Bond dissociation energies of the N-H bond and rate constants for the reaction with alkyl, alkoxyl, and peroxyl radicals of phenothiazines and related compounds

The results of a detailed thermodynamic and kinetic investigation on the ho molytic reactivity of phenothiazine, phenoxazine, and phenoselenazine, of s everal substituted phenothiazines, and of related tricyclic aromatic amines are reported. All these compounds give, by hydrogen atom abstraction from the N-H group, persistent aminyl radicals; Equilibration of each of these r adicals with the parent amine and a reference compound having an easily abs tractable hydrogen allowed us to determine, by using EPR spectroscopy, the N-H Bond Dissociation Energies (BDE) of the amines, These are characterized by low BDE values (in some cases lower than the O-H bond strength of alpha -tocopherol, i.e. 78.3 kcal/mol) and therefore are very good hydrogen atom transfer reagents. To check the efficiency of tricyclic amines as antioxida nts and as polymerization inhibitors, absolute rate constants were determin ed for their reaction with alkyl, alkoxyl, and peroxyl radicals by using co mpetitive techniques in the first two cases and by autoxidation studies und er controlled conditions in the last one. AU amines have been found to be h ighly reactive toward these radicals which makes them very good autoxidatio n and polymerization inhibitors.