The melting point alternation in alpha,omega-alkanedithiols

alpha,omega-Alkanedithiols show an alternation in their melting points with even members exhibiting systematically higher melting points than odd memb ers. In this study, single crystals of the first nine members of alpha,omeg a-alkanedithiols (HS-(CH2)(n)-SH, n = 2-10) have been grown in situ; using a miniature zone melting procedure, and their X-ray analyses have been perf ormed. The structural similarities and differences between even and odd mem bers have been analyzed based on the packing arrangements of hydrocarbon ch ains and SH functionalities. Whereas the hydrocarbon chains maintain an int ergrooving pattern, time SH groups associate in an antiparallel fashion in both the members. The SH groups do not form hydrogen bonds. In the even mem bers the molecules are offset along their length, whereas such an offset is not observed in the odd members, The densities of even members are systema tically higher than those of odd: members and show an alternating trend. Us ing a simple geometrical model, describing even and odd members as modified parallelograms and trapezoids, respectively, the density features of the p acking patterns in both the members are illustrated. The melting point alte rnation in dithiols is attributed to the geometry allowed or forbidden mole cular offset toward a higher packing density.