Stability of X4Y24q (X=C, Si; Y=B, Al, C, Si, N, P; q=-4 to 4) and C28X4 (X=H, F, Cl, Br, I)

Energetics, geometries and charge distributions of pure and hetero-fullerenes of 28 atoms as well as their derivatives, are studied using the MNDO method. The stability of C-28 isomers decreases in the order: monocyclic rings, bicyclic rings, cages, bowls, tricyclic rings, and polycyclic sheets. This order hints at the possible formation mechanism of cage fullerenes from graphite-like networks through to intermediate bowl structures. Among the cage structures, the Td series has higher stability than the D-2 series and the D-7d series. For exohedral clusters, C28F4 and C28H4 have higher stability than C28Cl4, C28Br4, C28I4 and C-28 For the Td cages, the hetero-fullerenes are not as stable as the pure carbon fullerene. Their stability decreases in the order: C-28, B4C24, P4C24, Si4C24, N4C24, Al4C24, N4Si24, C4Si24, P4Si24, B4Si24, Si-28 and Al4Si24. The energetics of charged fullerenes shows that cage isomers of C-28 can be reduced up to C-28(2-) It is interesting that site preference of heteroatoms can be correlated with charge distribution and radial distance from the site to the center of the cage. (C) 1997 Elsevier Science B.V.