Melting point alternation in α,ω-alkanedithiols

α,ω-Alkanedithiols show alternation in their melting points with even members exhibiting systematically higher melting points than odd members. In this study, single crystals of the first nine members of α,ω-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 performed. The structural similarities and differences between even and odd members have been analyzed based on the packing arrangements of hydrocarbon chains and SH functionalities. Whereas the hydrocarbon chains maintain an intergrooving pattern, the SH groups associate in an antiparallel fashion in both the members. The SH groups do not form hydrogen bonds. In the even members the molecules are offset along their length, whereas such an offset is not observed in the odd members. The densities of even members are systematically higher than those of odd members and show an alternating trend. Using a simple geometrical model, describing even and odd members as modified parallelograms and trapezoids, respectively, the density features of the packing patterns in both the members are illustrated. The melting point alternation in dithiols is attributed to the geometry allowed or forbidden molecular offset toward a higher packing density.