Structure, stability, MEP, NICS, reactivity, and NBO of Si–Ge nanocages evolved from C₂₀ fullerene at DFT

Abstract

We have performed systematic theoretical studies of C₂₀ and its Si_nGe_nC_20-2n heterofullerenic derivatives with n = 1–10, at the density functional theory. Vibrational frequency analysis confirms that except for Si₅Ge₅C₁₀, Si₈Ge₈C₄, and Si₁₀Ge₁₀, other systems are true minima and none of the 12 computed heterofullerenes collapses to open deformed as segregated nanocage. Isolating the Si–Ge hetero bonds through C=C double bond and/or one carbon atom is an appropriate approach for reaching highly substituted stable heterofullerenes, since it prevents weak Si–Si and Ge–Ge homo bonds. The calculated band gap, ionization potential, and atomization energy of Si₁Ge₁C₁₈ shows it as the best insulated heterofullerene. Binding energy, and absolute values of heat of atomization of computed heterofullerene decrease as number of substituting Si–Ge unit increases. Henceforth, in contrast to the common belief that thermodynamically stable species are not always kinetically stable, here isolable or extractable Si₁Ge₁C₁₈ heterofullerene is stable from both viewpoints. Contrary, Si₁₀Ge₁₀ that unfortunately suffers from lack of carbon participation, is revealed as the most chemically reactive heterofullerene and the best optically active via increasing its hyperpolarizability. Exclusive of Si₁₀Ge₁₀, substitutional doping of other heterofullerenes leads to a high NBO charge distribution upon their surfaces. We are very pleased to state that isolating the Si–Ge single bonds by intermediation of only one carbon atom is an applicable strategy for obtaining the highest atomic charge distributions on C, Si, and Ge atoms of Si₆Ge₆C₈ as suitable hydrogen storage. Evaluating nucleus independent chemical shift (NICS) values at the cage centers clearly show the highest aromatic character of Si₁Ge₁C₁₈ species followed by Si₂Ge₂C₁₆ (− 48.76 and − 35.42 ppm compared to − 19.61 for the pure carbon cage, C₂₀).

Graphic abstract