The spike in the C-60(center dot-) ESR spectrum: Oxygen effect and negative temperature dependence of the C60O2 center dot- isomerization rate

A stable, broad ESR signal (g = 1.998, Delta H-pp = 37.0 G) of the C-60(.-) anion radical was generated by irradiation of a C-60-toluene solution in the presence of organic salt, [(ph)(3)P](2)N+(ph)B-4(-), and 14.3% methanol. Upon exposure to molecular oxygen, the broad band gradually diminishes and a narrow band of g = 2.0008 and Delta H-pp = 3.32 G (hereafter, band b) grows, which further transforms to another narrow band of g = 2.0026 = and Delta H-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 C-60(.-) 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.