The Melting Point Alternation in <i>α</i>,<i>ω</i>-Alkanedicarboxylic Acids

Single-crystal X-ray diffraction analyses of α,ω-alkanedicarboxylic acids (HOOC−(CH2)n-2−COOH, n = 2−10) have been carried out at 130 and 298 K. Dimorphism is prevalent in odd carbon members, and the crystal structures of α- and β-forms of C7-diacid have been determined. Diacids show an alternation in their melting points with members containing an even number of carbon atoms exhibiting systematically higher melting points compared to odd ones. On the contrary, the solid-state densities of odd members with Cn ≥ 5 are higher than those of even members. Closest packing is therefore not the reason for alternating melting points in diacids. Diacids with Cn ≥ 5 show distinct packing regularities within even series and also within α- and β-series of odd members. The gross structural features are similar in even and (both forms of) odd diacids: (a) carboxy groups form hydrogen bonded dimers at both ends of the molecules, leading to infinite chains, and (b) methylene chains stack into columns through hydrophobic interactions. However, there are certain differences within these similar packing patterns that are important in the context of melting point alternation: (a) molecules are offset along their length within the columnar stacks in even members, whereas such an offset is absent in both forms of odd members, and (b) molecules in both modifications of odd members exhibit twisted molecular conformations with severe torsions as opposed to the non-twisted all-trans conformation in the even members. Energies of the ideal and observed conformations have been computed with the hybrid-DFT method B3LYP and 6-31G* basis set. A simple geometrical model has been developed wherein the even and odd members are described as modified parallelograms and trapezoids, respectively. It is shown that, whereas the packing of parallelograms allows an offset which reduces the repulsions between the carboxy dimers of adjacent hydrogen bonded chains, a similar offset is forbidden for the packing of trapezoids. The model also suggests the reason for the prevalence of dimorphism in odd diacids. Because the twisted molecular conformations in odd diacids are associated with high energy, they have lower melting points. The melting point alternation in diacids is therefore attributed to the geometry-allowed or -forbidden attainment of an offset packing with a non-twist molecular conformation.