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...
The effusion method, together with mass spectral analysis, has been used to study ion/molecule equilibria involving the negative ion BF;. The experimental data obtained have made it possible to calculate enthalpies for the reactions:
New accurate data for the series of 21 higher fullerenes including the first electron affinity (EA) measurements for [72]-and [74]-fullerenes were obtained by the recently developed ion/molecule equilibria method. The regularity of the EA change in the larger fullerene series is analyzed and correlation with the available data on the electronic structure of fullerene molecules and anions is discussed. © 1997 by John Wiley & Sons, Ltd. Received 12 December 1996; Revised 24 January 1997; Accepted 10 February 1997 Rapid. Commun. Mass Spectrom. 11, 662-663 (1997 The discovery of buckminsterfullerene, C 60 , in gaseous phase and the subsequent amazing success in preparing this substance in bulk amounts in 1990 has evoked a real boom in studies of the physical and chemical properties of this quite new form of carbon. Among these studies, fullerene gas-phase chemistry, and gasphase ion chemistry in particular, occupies a considerable place. A review of works on ion/molecule reactions involving mono-and multiply-charged fullerene cations can be found in a recent publication.1 Relatively little work has been focused on the fullerene negative ion chemistry, reflecting the fact that (as well as in the case of 'ordinary' inorganic compounds) negative ions are a more difficult topic for experimental study. 2-5Electron affinity (EA) being one of the principal properties of molecules could help in understanding both the electronic structure and reactive abilities of fullerenes. It is consequently not surprising that these values have attracted intense interest immediately after their discovery.In the present work we utilized the so-called ion/ molecule equilibria (IME) method, 6 which has recently been developed at the Chemistry Department of the Moscow State University and initially assigned for nonvolatile inorganic compounds. Among numerous examples of its successful application to less volatile substances we mention the studies of CsI-HoI 3 system, important for lamp technology, which were conducted in collaboration with the University of Budapest.7 The possibility of the application to fullerenes was by no means obvious because of the relatively high temperatures ( ≥ 700 K) necessary for the appearance of negative ions in the saturated vapour in measurable concentrations. Nevertheless, the first successful measurements of thermal fullerene anions were made using a special procedure. 8The IME method is, as a matter of fact, an extension of the conventional Knudsen method over the ionic component of the gaseous phase of compounds and related systems. Briefly, ions (either negative or positive) contained in the saturated vapour of the investigated system placed into an effusion cell, are extracted through the effusion orifice by a weak electrostatic field, accelerated to energies of several keV and introduced into a mass analyser with subsequent measurement of ion intensities. Neutral (molecular) species of the vapour are analysed by electron impact (EI) ionization in the same way as is done by conventional Knudsen-cell mas...
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