Theoretical study of N20, C20, and B20 clusters “squeezed” inside icosahedral C80 and He80 cages

Abstract

The equilibrium geometries and spectroscopic and energetic characteristics of model endohedral M20@C 80 n− clusters, in which the guest clusters M20 = N20, C20, and B20 are squeezed inside the fullerene C 80 n− cages (n = 0, 2, 4, and 6), have been calculated at the density functional theory B3LYP/6-31G and B3LYP/6-31G* levels. Analogous calculations with partial geometry optimization have been performed for their congeners M20@He 80 n− with a fixed icosahedral helium cage He80. According to the calculations, all the structures of the N20@C80, C20@C80, and B20@C80 series correspond to local minima of the potential energy surface (all vibrational frequencies are real). In the first cluster, the N20 guest has a structure of a dodecahedron with a diameter of ∼4.0 A. The alternative 10N2@C80 structure containing ten separated endohedral N2 molecules is considerably less favorable and transforms without a barrier to the dodecahedral N20@C80 isomer upon geometry optimization. It has been suggested that, under extreme supercompresison conditions, molecular nitrogen can be associated without barriers into highly endothermal chemically bound clusters of the N20 type. In the helium analogues, the relative position of the N20@He80 and 10N20@He80 structures on the energy scale is determined by the degree of compression and can change its sign with a change in the diameter of the external cage D(He80). The mechanism of gradual assembly of the N20 dodecahedron from the 10N2 set has been traced with a decrease in the diameter D(He80) in the range 7.5–8.6 A. In the C20@C80 cluster, the C20 guest has a structure of a distorted dodecahedron bound to the C80 cage through four “inner” (endohedral) bonds. In the B20@C80 cluster, the B20 guest is severely squeezed along the C5 axis. Its equatorial atoms form ten endohedral B-C bonds to C80 cage atoms. In similar systems, the division of the endoclusters into the internal guest and external cage becomes uncertain. Calculations predict that the isolated “salt” molecules of the L n C20 and L n B20 type in which the C20 and B20 clusters function as anions surrounded by the L atoms of alkali metals (n = 1–6) should be stable to stepwise dissociation accompanied by elimination of separate L atoms and L2 molecules.