Studies on the solution-phase ESR spectra of the C-60 monoanion under varying experimental conditions

ESR spectroscopic studies on C-60(-.) electrochemically generated from C-60 in toluene-acetonitrile solutions (with Bu4NPF6 as the supporting electrol yte) were performed under varying time (0-35 days), temperature (293-133 K) , concentration (0.2-1.0 mM), oxygen content, purity and photolytic conditi ons. It was found that the ESR signals frequently referred to as sharp (Del taH(pp) approximate to 0.1 mT), that are present in low abundance (similar to1-2%) in most samples of C-60(-.) (DeltaH(pp) approximate to 3-6 mT), can exist in several different forms with distinct linewidths and g-values dep ending on the temperature, the oxygen content of solution and the time foll owing the generation of C-60(-.) Only one sharp signal (in addition to the broad signal) was evident immediately after the electrochemical generation of C-60(-.), but with increasing time (hours-days), in the presence of ligh t and/or in the presence of atmospheric air, other sharp signals appeared, some of which were attributable to the formation of precipitate radicals. D ata obtained from a number of experiments suggest that the sharp linewidth signals behave independently of the broad linewidth signal, which reduces t he likelihood that the species responsible for the two types of signals (br oad and sharp) are in equilibrium, or that the paramagnetic species respons ible for the broad signal converts into the species responsible for the sha rp signals. Instead, the data are consistent with the sharp linewidth signa ls originating from an impurity in the original C-60 that is converted to a radical species upon reduction of C-60, which subsequently undergoes furth er chemical reactions to form other paramagnetic species. Copyright (C) 200 0 John Wiley & Sons, Ltd.