Radical Impurity Mechanisms for Helium Incorporation into Buckminsterfullerene

Radical impurity mechanisms for incorporating He into C₆₀ have been examined by semiempirical (MNDO) and density functional (BLYP/3-21G) calculations. The key step in these mechanisms is the insertion of He into C₆₀X or C₆₀X₂ intermediates generated by adding the impurity X to C₆₀ (X = H, Me in our model study). Contrary to C_60. several window-type structures with one broken C,C bond exist as local minima on the MNDO potential surfaces of C₆₀X and C₆₀X₂, but they are mechanistically irrelevant due to extremely facile ring closure. Effective activation barriers for the penetration of He through intact hexagons and various windows are reported for 65 different pathways in C₆₀X and C₆₀X₂. Window-type transition states are stabilized significantly when there is a C? X bond involving a C-atom from the broken C,C bond. The corresponding barriers in C₆₀X and C₆₀X₂ are much lower than in C₆₀, This provides some theoretical support for the suggested impurity mechanisms even though the computed barriers for X = H, Me are still higher than indicated by the experiment (X unknown).