Electron affinities of higher fullerenes

Abstract

Knudsen cell mass spectrometry has been used for the investigation of ion-molecule equilibria between gaseous neutral species and negative ions in fullerene vapors at 800-900 K. Equilibrium constants of one-electron exchange reactions between Cso and the higher fullerenes C70, C76, C78 and Cu have been measured. Electron afiinities of C70, C76, C78 and Cw were found to be 2.72k0.05, 2.88k0.05, 3.01+0.07, and 3.05+0.08eV, respectively. Negative ions of CS, Cm and Cg2 were observed, and the lower limit of electron affinities for these species was estimated as 3 3.0 eV.Recent studies revealed'. that fullerene Cm forms very stable negative ions C, . The ion chemistry of fullerenes is develo ing rapidly. The first theoretical predicted that the electron affinity (EA) of Cm must be not less than 2.2-2.6 eV. Some experimental attempts have been made to obtain EA(Cm).'-, The photoelectron spectroscopy method has been used to measure EA values for negative carbon-cluster ions., The result obtained for EA(C,) was consistent with the previous estimates, viz: 2.650+ 0.050 eV. There are some publications on the investigation of fullerene anions ('fullerides') in solution.' The electrospray ionization method has been used to obtain C, and C, through the reduction of fullerenes in benzene solution by NaK amalgam.' It was shown that C, is more reactive than C, , and the anions C,, C,, C, and C, have also been detected. Yang et al.' observed photoelectron spectra of negative cluster ions from C4 to CM. It was supposed that C, should have the lowest EA among the higher fullerenes, but up till now there have been no reliable estimates of EAs for C, (n >60).This report presents the first direct determination of the differences between the electron affinities of Cm and the higher fullerenes C7", C76, C78 and CM, and gives estimates for the cases of CR6, Cw and CW. EXPERIMENTAL Instrumentation.All experiments were performed using an MI-1201 magnetic mass spectrometer, modified for measuring both charged and neutral species in high temperature vapors. Details of the apparatus and measurement procedures have been described elsewhere."A nickel effusion cell, with an orifice/evaporation area ratio of approximately 500 was used in all experiments, and was resistively heated up to 950K. A Pt/Pt-Rh thermocouple was used for measuring temperature.Recording of neutral species. Electron impact (EI) mass spectrometry conditions were as follows: electronionizing energy, 70 eV; emission current, 0.3-0.5 mA; mass resolution, 840 (10% valley definition).Recording of negative ions. Negative ions in equilibrium with neutral fullerene species were extracted from the Knudsen cell, and their intensities measured after passing them through the magnetic analyzer. Reagents. Cm, more than 99.8% purity; temperature and heat of phase transition, 260 K and 8.7 J/g respectively; C70, more than 98.5% purity; mixed extract, 75% C, and 25% C70; mixture of the highest fullerenes: Cm, 6%, C70, 31%, C76, 27%, C78, l8%, CMr 18% (data obtained by HPLC). All samples were...

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