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 Ng2@C60 (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 Ng2@C60 have D3d symmetry when Ng=Ne, Ar, Kr, while the energy-minimum structure of Xe2@C60 has D5d symmetry. The precession movement of He2 in He2@C60 has practically no barrier. The Ng--Ng distances in Ng2@C60 are much shorter than in free Ng2. All compounds Ng2@C60 are thermodynamically unstable towards loss of the noble gas atoms. The heavier species Ar2@C60, Kr2@C60, and Xe2@C60 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 C60+2 Ng. The AIM method reveals that there is always an Ng--Ng bond path in Ng2@C60. There are six Ng--C bond paths in (D3d) Ar2@C60, Kr2@C60, and Xe2@C60, whereas the lighter D3d homologues He2@C60 and Ne2@C60 have only three Ng--C2 paths. The calculated charge distribution and the orbital analysis clearly show that the bonding situation in Xe2@C60 significantly differs from those of the lighter homologues. The atomic partial charge of the [Xe2] moiety is +1.06, whereas the charges of the lighter dimers [Ng2] are close to zero. The a2u HOMO of (D3d) Xe2@C60 in the 1A1g state shows a large mixing of the highest lying occupied sigma* orbital of [Xe2] and the orbitals of the C60 cage. There is only a small gap between the a2u HOMO of Xe2@C60 and the eu LUMO and the a2u LUMO+1. The calculations show that there are several triplet states which are close in energy to each other and to the 1A1g state. The bonding analysis suggests that the interacting species in Xe2@C60 are the charged species Xe2q+ and C60q-, where 1<q<2. The calculated Xe--Xe distance in the endohedral fullerene (2.494 A) is even shorter than the calculated value for free Xe2(2+) (2.746 A). 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 Ar2@C60 and Kr2@C60 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, He2@C60 and Ne2@C60 are weakly bonded van der Waals complexes.

X-ray observation of a helium atom and placing a nitrogen atom inside He@C60 and He@C70

“…Comprehensively, the He atom and the N atom interact with each other only through a van der Waals interaction inside the C 60 cage, whose situation is similar to that of He 2 @C 60 (ref. 37). The present result clearly showed that a single atom of helium in C 60 is observable by means of single crystal X-ray analyses.…”

On the Problem of He–He Bond in the Endohedral Fullerene He2@C60

“…2. In this complex, two He atoms are located at a distance of 1.979Å (unlike 1.948Å, as in work [90]) from each other. Each He atom is located at a distance of about 2.507Å (2.671Å in work [90]) from the C-C bond in the C 60 pentagon.…”

“…In this complex, two He atoms are located at a distance of 1.979Å (unlike 1.948Å, as in work [90]) from each other. Each He atom is located at a distance of about 2.507Å (2.671Å in work [90]) from the C-C bond in the C 60 pentagon. The Mulliken charge of an He atom equals = +0.011 e. The calculated frequency (He-He) = 69.6 cm −1 describes the motion of the diatomic dumbbell-like complex He-He, which was also mentioned earlier in work [90, p. 8260] 9 .…”

“…In order to continue our discussion, let us consider the ground state of a He dimer at two theoretical levels: MP2/aug-cc-pV5Z (see work [96], as well as work [90]) and M06-2X/aug-cc-pV5Z. The both are available in the Gaussian software package [80].…”

“…Actually, it is observed experimentally. It undoubtedly exists in silico: the recent work carried out at the theoretical level SCS-MP2/TZVPP //BP86/TZVPP by Krapp and Frenking [90] predicts the following parameters for the ground state of the complex He 2 @C 60 : the symmetry 3 , the binding energy Δ [He 2 @C 60 ] = −1.25 kcal/mole, the equilibrium distance (He-He) = 1.953Å, and inf ∀C∈C60 (He-C) = 2.696Å.…”