Estimating the entropy of melting from structure

Abstract

The total entropy of melting for a wide variety of compounds is estimated by a modification of Walden's rule. This modification accounts for the effects of both molecular rotational symmetry and molecular flexibility on entropy. These effects are combined into a single simple semi-empirical equation. The intercept of the equation was modified from Walden's rule (56.5 J/K·mol), which uses a small data set, to 50 J/K·mol, which uses a data set of 237 rigid and asymmetrical molecules. The molecular rotational symmetry number, σ, and molecular flexibility number, φ, are separately defined and evaluated for a wide variety of molecules and are shown to be related to the entropy of melting in Chapters II and III, respectively. The two effects are combined so that a single equation can be used to predict the entropy of melting for any nonelectrolyte compound. This semi-empirical equation is tested on an independent data set. For over 930 different molecules, including those which are both rigid and flexible, the average absolute error between the predicted and observed entropy of melting values is only 12.5 J/deg·mol. This difference is within experimental error.