THE STRUCTURE OF A 7-BLADED PROPELLER - C7PH7+ IS NOT PLANAR

The heptaphenylcycloheptatrienyl cation, C7Ph7+ in [C7Ph7+]-[CF3CO2-]. 2CF(3)CO(2)H (1), is the last species in the series of CnPhnx+/- (n = 3-7) molecular paddle wheels to be structurally characterized. In the solid state, the C-7 central ring of the 6-pi-electron perphenylated tropylium cation adopts a shallow boat conformation, with the peripher al phenyl groups possessing an average dihedral angle of 80 degrees re lative to the plane containing their attached central ring carbon and neighboring ring atoms. This geometry has been rationalized by semiemp irical (AM1) calculations, and represents a compromise between steric congestion and the fractional loss in aromaticity as the central ring distorts from planarity. A correlation of the C-13 NMR chemical shift with the calculated electron density for CnHnx+/- and CnPhnx+/- specie s (n = 3 to 7) yields proportionality factors of approximately 21 and 22 ppm per extra 0.1 electron at the central ring carbons, respectivel y.