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

Abstract

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 C 120O 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 EPRsilent. Signals previously ascribed to a quartet state of the C60 3-ion are ascribed to C120O 4-. 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, ∆Hpp ∼ 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 (∆Hpp ) 6-8 G). It is suggested that the EPR signal in airexposed 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.The addition of electrons to C 60 is its most characteristic chemical property. As a consequence, over the past decade, a great deal of effort has been put into characterizing the electronic structure of fulleride anions, C 60 n-. 1 In this paper, we show that the electron paramagnetic resonance (EPR) characterization of these ions has been significantly compromised by imprecise levels of reduction and by the presence of an unrecognized impurity in [60]-fullerene, namely, C 120 O.In addition, we question the widely held belief that the EPR signal commonly observed in solid "pristine" C 60 is due to the radical cation C 60•+ . [2][3][4][5][6] In contrast to the easy addition of electrons, the removal of electrons from C 60 is extremely difficult. 7 Thus, the formation of air-stable C 60 •+ radical cation centers is not easy to chemically rationalize. Now that a synthetic route to C 60•+ has become available, 8 a purposeful doping experiment into C 60 can be performed.The C 60 3-ion is a particularly important fulleride ion to characterize well because it gives rise to superconductivity in A 3 C 60 fulleride phases (A ) alkali metal). The triply degenerate nature of the t 1u lowest unoccupied molecular or...

Spin dynamics and charge transfer in C60·2ferrocene studied by electron spin resonance

“…Such ESR spectra resemble closely those reported at high temperatures for many fullerides, comprising a very narrow 'spike' ESR line with DH pp of 1-2 G, frequently superimposed on a much broader signal of C À 60 ions [1]. Recent experiments have shown that the frequently debated origin of this sharp ESR signal is related to the presence of C 120 O of an unrecognized impurity in the air-exposed samples of C 60 [13,14]. Specifically, this narrow ESR signal has been assigned to a doublet spin state (S = 1/2) of C 120 O À which also accounts for the frequent temporal reduction of the C À 60 ESR line intensity, as C 60 is more easily Fig.…”

Jahn-Teller effect versus Hund’s rule coupling inC60N−

“…However, there was a controversy 27 on whether the low-spin/high-spin gap is very small (below one wavenumber 28 ) or rather of the order of 600 wavenumbers 29 . Recently, this problem was carefully reconsidered and it was shown that activated behaviors of C N − 60 which were observed so far and used to determine the gap are in fact due to C 120 O impurities 30,31 . However, the work clearly reconfirms that iso-lated C N − 60 ions (N = 2, 3) are in the low-spin state.…”

“…With these selection rules and the spectrum of the top given in (46), we find that the ground state and lowest excitations have quantum numbers (LK) = (10), (2 −2), (4 −4), (32), (30). The next higher state in energy is (6−6) which, according to (50), is allowed to mix with (6 −2).…”

Development of Conductive Organic Molecular Assemblies: Organic Metals, Superconductors, and Exotic Functional Materials

“…1070 Recently, we prepared a single crystal of PhCN) 2 , which forms a 3D framework built of alternating Cp à Co þ and C 60 2À ions with channels accommodating solvent molecules (Fig. 154).…”