Using a convenient, quantitative model for torsional entropy to establish qualitative trends for molecular processes that restrict conformational freedom

A non-quantum-mechanical, readily applied model is described that estimates torsional entropy (S-tor, the entropy associated with torsional motions about a single bond) quantitatively. Using this model, torsional entropies are evaluated for a variety of molecular arrangements. Qualitative trends emerge from these estimates that are consistent with chemical intuition. The entropy associated with torsional motion is not constant: values of S-tor range from 0 to 15 J mol(-1) K-1 and are sensitive to details of the bond around which the torsion occurs Important characteristics include the bond length, the hybridization, the symmetry, the sizes of these stems or groups of atoms, and the extent of conjugation to adjacent bonds. These values are relatively independent of one another in a number of important cases, and therefore the total change in conformational entropy for a given process may be estimated by adding changes in entropy due to restricting torsions around individual bonds. A model that permits quantitative estimations of changes in conformational entropy will be useful in a wide range of chemical and biochemical applications that include the design of tight-binding polyvalent pharmaceuticals and stable multiparticlemolecular assemblies, as well as in the kinetic and thermodynamic analysis of almost any chemical reaction that involves the restriction of the torsions of rotors.