GAS-PHASE ASSOCIATION REACTIONS OF FULLERENE CATIONS - MODELING THE INFLUENCE OF CHARGE-STATE AND OTHER MOLECULAR-PARAMETERS ON ASSOCIATION EFFICIENCY

The efficiency of association reactions of fullerene cations observed in helium at pressures of 0.35-0.4 Torr sustained in a Selected-Ion Flow Tube (SIFT) at room temperature has been examined with a view to the influence of the charge state and the size of the fullerene ion and the size, structure, and degree of unsaturation of the neutral. The association efficiency is found to be strongly dependent on these molecular parameters. The calculated lifetime of the intermediate collision complex provides insight into the well depth associated with the formation of this complex and the effectiveness of vibrational and (or) rotational degrees of freedom of the reactants in the dispersal of the energy of complex formation. The results are considered in terms of single-well and double-well models for the potential energy surface leading to association and are best accommodated by the double-well model involving the formation of an initial electrostatic collision complex that must overcome a barrier to proceed to the formation of a chemically bonded adduct. The observed competition between association and charge transfer can also be accommodated with a double-well model, which provides a lower limit for the depth of the electrostatic well for those systems that exhibit such competition.