Multifrequency CW EPR spectroscopy of extraterrestrial carbonaceous matter

In this work, the carbonaceous matter of Orgueil, Murchison and Tagish Lake carbonaceous meteorites and a reference coal is studied by multifrequency continuous-wave electron paramagnetic resonance (EPR) spectroscopy from 4 to 285 GHz. It is found that the shape of the EPR line of the radicals in meteoritic carbonaceous matter is Lorentzian in all the frequency range, while the line shape of the coal is Lorentzian only below 95 GHz and becomes inhomogeneously broadened at higher frequency, as previously observed for coals by other authors. This points to strong exchange interactions in meteoritic carbonaceous matter, resulting from a pronounced spin clustering that does not occur in biogenic carbonaceous matter (coals). The temperature dependence of the EPR line width has been studied in detail at X- and W-bands for the Orgueil meteorite. It confirmed our previous model of the presence of radicals with thermally accessible triplet states (TATS) in meteorites. These TATS, which were attributed to diradicaloids moieties on the basis of molecular quantum DFT calculations (L. Binet, D. Gourier, S. Derenne, F. Robert, I. Ciofini: Geochim. Cosmochim. Acta 68, 881-891, 2004) do not exist in biogenic carbonaceous matter. This analysis also precised the strength of the clustering effect in meteorites, yielding an estimated local spin concentration N = 5 center dot 10(20) spin/g, which is two orders of magnitude higher than the average spin concentration in the Orgueil meteorite. It is important to note that such spin clustering has also been observed by other authors in synthetic hydrogenated amorphous carbon. It seems that the clustering of radicals is a common feature of synthetic and extraterrestrial (abiotic) carbonaceous matters, while radicals are homogeneously distributed in biogenic carbonaceous matter.