Using Nitrogen Isotope Analysis to Investigate the Oxidation of Substituted Aromatic Amines

Skarpeli-Liati, M.; Jiskra, M.; Turgeon, A.; Garr, A. N.; Arnold, W. A.; Cramer, C. J.; Schwarzenbach, R. P.; Hofstetter, T. B.
Environ. Sci. Technol. 2011, 45, 5596 (doi:10.1021/es200743t).

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 oxidation of various substituted anilines in suspension of manganese oxide (MnO₂) and compared to reference experiments in homogeneous solution and at electrode surfaces, as well as to density functional theory calculations of intrinsic KIE_N for 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 AKIE_N-values provide a new line of evidence for the identification of contaminant degradation processes via oxidations of primary aromatic amino groups.