Is this a chemical bond? A theoretical study of Ng(2)@C-60 (Ng = He, Ne, Ar, Kr, Xe)

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 D-3d sym metry when Ng=Ne, Ar, Kr, while the energy-minimum structure of Xe-2@C-60 has D-5d 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 N-92@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 kcalmol(-1) less stable than free C-60+2Ng. 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 (D-3d) Ar-2@C-60, Kr-2@C-60, and Xe-2@C-60, whereas the lighter D3d 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,, HOMO of (D3d) Xe2@C60 in the (1)A(1g) state shows a large mixing of the highest lying occupied sigma* orbital of [Xe2] 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-2(q+) and C-60(q-), where 1 < q < 2. The calculated Xe-Xe distance in the endohedral fullerene (2.494 angstrom) is even shorter than the calculated value for free Xe-2(2+) (2.746 angstrom). Thus, the Xe-C and Xe-Xe interactions in Xe2@C60 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 C60 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.