Stable perinaphthenyl radicals in flints

The technique of electron spin resonance (ESR) spectroscopy is proving useful in determining dates by the study of radical species in hard geological and archaeological samples. The key aspect of this work is that normally unstable radical centres, when formed in rigid solids, are rendered inactive by their immobility. Such species may be formed by ionizing radiation, or by thermal treatment. Flint, being an extremely hard form of polycrystalline silica, provides an ideal trapping medium, and we have recently established that methyl radicals (CH3), formed thermally in various flint samples from unknown precursors, are trapped in cavities and are stable up to at least 350 °C. We have now found that another organic radical, perinaphthenyl (or phenalenyl, compound I in Fig. 1) is also formed in a variety of flint samples. ESR spectra provide an unambiguous detection of these radicals, and show that they are rotating rapidly at room temperature. Dimers are not observed, indicating that they are individually isolated in flint cavities.