Two New Parameters for Predicting the Entropy of Melting: Eccentricity (epsilon) and Spirality (mu)

The prediction of the entropy of fusion of organic compounds is studied. Rotational symmetry (sigma) and flexibility (phi) have already been shown to contribute to the orientational and conformational entropy of fusion, respectively. Eccentricity (epsilon) and spirality (mu) are two new parameters that influence translational and configurational entropy. (epsilon)is defined as the ratio of the volume of a box around a rigid molecule to the cubed radius of a sphere containing the same molecular van der Waals volume. (Mu)is defined as the number of benzo[c]phenanthrene regions present in the molecule. This moiety results in repulsion and out of plane twisting to maximize the distance between hydrogens. Enthalpy of melting, melting point, and geometrical data for 117 compounds were gathered from the literature. The database consists of alkanes, polycyclic aromatic hydrocarbons, alkyl benzenes, and other alkyl aromatics. The addition of and (mu)to and (phi)as predictors for the entropy of melting revealed a best fit melting point prediction line with a slope close to unity and a coefficient of determination (R^2) of 0.90. In addition, the average absolute error of melting point estimation was improved from 90 K to 43 K. Hence, the newly defined and (mu)parameters offer significant improvement when combined with and in the prediction of the entropies of melting and, consequently, melting points of organic molecules.