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Abstract
Simple alkanes feature fully extended conformations as their lowest-energy shapes but can assume coiled, compressed conformations in small spaces. A series of normal alkanes, C16 to C19, were encapsulated in self-assembled, hydrogen-bonded complexes. Coiling of the longer alkanes was observed by NMR spectroscopy. The coiling exerts pressure on the interior; the hydrogen bonding seams are loosened, and rotation of the capsule's components occurs on the NMR timescale. The rotation results in interconversion of mirror-image capsule assemblies (racemization). The racemization rates were determined and shown to increase with the length of the alkane, the longer alkanes exerting more pressure. Free energies of activation for racemization were determined at the coalescence temperatures, and were ΔG‡ = 15.7, 16.7 and 17.2 kcal mol−1 for C19, C18 and C17, respectively. The shorter C16 was encapsulated in its fully extended conformation, and does not seem to exert pressure inside the capsule. Molecules confined to small volumes can contort themselves into unusual conformations that differ from those usually observed when no constraints are placed on them. It is now shown that when normal alkanes are encapsulated inside self-assembled capsules, they adopt a coiled conformation and exert pressure inside their hosts. Cite this article
Ajami, D., Rebek, J. Compressed alkanes in reversible encapsulation complexes. Nature Chem 1, 87–90 (2009). https://doi.org/10.1038/nchem.111 