Getting a theoretical handle on fullerene ions: Quantum chemical calculations on the reactions of C60+, C602+ and C603+ with ammonia

Hybrid density functional theory calculations at the B3-LYP/6-31G** level of theory are used to explore the interactions between singly, doubly, and triply charged fullerene cations and ammonia. The calculations illuminate (and generally support) several aspects of the previously-reported experimental results for these systems. Primary adduct formation is exothermic but is hindered by an energetically costly localized distortion of the fullerene cage at the site of addition, as the 'chosen' carbon atom shifts from strained sp2 to sp3 coordination. The 50-75 kJ mol−1 'deformation energy' is substantially larger than the residual strength of the bond between C60+ and NH3. Although the deformation energy rises with increasing fullerene ion charge state, it does so less steeply than does the electrostatic attraction between C60n+ and NH3, so that the overall bond strength to NH3 is progressively and substantially larger for the di- and tricationic adduct ions. For all charge states, a proton-bound structure is found to be the energetically preferred secondary adduct, but for dicationic and tricationic adducts formation of a secondary adduct is significantly less exothermic than proton transfer. It appears that the failure of C60+ to add measurably to nucleophiles weaker than NH3 arises because such nucleophiles are not able to overcome the required deformation energy to effect bond formation. We find also that, in contrast to a simple electrostatically-driven model of 'handle' formation (in which it was proposed that the most strongly bound doubly-derivatized fullerene dications would be those for which the 'handles' were most widely separated across the fullerene framework) the lowest-energy double-handled adducts are in fact those for which the remaining resonance stabilization is greatest, with electrostatic considerations apparently taking a back seat on the question of relative stability. Similar considerations appear to apply to the double-handled tricationic adducts.