Equilibrium yield for helium incorporation into buckminsterfullerene: Quantum-chemical evaluation

The binding energy and equilibrium constant for the endohedral He@C60 compound have been determined from ab initio and density functional(DFT) calculations. Very large grids for the numerical integration are necessary to converge the DFT results to within 0.1 kcal/mol. Gradient-corrected DFT methods incorrectly predict He@C60 to be less stable than He+C60. At the highest ab initio level employed, i.e., second-order Mo/ller–Plesset perturbation theory (MP2) with extended basis sets and counterpoise corrections, He@C60 is bound by 2.0 kcal/mol. The equilibrium constant for He incorporation into C60 has been evaluated from Hartree–Fock and DFT interaction potentials adjusted to reproduce the MP2 binding energy. Computed equilibrium yields at 3000 atm and 900 K exceed 10%, compared with 0.1% observed in the experiment, which indicates that suitable catalysts could increase the observed yield significantly