Theoretical analysis of torsional potential functions. application to the CXS-CXS (X = H, F) series of molecules

Abstract

A detailed investigation of the general behaviour and properties of potential functions hindering the internal rotation of one-rotor molecules is presented. An interpolated three-parameter Fourier representation of torsional potential functions is shown to be adequate to describe rotational isomerization processes. Simple analytic expressions to explore quantitatively and qualitatively the main characteristics of transition states are formulated. It is shown that the barrier to internal rotation is given by a two-term expression involving an intrinsic barrier associated with the shape of the potential wells, and a contribution due to the energy difference between the stable isomers. Conformational functions are introduced and used together with the Brønsted coefficient to get a quantitative version of the Hammond postulate. These concepts are applied to the analysis of ab-initio SCF-MO calculations on the rotational isomerism in a series of dithioglyoxal molecules.