Paramagnetic rings intermolecular interactions

Since the relaxation times depend to the sixth power on the distance of the nuclei from the paramagnetic center, the free radical site must be further away from the H-4 and H-6 protons. This would eliminate the sulfur atom as a possible free radical site, leaving the nitrogen as the most likely candidate. Furthermore, it is not possible for the tricyclic ring to assume a conformation such that the protons at positions 4 and 6 are further away from the nitrogen free radical site than their immediate neighboring protons at positions 3 and 7, suggesting that intermolecular interaction must be important. 

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