Time-resolved resonance Raman observation of the extreme protonation forms of a radical zwitterion in water

The reactions of the aqueous proton with the zwitterionic p-aminophenoxyl radical in strongly basic to extremely acidic aqueous solutions have been investigated using time-resolved resonance Raman spectroscopy. The dynamic stability of the different protonation forms of the radical, observed on the microsecond time scale in this work, has been achieved by controlling the proton exchange rate in water. In strongly acidic solutions we observe a rare ring-H+ bonded dication species, a key intermediate in the amine hydrolysis. The neutral p-aminophenoxyl radical undergoes NH2-deprotonation in strongly basic aqueous solutions, which has no analogues in closed-shell amines.