Theoretical identification of the lowest energy structure of C70−n Si n , n = 1, 2, 6, 10, and 20 heterofullerenes

Abstract

We have applied DFT calculations to devise stable atomic arrangements of C70−n Si n heterofullerenes with n = 1, 2, 6, 10, and 20. In general, it seems that among the considered arrangements, C atoms that are located at the polar caps of C70 prefer to be substituted by Si atoms. In the case of C68Si2, an extensive isomer search shows that the most stable configuration is that in which the Si atoms are located at the para positions of a hexagon in a polar cap. In the highly Si-doped C70 fullerenes, C and Si atoms form separate subnetworks. Doping ten Si atoms in each cap of C70, following the most stable structure of C60Si10, leads to three distinct subnetworks, a tubular carbon belt C50 surrounded by two Si10 caps. C atoms in the tubular belt preserve the conjugated pattern of distances typical of carbon nanotubes in a regular arrangement. Nucleus-independent chemical shifts (NICS) calculated at the cage centers of heterofullerenes (−21.8 to −11.0) demonstrate that all the substituted species are aromatic, but less than C70 (−27.3). The calculated NICS values may be useful for the characterization of these heterofullerenes through their endohedral 3He chemical shifts. .