Dynamical Jahn-Teller effect of fullerene anions

The dynamical Jahn-Teller effect of ${\mathrm{C}}₆₀^{n\ensuremath{-}}$ anions ($n=1$--5) is studied using the numerical diagonalization of the linear ${p}ⁿ\ensuremath{\bigotimes}8d$ Jahn-Teller Hamiltonian with the currently established coupling parameters. It is found that in all anions the Jahn-Teller effect stabilizes the low-spin states, resulting in the violation of Hund's rule. The energy gain due to the Jahn-Teller dynamics is found to be comparable to the static Jahn-Teller stabilization. The Jahn-Teller dynamics influences the thermodynamic properties via strong variation of the density of vibronic states with energy. Thus the large vibronic entropy in the low-spin states enhances the effective spin gap of ${\mathrm{C}}₆₀^{3\ensuremath{-}}$ quenching the spin crossover. From the calculations of the effective spin gap as a function of the Hund's rule coupling, we found that the latter should amount $40\ifmmode\pm\else\textpm\fi{}5$ meV in order to cope with the violation of Hund's rule and to reproduce the large spin gap. With the obtained numerical solutions, the matrix elements of electronic operators for the low-lying vibronic levels and the vibronic reduction factors are calculated for all anions.