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.