The Melting Point Alternation in ,-Alkanedicarboxy
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- 作者：Venkat R. Thalladi ; Markus Nü ; sse ; and Roland Boese
- 刊名：Journal of the American Chemical Society
- 出版年：2000
- 出版时间：September 27, 2000
- 年：2000
- 卷：122
- 期：38
- 页码：9227 - 9236
- 全文大小：194K
- 年卷期：v.122,no.38(September 27, 2000)
- ISSN：1520-5126
文摘
Single-crystal X-ray diffraction analyses of 
rs/alpha.gif" BORDER=0>,rs/omega.gif" BORDER=0 >-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 thecrystal structures of rs/alpha.gif" BORDER=0>- and rs/beta2.gif" BORDER=0 ALIGN="middle">-forms of C7-diacid have been determined. Diacids show an alternation in theirmelting points with members containing an even number of carbon atoms exhibiting systematically highermelting points compared to odd ones. On the contrary, the solid-state densities of odd members with Cn 
5are higher than those of even members. Closest packing is therefore not the reason for alternating meltingpoints in diacids. Diacids with Cn 5 show distinct packing regularities within even series and also within rs/alpha.gif" BORDER=0>-and rs/beta2.gif" BORDER=0 ALIGN="middle">-series of odd members. The gross structural features are similar in even and (both forms of) odddiacids: (a) carboxy groups form hydrogen bonded dimers at both ends of the molecules, leading to infinitechains, and (b) methylene chains stack into columns through hydrophobic interactions. However, there arecertain differences within these similar packing patterns that are important in the context of melting pointalternation: (a) molecules are offset along their length within the columnar stacks in even members, whereassuch an offset is absent in both forms of odd members, and (b) molecules in both modifications of odd membersexhibit twisted molecular conformations with severe torsions as opposed to the non-twisted all-trans conformationin 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 evenand 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 carboxydimers of adjacent hydrogen bonded chains, a similar offset is forbidden for the packing of trapezoids. Themodel also suggests the reason for the prevalence of dimorphism in odd diacids. Because the twisted molecularconformations in odd diacids are associated with high energy, they have lower melting points. The meltingpoint alternation in diacids is therefore attributed to the geometry-allowed or -forbidden attainment of an offsetpacking with a non-twist molecular conformation.
rs/alpha.gif" BORDER=0>,rs/omega.gif" BORDER=0 >-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 thecrystal structures of rs/alpha.gif" BORDER=0>- and rs/beta2.gif" BORDER=0 ALIGN="middle">-forms of C7-diacid have been determined. Diacids show an alternation in theirmelting points with members containing an even number of carbon atoms exhibiting systematically highermelting points compared to odd ones. On the contrary, the solid-state densities of odd members with Cn 5are higher than those of even members. Closest packing is therefore not the reason for alternating meltingpoints in diacids. Diacids with Cn 5 show distinct packing regularities within even series and also within rs/alpha.gif" BORDER=0>-and rs/beta2.gif" BORDER=0 ALIGN="middle">-series of odd members. The gross structural features are similar in even and (both forms of) odddiacids: (a) carboxy groups form hydrogen bonded dimers at both ends of the molecules, leading to infinitechains, and (b) methylene chains stack into columns through hydrophobic interactions. However, there arecertain differences within these similar packing patterns that are important in the context of melting pointalternation: (a) molecules are offset along their length within the columnar stacks in even members, whereassuch an offset is absent in both forms of odd members, and (b) molecules in both modifications of odd membersexhibit twisted molecular conformations with severe torsions as opposed to the non-twisted all-trans conformationin 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 evenand 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 carboxydimers of adjacent hydrogen bonded chains, a similar offset is forbidden for the packing of trapezoids. Themodel also suggests the reason for the prevalence of dimorphism in odd diacids. Because the twisted molecularconformations in odd diacids are associated with high energy, they have lower melting points. The meltingpoint alternation in diacids is therefore attributed to the geometry-allowed or -forbidden attainment of an offsetpacking with a non-twist molecular conformation.