Effect of ultrasound and ionizing radiation on a sterically hindered cyclic secondary amine: an ESR study

The possible use of 2,2,6,6-tetramethyl-4-piperidone (TMPone) for the detection of singlet oxygen was investigated by gamma radiolysis and sonolysis of oxygen-saturated aqueous solutions. Formation of 2,2,6,6-tetra-methyl-4-piperidone-N-oxyl (TAN) was observed with both gamma radiolysis and sonolysis with a similar dependence on the concentration of TMPone up to 20 mM and a strong dependence on pH. In oxygen-saturated solutions the sonolysis of TMPone leads to the formation of the cyclic hydroxylamine (approx. 30% of the yield of TAN) while radiolysis does not. In the low pH range (5-6.5) and at high concentrations of OH radical scavengers (azide or formate), TAN is produced by sonolysis but not by radiolysis. Sonolysis of argon-saturated solutions of TMPone produces methyl radicals due to the high-temperature regions of the collapsing cavitation bubbles. The methyl radicals were detected by ESR (electron spin resonance) and spin trapping with 3,5-dibromo-2,6-dideuterio-4-nitroso-benzene sulfonate. Since the reaction of singlet oxygen with TMPone is also strongly dependent on pH, it does not seem likely that TMPone could be used for the detection of singlet oxygen in sonochemistry.