Radical impurity mechanisms for helium incorporation into buckminsterfullerene

Radical impurity mechanisms for incorporating He into C-60 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 C(60)X or C(60)X(2) intermediates generated by adding the impurity X to C-60 (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 C(60)X and C(60)X(2), 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 C(60)X and C(60)X(2). 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 C(60)X and C(60)X(2) are much lower than in C-60. 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).