of using Bigeleisen formula for reaction intermediates with low-frequency internal rotation modes is discussed.