Computer Simulations of Fulleride Anions in Metal-Ammonia Solutions

Monte Carlo computer simulation has been used to study the energetics and local structure of fulleride anions C-60(n-) (n = 0, 2, 4, 6) in metal-ammonia solutions. We find that the enthalpy of dissolution is markedly favorable only for n = 2 and 4, which is in line with experimental observations. Analysis of the structure developed around the fulleride anions shows two strong solvation shells of ammonia at distances of around 6.75 and 9.5 angstrom from the fulleride center of mass. This is in excellent agreement with high-resolution neutron diffraction studies of K5C60 in ammonia. The uncharged fullerene (n = 0) induces no discernible orientational order in the neighboring solvent. In contrast to this, there is progressively stronger hydrogen-bonding of the first-shell solvent to the anions (n = 2, 4, 6), approaching one hydrogen-bond per molecule for n = 6. This maximum of one hydrogen bond per ammonia to the fulleride anion is found to permit intersolvent hydrogen bonding within and across the solvation shells similar to that found in bulk liquid ammonia. Comparison of the cations Li+, Na+, K+, and Ca2+ shows that only the potassium has a tendency to form direct ion-pair complexes with the fulleride anion. This work therefore highlights the mechanisms by which metal-ammonia solutions are able to dissolve high concentrations of fullerene salts.