The Spike in the C₆₀^•- ESR Spectrum:  Oxygen Effect and Negative Temperature Dependence of the C₆₀O₂^•- Isomerization Rate

Abstract

A stable, broad ESR signal (g = 1.998, ΔH pp = 37.0 G) of the C60 •- anion radical was generated by irradiation of a C60−toluene solution in the presence of organic salt, [(ph)3P]2N+(ph)4B-, and 14.3% methanol. Upon exposure to molecular oxygen, the broad band gradually diminishes and a narrow band of g = 2.0008 and ΔH pp = 3.32 G (hereafter, band b) grows, which further transforms to another narrow band of g = 2.0026 and ΔH pp = 1.67 G (hereafter, band c). The transformation rate of bands b to c was found to be negative temperature dependent, i.e., the higher the temperature, the slower the transformation rate. At high temperatures (e.g., 365 K) and in polar solvents (e.g., 30% methanol in toluene), band c can reversibly transform back to band b. Microwave power saturation experiments show that band c has much longer relaxation times than band b. Both bands b and c resemble the “spike” commonly observed in the C60 •- anion radical ESR spectra and were designated to two isomers of the C60O2 •- anion radical. A kinetic model was derived to account for the negative temperature-dependent transformation rate of bands b to c.

Discrete Fulleride Anions and Fullerenium Cations

“…It has been proposed that O 2 can add to C 60 -to form stable adducts. 366 This seems doubtful in the light of the above studies and the observation that C 60 O -rapidly deoxygenates to give C 60 . 374 It is possible, however, that peroxide is formed as a byproduct of simple reduction and this could attack C 60 to form oxygenated products such as fullerene epoxides or fullerenols.…”

“…175,299,357,362 Covalently bonded dioxygen adducts of C 60 -have been recently proposed as the source of sharp signals. 366 To be a serious contender, this proposal needs a much firmer chemical precedent. It is contrary to reports that simple oxidation back to neutral C 60 is the major consequence of exposure of C 60 -to air, 116,117 although polyoxygenated fullerenes have been reported under aerobic electrolysis conditions at the C 60 2-level.…”

“…We have only found one published solution spectrum that is apparently free of sharp signal, although it develops a spike immediately upon air exposure. 366 Indeed, it has been commonly observed that the sharp signal increases at the expense of the broad one upon standing. 133,345,366,377 This can be readily interpreted in terms of C 60 -being more easily oxidized than C 120 O -to an EPR-silent species (C 60 ), even to the point that the spike is the only remaining trace signal after extensive aging in air.…”

Studies on the solution-phase ESR spectra of the C60 monoanion under varying experimental conditions

“…3b A comparison of the spectra shown in Fig. 3 at decreasing (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14) and increasing (15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28) temperatures illustrates that there is a high degree of reversibility in the temperature-dependent response. Unfortunately, it is very difficult to correlate changes in the signal intensity of the sharp signals with that of the broad signal (2) with changing temperature, because the sharp signals are present in such low abundance compared with the broad signal (especially before the decomposition of the broad signal that occurs after several days in normal light conditions).…”

“…The g-value of 1c (2.0028) is very similar to that of a species obtained by the reaction of C 60 ž with O 2 reported previously (2.0026). 7 Attempts to isolate and characterize 1c were not successful, mainly owing to the low solubility of C 60 , which made it difficult to scale the reactions up to a degree that would enable a species present in 1% abundance to be isolated.…”