Rotation Barriers in Pyridinium Salts Depend on the Number of Available Ground State Conformations

The enthalpy and entropy of activation for the rotation about the C–N bond were measured in a series of five N-(1-hydroxybutan-2-yl)pyridinium salts, the 2,6-substituents being methyl, ethyl and isopropyl. Replacement of a methyl by an ethyl does not change the activation parameters, while replacement by an isopropyl increases both the activation enthalpy and the activation entropy. The latter is due to the decreased number of conformations available in the ground state. The activation entropies fit a simple model based on entropy of mixing