Comment on “Theoretical Investigation of the Formation Mechanism of Metallofullerene Y@C82”

Abstract

A recent study1 by Gan and Wang characterized a formation mechanism for the metallofullerene Y@C82, and the energetically favorable path was determined to be a reaction C76 + YC6 → Y@C82. The YC6 reactant was portrayed as a six-membered ring of carbon coordinated to an yttrium atom in η6 fashion, with C6v or near-C6v point group symmetry. However, what is the stability of this proposed reactant relative to other YC6 isomers? Will the YC6 exist in the proposed form with a sufficient lifetime to perform its proposed role in the mechanism? Using the B3LYP/LANL2DZ method2,3 of the previous study, this isomer of YC6 is compared to two other YC6 isomers from a previous study4 by Strout and Hall. These two other isomers are shown in Figures 1 and ​and2.2. Figure 1 shows a planar isomer in which the yttrium atom is coordinated to a six-membered carbon ring in η2 fashion. Figure 2 shows the so-called “fan isomer” is which the yttrium atom is coordinated to a linear chain of six carbons in such a way as to have Y-C bonding distances with all six carbon atoms. Figure 1 Fan isomer of YC6 (C2v point group symmetry). Figure 2 Planar ring isomer of YC6 (C2v point group symmetry). The first major result is that the geometry optimization of a C6v isomer was unsuccessful due to gradients that suggest that the six Y-C distances should be non-identical. The stationary point most similar to the previous authors’ C6v isomer was found in C2v symmetry and is shown in Figure 3. Even this structure is not quite a local minimum, having a single imaginary frequency of 135i. The molecules in Figures 1 and ​and22 are local minima at the B3LYP/LANL2DZ level of theory. Table 1 shows the relative energies of these three stationary points. The fan isomer is the lowest in energy, followed by the planar η2 ring isomer, with the nonplanar isomer lying much higher in energy. Given this energy ordering of the isomers, it is plausible to envision a reaction path whereby the yttrium atom of the nonplanar isomer slides down to its position on the planar ring, followed by insertion of the yttrium into the ring to form a fan isomer. If the barrier between nonplanar ring isomer and planar ring isomer is a high one, then the nonplanar ring isomer may be stable enough to perform its proposed role in the formation of Y@C82. However, that would have to be demonstrated to be the case in order for the nonplanar YC6 to be a plausible reactant in a Y@C82 reaction mechanism. Figure 3 Nonplanar ring isomer of YC6 (C2v point group symmetry). This structure resulted from unsuccessful attempts to optimize the structure with C6v symmetry. TABLE 1 Relative energies of three isomers of YC6 (calculated with B3LYP/LANL2DZ method, energies in kcal/mol).