Prediction of endohedral borafullerenes X@B₃₂C₃₆ (X = CH₄, BH₄^-, H₂O, and NH₃) with a B₃₂C₃₆ shell isovalent with C₆₀

Context: Inspired by the newly synthesized endohedral fullerene T CH₄@C₆₀ (1) and based on extensive density functional theory calculations, we predict herein a series of endohedral borafullerenes C₃ CH₄@B₃₂C₃₆ (4), T BH₄@B₃₂C₃₆^- (5), C₁ H₂O@B₃₂C₃₆ (6), C₃ NH₃@B₃₂C₃₆ (7), and T C₈@B₃₂C₃₆^2- (8) which possess a B₃₂C₃₆ (3) shell isovalent with C₆₀, with the neutral D₂ C₈@B₂₄C₄₄ (9) obtained from C₈@B₃₂C₃₆^2- (8) by symmetric C─B substitutions. Detailed adaptive natural density partitioning (AdNDP) bonding analyses and iso-chemical shielding surfaces (ICSSs) calculations indicate that these core-shell species are spherically aromatic in nature, rendering high stability to the systems. More interestingly, based on the calculated effective donor-acceptor interaction between LP(O) → LV(B@B₃C₃) in H₂O@B₃₂C₃₆ (6), we propose the concept of boron bond (BB) in chemistry which is defined as the in-phase orbital overlap between an electronegative atom A as lone-pair (LP) donor and an electron-deficient boron atom with a lone vacant (LV) orbital as LP acceptor. A boron bond appears to possess about 20 ~ 30% of the bond dissociation energy of a typical A-B covalent bond.

Methods: Extensive density functional theory investigations at the hybrid M06-2X-D3 and PBE0-D3 levels with the basis set 6-311 + G(d) were employed to fully optimize the structures of endohedral C₃ CH₄@B₃₂C₃₆ (4), T BH₄@B₃₂C₃₆^- (5), C₁ H₂O@B₃₂C₃₆ (6), C₃ NH₃@B₃₂C₃₆ (7), T C₈@B₃₂C₃₆^2- (8), and D₂ C₈@B₂₄C₄₄ (9), with natural bonding orbital (NBO) and adaptive natural density partitioning (AdNDP) analyses performed to analyze the bonding patterns of the concerned species and the non-covalent interactions reduced density gradient (NCI-RDG) approach utilized to identify the types of the intramolecular non-covalent bonding interactions.