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Do 12-Membered Cycloalkane Rings Only Exist As One Conformation in the Solid-State? A Detailed Solid-State Analysis Involving Polymorphs of N,N′‑Biscyclododecyl Pyromellitic Diimide

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posted on 05.12.2012, 00:00 by Sanaz Khorasani, Manuel A. Fernandes, Christopher B. Perry
Conformational flexibility in molecules plays a key role in many chemical and biological processes. It is a common belief that the larger the cycloalkane the more flexible it will be, and the more conformations it will adopt. While theoretical studies have shown that cyclododecane has many possible conformations, they have also consistently shown that one conformation is slightly more stable. In this work, we examine the effect of substitution and crystal packing on the conformation of singly substituted cyclododecane rings. This has been done by exploiting polymorphism in an attempt to induce new conformations in a specific molecule, as well as by examining structures reported in the Cambridge Structural Database (CSD). To this end, three polymorphs of N,N′-biscyclododecyl pyromellitic diimide (PMDI-12) have been identified and their structures elucidated. To rationalize the differences between the various polymorphs, molecule···molecule interaction energies have been calculated using atom–atom potential methods. Though the conformation of the PMDI-12 molecules as a whole may differ, examination of the conformation of the 12-membered ring indicates that it is conformationally identical in all three polymorphs. Examination of 20 other organic and organometallic structures containing this group in the CSD, indicates that they have the same conformation (only one possible exception in the 34 rings examined in this work), which suggests that the 12-membered ring adopts a single conformation ([3333] with D2 symmetry) in the solid-state that is relatively unaffected by crystal packing.