Using nitrogen isotope fractionation to assess the oxidation of substituted anilines by manganese oxide

We explored the N isotope fractionation associated with the oxidation of substituted primary aromatic amines, which are often the position of initial attack in transformation processes of environmental contaminants. Apparent ¹⁵N-kinetic isotope effects, AKIE_N, were determined for the oxidation of various substituted anilines in suspensions of manganese oxide (MnO₂) and compared to reference experiments in homogeneous solutions and at electrode surfaces, as well as to density functional theory calculations of intrinsic KIE_Nfor electron and hydrogen atom transfer reactions. Owing to the partial aromatic imine formation after one-electron oxidation and corresponding increase in C-N bond strength, AKIE_N-values were inverse, substituent-dependent, and confined to the range between 0.992 and 0.999 in agreement with theory. However, AKIE_N-values became normal once the fraction of cationic species prevailed owing to ¹⁵N- equilibrium isotope effects, EIE_N, of 1.02 associated with N atom deprotonation. The observable AKIE_N-values are substantially modulated by the acid/base pre-equilibria of the substituted anilines and isotope fractionation may even vanish under conditions where normal EIE _N and inverse AKIE_N cancel each other out. The pH-dependent trends of the AKIE_N-values provide a new line of evidence for the identification of contaminant degradation processes via oxidation of primary aromatic amino groups.