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 esti mates torsional entropy (S-tor, the entropy associated with torsional motions about a single bond) quantitatively. Using this model, torsion al entropies are evaluated for a variety of molecular arrangements. Qu alitative trends emerge from these estimates that are consistent with chemical intuition. The entropy associated with torsional motion is no t constant: values of S-tor range from 0 to 15 J mol(-1) K-1 and are s ensitive to details of the bond around which the torsion occurs Import ant characteristics include the bond length, the hybridization, the sy mmetry, the sizes of these stems or groups of atoms, and the extent of conjugation to adjacent bonds. These values are relatively independen t of one another in a number of important cases, and therefore the tot al change in conformational entropy for a given process may be estimat ed by adding changes in entropy due to restricting torsions around ind ividual bonds. A model that permits quantitative estimations of change s 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 assembli es, as well as in the kinetic and thermodynamic analysis of almost any chemical reaction that involves the restriction of the torsions of ro tors.