Molecular origin of the stability of violuric acid radicals at high pH-values

Violuric acid forms an iminoxy radical by electrochemical oxidation which shows an astonishing lifetime of more than 6 h at pH 2 and still of about 0.5 h at pH 12 in aqueous solution without any observable dimer formation. The stabilization of the radical over such a wide pH-range is related to amino groups in the ring which cause at high pH-values a negatively charged radical shielded from nucleophilic attacks. This finding is based on the comparison of the pH dependence of the radical lifetime of violuric acid and N,N′-dimethylvioluric acid which were both isolated in solid form.