Fluorinated fullerenes have been reported by several groups recently.[', Weight uptake and mass spectrometry studies by Selig et al. ['] have shown that direct fluorination of C,, leads to the formation of C,,F, species with n being an even number (such as C,,F,, , C,,F,, , C,,F4,, etc.) and a negligibly small contribution of fullerenes with odd n values. This does not necessarily mean that odd fluorinated fullerenes are absent in the solid state. It may suggest, however, that odd C,,F, are less stable at T = 350 "C at which the mass spectrometry was performed.
Generally speaking, C,,F, species with odd n should be paramagnetic, because the formation of an odd number of C-F bonds should leave an extra "dangling bond" or unpaired electron. By contrast, C,,F,, with even n is a diamagnetic singlet because of the closed shell. The unpaired electron in odd C,,F, is expected to be localized on the neighboring carbon atom, and thus a large transferred hyperfine interaction due to 19F nuclei is expected.
Estimates based on dipolar interaction assuming the distance between adjacent carbon atoms to be 1.45 A (on the pentagons) and 1.40 A (between the hexagons) yield hyperfine constants of 19A % 160 G and 19A z 180 G, respectively. Thus, odd C,,F, are paramagnetic and may be detected by ESR. This communication reports on ESR and in-situ ESR measurements of fluorinated fullerenes. Unlike previously reported ESR spectra of fullerenes which exhibit a single unresolved line,[31 the ESR spectra of fluorinated fullerenes show clear structure which can be attributed to hyperfine splitting and to the formation of odd C,,F,, .