Compressed alkanes in reversible encapsulation complexes.

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, C(16) to C(19), 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 C(19), C(18) and C(17), respectively. The shorter C(16) was encapsulated in its fully extended conformation, and does not seem to exert pressure inside the capsule.