Mass spectrometric study of unimolecular decompositions of endohedral fullerenes

Unimolecular decompositions of noble gas containing endohedral fullerenes as well as metallofullerenes were studied using tandem mass spectrometry techniques. Endohedral fullerenes do not lose the endohedral atom unimolecularly but fragment via the loss of C₂ units. Kinetic energy release distributions were measured for the emission of C₂ units from the positive ions of C₆₀, Ne@C₆₀, Ar@C₆₀, Kr@C₆₀, C₈₂, La@C₈₂, Tb@C₈₂, C₈₄, and Sc₂@C₈₄. These distributions were analyzed using both a model free approach, and a formalism developed by Klots, based on decomposition in a spherically symmetric potential. The C₂ binding energies were deduced from the models. Noble gas atoms are shown to stabilize the fullerene cage. The C₂ binding energies increase in the order: ΔE_vap(C₆₀⁺) < ΔE_vap(Ne@C₆₀⁺) < ΔE_vap(Ar@C₆₀⁺) < ΔE_vap(Kr@C₆₀⁺). Endohedral metal atoms have a strong effect on the cage binding. The C₂ binding energy in La@C₈₂⁺ is about 1.5 eV higher than that in C₈₂⁺. The Tb atom has an even stronger effect with a binding energy of about 3 eV higher than for C₈₂⁺. The emission of a C₂ unit from the dimetallofullerenes Sc₂@C₈₄⁺ and Tb₂@C₈₄⁺ was studied as well. Two Sc atoms have a slight destabilizing effect on C₈₄, whereas two Tb atoms stabilize the cage.