Carbide Clusterfullerene Gd₂C₂@C₉₂ vs Dimetallofullerene Gd₂@C₉₄: A Quantum Chemical Survey

The geometric, electronic structure, and thermodynamic stability of Gd₂C₉₄ species, including dimetallofullerenes Gd₂@­C₉₄ and carbide clusterfullerenes Gd₂C₂@­C₉₂, have been systematically investigated by a density functional theory approach combined with statistical mechanics calculations. Although the Gd₂@C₂(153480)-C₉₄ is determined to possess the lowest energy, its molar fraction at the temperature region of fullerene formation is extremely low if the temperature effect is taken into consideration. Meanwhile, three C₉₂-based carbide clusterfullerene species, Gd₂C₂@­D₃(126408)-C₉₂, Gd₂C₂@­C₁(126390)-C₉₂, and Gd₂C₂@­C₂(126387)-C₉₂, with some higher energy are exposed to possess considerable thermodynamic stabilities within a related temperature interval, suggesting that carbide clusterfullerenes rather than dimetallofullerenes could be isolated experimentally. Although one isomer, Gd₂C₂@­D₃(126408)-C₉₂, has been indeed obtained experimentally, a novel structure, Gd₂C₂@­C₁(126390)-C₉₂, behaving as the most abundant isomer at more elevated temperatures with the largest SOMO–LUMO gap, is predicted for the first time to be another proper isomer isolated in the experiment. Moreover, in order to further analyze the interaction between gadolinium atoms and carbon atoms in either a carbide cluster or a fullerene cage, frontier molecular orbital, natural bond orbital, and Mayer bond order analyses have been employed, and the results show that the covalent interaction cannot be neglected. The IR spectra of Gd₂C₂@­C₉₂ have been simulated to provide some valuable guidance for future experiments.