ENDOR of Fremy's salt

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Abstract
The 14N ENDOR of the radical anion (SO3)2NO= has been observed in fluid solutions in water and in water-ethanol mixtures, and trapped in irradiated single crystals of potassium hydroxylamine disulphonate. The quadrupole coupling has been estimated from the solid-state spectra and it is argued that this interaction is too weak to provide a significant relaxation mechanism for the radical in solution. Spectra have been calculated assuming relaxation occurs through modulation of the g and dipolar tensors and the spin-rotation coupling, together with Heisenberg exchange, and the results agree quite well with the spectra observed for the radical in water. An additional relaxation mechanism seems to operate in the water-ethanol mixtures.