Molecular symmetry depresses the entropy of fusion of organic molecules with regard to their expected values when comparing structural isomers

Using published data, the symmetry effect on the entropy of fusion, ΔSf, of rigid organic molecules has been investigated by comparing structural isomers. Because ΔSf is dependent on the enthalpy of fusion, ΔHf, [A.S. Gilbert, Thermochim. Acta 339 (1999) 131–142] this dependence must be allowed for as ΔHf values can vary quite widely. When this is done, it is found that more symmetric isomers usually have lower than expected values of ΔSf, in relation to ΔHf, than their less symmetric counterparts. This is the reason that more symmetric isomers generally melt at higher temperatures, a fact noted by many workers over the years.