C78 cage isomerism defined by trimetallic nitride cluster size: a computational and vibrational spectroscopic study

Molecular structure of Dy3N@C78 and Tm3N@C78 clusterfullerenes are addressed by the IR and Raman vibrational spectroscopic studies and DFT computations. Firstly, extensive semiempirical calculations of 2927 isomers of C78 hexaanions followed by DFT optimization were applied to establish their relative stability. Then, DFT calculations of a series of M3N@C78 (M=Sc, Y, Lu, La) isomers were performed which have shown that the stability order of the isomers depends on the cluster size. While the Sc3N cluster is planar in the earlier reported Sc3N@C78 (D3h: 24109) clusterfullerene, relatively large Y3N and Lu3N clusters would be forced to be pyramidal inside this cage, which would result in their destabilization. Instead, these clusters remain planar in the non-IPR C2: 22010 isomer making Y3N@C78 and Lu3N@C78 clusterfullerenes with this cage structure the most stable ones. Finally, on the basis of a detailed analysis of their IR and Raman spectra supplemented with DFT vibrational calculations, the recently isolated Tm3N@C78 and the major isomer of Dy3N@C78 are assigned to the non-IPR C2: 22010 cage structure. A detailed assignment of their experimental and computed IR and Raman spectra is provided to support this conclusion and to exclude other cage isomers.