On the use of the rotational isomeric state approximation in studies of internal rotation

A critical appraisal is made of the validity of the rotational isomeric state (RIS) approximation in studies of rotational isomerism, especially as far as the determination of energy differences for the stable conformers is concerned. By using simple model potential energy functions for internal rotation appropriate to some ethane derivatives, a comparison is made between the thermodynamic parameters that can be extracted from such potential functions and those obtained by applying the RIS method to continuum averaged values of conformation dependent properties. The effects due to the form of this conformation dependence, to the features of the potential energy curve, and to the temperature are discussed. The errors can be very large for realistic situations, and variable with temperature and with the property being studied; cases for which the errors are small are usually accidental.