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Structure, Stability, and Cluster-Cage Interactions in Nitride Clusterfullerenes M3N@C2n (M = Sc, Y; 2n = 68−98):  a Density Functional Theory Study

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journal contribution
posted on 26.09.2007, 00:00 authored by Alexey A. Popov, Lothar Dunsch
Extensive semiempirical calculations of the hexaanions of IPR (isolated pentagon rule) and non-IPR isomers of C68−C88 and IPR isomers of C90−C98 followed by DFT calculations of the lowest energy structures were performed to find the carbon cages that can provide the most stable isomers of M3N@C2n clusterfullerenes (M = Sc, Y) with Y as a model for rare earth ions. DFT calculations of isomers of M3N@C2n (M = Sc, Y; 2n = 68−98) based on the most stable C2n6- cages were also performed. The lowest energy isomers found by this methodology for Sc3N@C68, Sc3N@C78, Sc3N@C80, Y3N@C78, Y3N@C80, Y3N@C84, Y3N@C86, and Y3N@C88 are those that have been shown to exist by single-crystal X-ray studies as Sc3N@C2n (2n = 68, 78, 80), Dy3N@C80, and Tb3N@C2n (2n = 80, 84, 86, 88) clusterfullerenes. Reassignment of the carbon cage of Sc2@C76 to the non-IPR Cs:  17490 isomer is also proposed. The stability of nitride clusterfullerenes was found to correlate well with the stability of the empty 6-fold charged cages. However, the dimensions of the cage in terms of its ability to encapsulate M3N clusters were also found to be an important factor, especially for the medium size cages and the large Y3N cluster. In some cases the most stable structures are based on the different cage isomers for Sc3N and Y3N clusters. Up to the cage size of C84, non-IPR isomers of C2n6- and M3N@C2n were found to compete with or to be even more stable than IPR isomers. However, the number of adjacent pentagon pairs in the most stable non-IPR isomers decreases as cage size increases:  the most stable M3N@C2n isomers have three such pairs for 2n = 68−72, two pairs for n = 74−80, and only one pair for n = 82, 84. For C86 and C88 the lowest energy IPR isomers are much more stable than any non-IPR isomer. The trends in the stability of the fullerene isomers and the cluster-cage binding energies are discussed, and general rules for stability of clusterfullerenes are established. Finally, the high yield of M3N@C80 (Ih) clusterfullerenes for any metal is explained by the exceptional stability of the C806- (Ih:  31924) cage, rationalized by the optimum distribution of the pentagons leading to the minimization of the steric strain, and structural similarities of C80 (Ih:  31924) with the lowest energy non-IPR isomers of C766-, C786-, C826-, and C846- pointed out.