Artifacts in the Electron Paramagnetic Resonance Spectra of C60 Fullerene Ions:  Inevitable C120O Impurity

• Published in: Journal of the American Chemical Society Vol. 124; no. 16; pp. 4394 - 4401
• Main Authors: Paul, Parimal, Kim, Kee-Chan, Sun, Dayong, Boyd, Peter D. W, Reed, Christopher A
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 24.04.2002
• Subjects: Anions; Carbon - chemistry; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Dimerization; Electron paramagnetic resonance and relaxation; Electron Spin Resonance Spectroscopy; Ethers, Cyclic - chemistry; Exact sciences and technology; Free radicals; Fullerenes; Magnetic resonances and relaxations in condensed matter, mössbauer effect; Physics; Temperature dependence; Inorganic compounds; Ionic cluster; Fullerene compounds; Molecular clusters; Magnetic impurities; Magnetic field effects; Experimental study; EPR spectroscopy
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja011832f

Aspects of the electron paramagnetic resonance (EPR) spectra of C60 n - fulleride ions (n = 2, 3) and the EPR signal observed in solid C60 are reinterpreted. Insufficient levels of reduction and the unrecognized presence of C120O, a ubiquitous and unavoidable impurity in air-exposed C60, have compromised most previously reported spectra of fullerides. Central narrow line width signals (“spikes”) are ascribed to C120O n - (n = odd). Signals arising from axial triplets (g ∼ 2.0015, D = 26−29 G) in the spectrum of C60 2- are ascribed to C120O n - (n = 2 or 4). Their D values are more realistic for C120O than C60. Less distinct signals from “powder” triplets (D ∼ 11 G) are ascribed to aggregates of C120O n - (n = odd) arising from freezing nonglassing solvents. In highly purified samples of C60, we find no evidence for a broad ∼30 G signal previously assigned to a thermally accessible triplet of C60 2-. The C60 2- ion is EPR-silent. Signals previously ascribed to a quartet state of the C60 3- ion are ascribed to C120O4-. Uncomplicated, authentic spectra of C60 - and C60 3- become available when fully reduced samples are prepared under strictly anaerobic conditions from freshly HPLC-purified C60. Solid off-the-shelf C60 has an EPR signal (g ∼ 2.0025, ΔH pp ∼ 1.5 G) that is commonly ascribed to the radical cation C60 •+. This signal can be reproduced by exposing highly purified, EPR-silent C60 to oxygen in the dark. Doping C60 with an authentic C60 •+ salt gives a signal with much greater line width (ΔH pp = 6−8 G). It is suggested that the EPR signal in air-exposed samples of C60 arises from a peroxide-bridged diradical, •C60−O−O−C60 • or its decomposition products rather than from C60 •+. Solid-state C60 is more sensitive to oxygen than previously appreciated such that contamination with C120O is almost impossible to avoid.