More than half of the known elements in the periodic table have been successfully added into fullerene frameworks, but only a few endohedral fullerenes are known with more than two atoms inside the carbon cage. A team around Ralf Tonner and Peter Schwerdtfeger, Philipps University, Marburg, Germany, addressed the question: How many rare gas atoms can be placed into a fullerene cage until the pressure becomes large enough to break the C₆₀ framework?

Quite surprisingly, they found by density functional and ab initio computations that one can fit up to 40 He, 17 Ne, 7 Ar, 6 Kr or 6 Xe atoms inside the carbon cage before the fullerene becomes unstable. That is, before it breaks open in the course of the calculation at rather high energies of 1000 to 2000 kcalmol^–1, which underlines the high stability of this unique carbon structure.

• Packed to the Rafters: Filling up C₆₀ with Rare Gas Atoms,
  Ralf Tonner, Gernot Frenking, Matthias Lein, Peter Schwerdtfeger,
  ChemPhysChem 2011, 12, 2081-2084.
  DOI: 10.1002/cphc.201100360