SELECTED-ION FLOW TUBE STUDY OF CHARGE-TRANSFER FROM FULLERENE DICATIONS - BRACKETING THE 2ND-IONIZATION ENERGIES OF C-60 AND C-70

Rate coefficients are reported for charge-transfer reactions of C60(2+) and C70(2+) with various neutral molecules at 294 +/- 2 K. Charge transfer is not observed for molecules with ionization energy (IE) greater-than-or-equal-to 9.69 eV (for C60(2+)) and IE greater-than-or-equal-to 9.26 eV (for C70(2+)), in keeping with a recent Fourier transform-ion cyclotron resonance (FT-ICR) study. Nevertheless, our observations support a number of experimental and theoretical determinations of IE(C60.+) for which a substantially larger value was obtained, and we propose an upper limit of IE(C60.+) < 11.8 eV. The absence of charge transfer from C60(2+) to molecules having ionization energies in the range 9.7-11.8 eV is accounted for by a reaction barrier arising from the Coulombic repulsion between the two adjacent monocationic fragments of such a charge-transfer process.