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 SinGenC20-2n 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).