The Melting Point Alternation in a,w-Alkanedithiols.
a,w-Alkanedithiols show an alternation in their m.ps. with even members exhibiting systematically higher m.ps. than odd members. Single crystals of the 1st nine members of a,w-alkanedithiols (HS-(CH2)n-SH, n = 2-10) were grown in situ using a miniature zone melting procedure, and their x-ray crystal structure analyses were performed. The structural similarities and differences between even and odd members were analyzed based on the packing arrangements of hydrocarbon chains and SH functionalities. Whereas the hydrocarbon chains maintain an intergrooving pattern, the SH groups assoc. in an antiparallel fashion in both the members. The SH groups do not form H bonds. In the even members the mols. are offset along their length, whereas such an offset is not obsd. 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, resp., the d. features of the packing patterns in both the members are illustrated. The m.p. alternation in dithiols is attributed to the geometry allowed or forbidden mol. offset toward a higher packing d.
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