Tolerance, homologous series

Ciystallograms of structure-type line diagrams have been developped by Smirnova (1971,1975a, b, 1976,1977) to recognise relations between structure types and homologous series based on structural elements. Tolerance classes of elements of a-Fe-, Cu-, Mg- and a-La-type are discussed by Smirnova, Kurashkovskaya and Below (1977 a, b, c) on the basis of miscibility properties. 

What can be said about either series when studied as solids From Eqs. 29 and 30 we see that our question can be answered in terms of the enthalpies of fusion. Consider the first series. For benzene, the enthalpy of fusion is 9.9 kJ mol" while it is but 6.6 kJ mol" for toluene. Extrapolating to 3.3 for the xylenes seems wrong if for no other reason than we would further extrapolate to impossible zero or negative values for a species with four or more methyl groups. Indeed, the fusion enthalpies for the three xylenes are 13.6,11.6, and 17.1 kJmol" for the o-, m-and p-isomers, respectively. Homology is lost, at least suspect, or said more optimistically, one must be tolerant of greater disparities. 

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