THE stability of organic free radicals is generally attributed to electron delocalization, steric inhibition of dimerization, or both (for example, 2,4,6-tri-t-butylphenoxyl), and the nitroxide radicals appear to accord well with these views. In the well-known diaryl nitroxides (I)1,2 the group is attached to two conjugated systems, in (II)3 and (III)4 and related radicals5 it is highly hindered, while in (IV)6 and (V)7 both conjugative and steric factors may contribute to radical stabilization.