Study On The Mechanism Of TEMPO Capturing Peroxy Radicals

With the development of industry,organic materials have been widely used all over the world.However,the peroxyl radicals can propagate the chain reactions,resulting in the oxidative degradation of organic materials.The hindered amine light stabilizers(HALS)inhibit strongly the autoxidation of hydrocabons by trapping the peroxyl radicals,protecting the organic materials.At this time,the detailed mechanism by which the active intermediate of piperidine-HALS eliminates the peroxyl radicals is still not completely clear.Herein,combined with the reported experimental results,we performed quantum chemical calculations at the different levels to uncover this reaction mechanism in the acid and neutral condition.The main conclusion includes(1)In the acid condition,the protonated TEMPO(2,2,6,6-Tetramethyl-1-piperidinyloxy)undergoes proton coupled and then pass single electron transfer process to eliminate the peroxyl radical and regenerate TEMPO.During the reaction,the proton coupled electron transfer step is rate-determining.(2)In the neutral condition,the reaction consists of four steps,the formation of alkoxyamine,beta-hydrogen atom transfer,subsequently,the two reaction steps of carbon center radical dissociation and TEMPO regeneration are carried out,and finally TEMPO restores antioxidant activity.The second step,beta-hydrogen atom transfer is determined to be rate-limit.(3)By comparing the optimized geometries and the energy calculations(including single-point energy,thermal correction,and solvation correction),we analyzed the influence on the computational results by the different levels.In addition,we found that the energy barrier in the acid condition,which is in good agreement with the experimental value,is much lower than that in the neutral condition,indicating that the reaction can be facilitated in the acid condition.(4)Through the detailed analysis,we found that TEMPO can be regenerated during the reaction with the peroxyl radical,explain the high-efficiency antioxidative activity observed in the experiments.The results in the present study will provide some valuable guidelines for the future discovery and development of high-activity antoxidants.