Substituent effects on nitrogen isotope fractionation during abiotic reduction of nitroaromatic compounds

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 of a series of nitroaromatic compounds (NACs) with intrinsic reactivities covering almost 6 orders of magnitude to evaluate substituent effects on ¹⁵N 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 KIEM. Apparent KIE_N values for reduction of NACs by structural Fe(II) in octahedral layers of an iron-rich clay mineral were substantial (average ±1σ of 1.038 ± 0.003), independent of the NACs' reactivity and ring substituent and larger than reported previously for reduction by Fe(II) species bound to Fe(III)(oxy)hydroxides and mercaptojuglone species (1.031 ± 0.002). DFT-calculations accounting for semiclassical contributions and quantum-mechanical tunneling yielded a KIE_N for N-O bond cleavage between 1.031 and 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 the apparent KIE_N of NAC reduction may simplify the practical application of ¹⁵N-fractionation data for the quantification of contaminant transformation in the environment.