Using Nitrogen Isotope Fractionation to Assess the Oxidation of Substituted Anilines by Manganese Oxide

Abstract

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 (15)N-kinetic isotope effects, AKIE(N), were determined for the oxidation of various substituted anilines in suspensions of manganese oxide (MnO(2)) and compared to reference experiments in homogeneous solutions 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 (15)N-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.

Isotope Fractionation Associated with the Photochemical Dechlorination of Chloroanilines

“…We can also exclude the contribution of H + exchange reactions to the observed N isotope fractionation at pH 2.0 as in processes, where only the neutral species determines the reactivity of a substituted aniline (e.g., mineralcatalyzed oxidation). Such cases are governed by large normal N isotope fractionation (i.e., ϵ N < 0 43,44 ) in contrast to what is shown in Figure 2b.…”

“…Polar transformation products were analyzed with LC/MS using a LTQ (Linear Trap Quadrupole) Orbitrap mass spectrometer (Thermo) with electrospray ionization as documented previously. 42,44 Gradient elutions from 90/10 to 5/ 95 vol % ratios of H 2 O/MeOH were run on Atlantis C-18 (15 cm × 3.0 mm, 3 μm) and XBridge C-18 (5 cm × 2.1 mm, 3.5 μm) columns from Waters. Both eluents contained 0.1 vol % formic acid.…”

Oxidation potentials of phenols and anilines: correlation analysis of electrochemical and theoretical values

“…First, to obtain a dataset with maximum overlap with the phenols and anilines for which there are electrochemical potentials from Suatoni et al and/or the newly-measured values reported in this study, we included most of the phenols and anilines in Tables S2 and S3. † Second, to represent the putative initial oxidation step for phenols and anilines [38][39][40] at the pH of Suatoni's work, E 1 was calculated for simple electron transfer from the neutral form of the phenols and anilines to the corresponding phenoxy or aryl amino radicals (i.e., PhOH # PhOH + + e À and ArNH 2 # ArNH 2 + + e À ) assuming no atom transfers. Third, to provide an avenue for extending the coverage of substituent combinations in future work, we chose moderately-high, but accessible levels of theory, so calculations could be done for many compounds without special accommodations (such as for the larger or more exible compounds).…”

Current challenges in compound-specific stable isotope analysis of environmental organic contaminants

“…More recently, isotope fractionation factors have become available for a wider range of contaminants including pesticides [62][63][64][65], anilines [66], and explosives [67][68][69]. Overall, most studies have reported isotope fractionation factors for C; data for other elements (H, N, Cl, O, S) are less frequent in this field.…”

“…3 Instrumentation for nitrogen-isotope analysis by GC-IRMS and substituted anilines [66] in water samples, by using different enrichment procedures (solid phase (micro)extraction, solvent extraction, etc.) to compensate for the poor sensitivity of CSIA of nitrogen.…”