Is This a Chemical Bond? A Theoretical Study of Ng2@C60 (Ng=He, Ne, Ar, Kr, Xe)

Abstract

Quantum‐chemical calculations using DFT (BP86) and ab initio methods (MP2, SCS‐MP2) have been carried out for the endohedral fullerenes Ng 2@C 60 (Ng=He–Xe). The nature of the interactions has been analyzed with charge‐ and energy‐partitioning methods and with the topological analysis of the electron density (Atoms‐in‐Molecules (AIM)). The calculations predict that the equilibrium geometries of Ng 2@C 60 have D3 d symmetry when Ng=Ne, Ar, Kr, while the energy‐minimum structure of Xe 2@C 60 has D5 d symmetry. The precession movement of He 2 in He 2@C 60 has practically no barrier. The NgNg distances in Ng 2@C 60 are much shorter than in free Ng 2. All compounds Ng 2@C 60 are thermodynamically unstable towards loss of the noble gas atoms. The heavier species Ar 2@C 60, Kr 2@C 60, and Xe 2@C 60 are high energy compounds which are at the BSSE corrected SCS‐MP2/TZVPP level in the range 96.7–305.5 kcal mol−1 less stable than free C 60 + 2 Ng. The AIM method reveals that there is always an NgNg bond path in Ng 2@C 60. There are six NgC bond paths in (D3 d) Ar 2@C 60, Kr 2@C 60, and Xe 2@C 60, whereas the lighter D3 d homologues He 2@C 60 and Ne 2@C 60 have only three NgC 2 paths. The calculated charge distribution and the orbital analysis clearly show that the bonding situation in Xe 2@C 60 significantly differs from those of the lighter homologues. The atomic partial charge of the [Xe 2] moiety is +1.06, whereas the charges of the lighter dimers [Ng 2] are close to zero. The a 2u HOMO of (D3 d) Xe 2@C 60 in the 1 A 1g state shows a large mixing of the highest lying occupied σ* orbital of [Xe 2] and the orbitals of the C 60 cage. There is only a small gap between the a 2u HOMO of Xe 2@C 60 and the e u LUMO and the a 2u LUMO+1. The calculations show that there are several triplet states which are close in energy to each other and to the 1 A 1g state. The bonding analysis suggests that the interacting species in Xe 2@C 60 are the charged species Xe 2q+ and C 60q−, where 1<q<2. The calculated XeXe distance in the endohedral fullerene (2.494 Å) is even shorter than the calculated value for free Xe 22+ (2.746 Å). Thus, the XeC and XeXe interactions in Xe 2@C 60 should be considered as genuine chemical bonds which are enforced by the compression energy. The NgNg and NgC interactions in the lighter homologues Ar 2@C 60 and Kr 2@C 60 may also be considered as chemical bonds because the theoretically predicted properties of the endohedral fullerenes are significantly different from the free C 60 and noble gas atoms. According to the bonding analysis, He 2@C 60 and Ne 2@C 60 are weakly bonded van der Waals complexes.