The Melting Point Alternation in a,w-Alkanedicarboxylic Acids.
Single-crystal x-ray diffraction analyses of a,w-alkanedicarboxylic acids (HOOC-(CH2)n-2-COOH, n = 2-10) were carried out at 130 and 298 K. Dimorphism is prevalent in odd C members, and the crystal structures of a- and b-forms of C7-diacid were detd. Diacids show an alternation in their m.ps. with members contg. an even no. of C atoms exhibiting systematically higher m.ps. 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 m.ps. in diacids. Diacids with Cn >= 5 show distinct packing regularities within even series and also within a- and b-series of odd members. The gross structural features are similar in even and (both forms of) odd diacids: (a) carboxy groups form H bonded dimers at both ends of the mols., 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 m.p. alternation: (a) mols. 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) mols. in both modifications of odd members exhibit twisted mol. conformations with severe torsions as opposed to the nontwisted all-trans conformation in the even members. Energies of the ideal and obsd. conformations were computed with the hybrid-DFT method B3LYP and 6-31G* basis set. A simple geometrical model was developed wherein the even and odd members are described as modified parallelograms and trapezoids, resp. Whereas the packing of parallelograms allows an offset which reduces the repulsions between the carboxy dimers of adjacent H 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 mol. conformations in odd diacids are assocd. with high energy, they have lower m.ps. The m.p. alternation in diacids is therefore attributed to the geometry-allowed or -forbidden attainment of an offset packing with a nontwist mol. conformation.
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