First-principles conformational analysis of the C36H36 spheriphane, a prototype hydrocarbon host cage

Comprehensive B3LYP/6-311G** electronic structure calculations establish th at, unlike closely related species such as cyclophanes, the C36H36 spheriph ane (heptacyclo[13.13.2(1,15).2(8,22).1(3,27).1(6,10).1(13,17).1(20,24)] he xatriaconta-1,3(33),6,8,10(34),13,15,17(35),20,22,24(36),27-dodecaene) poss esses only seven energetically distinct conformers, out of which five exist in enantiomeric pairs and two are achiral. These local energy minima are i nterrelated through an intricate net of 20 reaction paths involving single inversions at the -CH2-CH2-bridges. In particular, the T and C-3 conformers , which are predicted to coexist in comparable concentrations at ambient te mperatures, are linked through three consecutive single-bridge inversions t hat proceed through the overall barrier of only 5.3 (kcal/mol). This barrie r is compatible with coalescence temperatures well below those employed in the recently measured H-1 and C-13 NMR spectra, explaining the observed lac k of line splittings. Both the T --> C-3 pathway and the racemizations of t he low-energy conformers involve C-1 and C-2 intermediates that are expecte d to be present in detectable amounts. (C) 2001 John Wiley & Sons, Inc.