On the relationship between the potential barrier and the activation energy for the internal rotation of a methyl group

Abstract

The absolute rate theory expression for the rate of the internal rotation of a methyl group, or a similar internal rotor, hindered by a three-fold barrier, is developed. This expression is used to derive the activation energy and activation enthalpy for the process. Numerical calculations are performed using two assumptions regarding the form of the potential for two typical methyl barrier heights at three temperatures. The Arrhenius activation energies ( E a) are found to be nearly temperature independent in the range 200–333 K and only 2–4% lower than the barrier height values ( V 3). The effect of tunneling is estimated to be about 2–3%.