Polyhedral Fluorocarbon (CF)n—Structures and Energies

Abstract

The structures and energies of selected (CX)n (n = even, 6−60, X = F or H) polyhedra, computed at the B3LYP/6-311++G(d,p)//B3LYP/6-31G(d,p) level, show (CH)n isomers to be generally less strained than the corresponding (CF)n analogues. For n = 6−24, both (CH)n and (CF)n favor the same geometric patterns. The tube isomers (t-(CX)n) of large (n = 26−60) species with fused five-membered rings at the tube ends are thermodynamically more stable than the cage isomers (c-(CX)n), deduced from the most stable carbon cages. The stabilities of tube isomers are determined by the distortion of the bond angles (angle strain) and the nonbonded F···F repulsive interactions. The latter have been quantified by means of homodesmotic equations. Four structural types of t-(CX)n structures are built on the basis of the number of fused five-membered rings, and the thermodynamically more stable t-(CX)n structures have more fused five-membered rings, in opposition to the isolated pentagon rule governing the stability of carbon fullerenes.