Internal Rotation in the Singly Bonded Dimers of Substituted C60. A Molecular Lever

The rotation about the intercage bond of singly bonded C 60 dimers RC 60 C 60 R (R = H, Me, and t -Bu attached at ortho or para positions from pivot carbon atoms) obtained by the dimerization of RC 60 • radicals, has been studied by molecular mechanics. The predicted rotational barriers are high, ranging from 15 kcal/mol for the least hindered p, p‘ -(HC 60 ) 2 to the almost forbidding values for those carrying tert -butyl groups. Contrary to the general notion, rotamers with R groups approached close to each other like the gauche form are predicted to be consistently more stable than those with maximally separated Rs (trans), by as much as 10 kcal/mol in the case of R = tert -butyl. The reasons for the unexpected stability of the gauche-like conformation as opposed to the trans-like conformation in these fullerene dimers have been analyzed in detail and interpreted in terms of a lever effect peculiar to the dumbbell shape of the molecule.