Comparative theoretical study of methano- and imino-[50] fullerenes: C50CH2 and C50NH

Total of seven possible isomers for both methano- and imino-[50]fullerenes have been investigated by ab initio calculations. An insight into the relationship between structures and stabilities of these isomers is presented. The results show that the [5,6]-closed structure with CH2 group bridging across a [5,6] type of C-C bond near the equatorial belt is the ground state of C50CH2 while that of the C50NH is [5,6]-open fashioned with NH group bridging across a [5,6] type of C-C bond at the site between the pole and the equatorial belt. The underlying reason of this structural difference may be the larger atomic size and the greater Pauli repulsion of carbon atom compared with those of nitrogen atom. The relative stabilities of the isomers are determined by the strain of the cage, especially the strain at the equator. And although the contribution of the cyclic phenylene substructure at the equatorial belt to the stabilization of the favored isomers probably can not be ignored, the contribution of the conjugation effect to the stabilization is not competitive. Energy difference among three favored isomers is not very large, hence it can be expected that some isomers will co-exist in a mixture once methano- and imino-[50]fullerenes are synthesized. The calculated NICSs for the isomers of C50CH2 and C50NH will be useful for their identification and characterization.