Substituent Effects on Nitrogen Isotope Fractionation During Abiotic Reduction of Nitroaromatic Compounds

Hofstetter, T. B.; Neumann, A.; Arnold, W. A.; Hartenbach, A. E.; Bolotin, J.; Cramer, C. J.; Schwarzenbach, R. P.
Environ. Sci. Technol. 2008, 42, 1997.

Compound-specific analysis of nitrogen isotope fractionation is an important tool for assessing transformation pathways of N-containing organic contaminants. We investigated ¹⁵N-fractionation during the abiotic reduction a series of nitroaromatic compounds (NACs) with intrinsic reactivities covering almost 6 orders of magnitude to evaluate substituent effects on nitrogen kinetic isotope effects, KIE_N. Insights into reaction mechanisms and isotopic elementary reactions of NAC reduction were obtained from comparison of experimental results to density-functional theory (DFT) calculations of intrinsic KIE_N. Apparent KIE_N for reduction of NACs by structural Fe(II) in octahedral layers of an iron-rich clay mineral, were substantial (1.0385 +/- 0.0026, +/- 1σ), independent of the NACs reactivity and ring substituent, and larger than reported previously for Fe(II) species bound to Fe(III)(oxy)hydroxides and mercaptojuglone species (1.0309 +/- 0.0010). DFT-calculations accounting for semiclassical contributions and quantum-mechanical tunneling yielded KIE_N for N-O bond cleavage between 1.031-1.041, showed no substituent effect, and thus agreed well with experimental observations. Calculated transition-state structures of NAC reduction intermediates were consistent with H₂O elimination from substituted N,N- dihydroxyanilines as the predominant ¹⁵N-fractionating elementary reaction. The absence of substituent effects on apparent KIE_N of NAC reduction may simplify the practical application of ¹⁵N-fractionation data for the quantification of contaminant transformation in the environment.

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