Reduction Rate Constants for Nitroaromatic Compounds Estimated from Adiabatic Electron Affinities

Nitroaromatic compounds (NACs) are ubiquitous environmental contaminants and predicting their environmental fate is important. Linear free energy relationships (LFERs) have been reported relating one-electron reduction potentials (E^o_H) of NACs in water to their reduction rate constants (k) measured under various conditions. In a recent effort to calculate E^o_H, we found that E^o_H values of NACs are also linearly correlated with their adiabatic electron affinities (EA). This suggests that the reactivity of NACsand hence their half-livescan be predicted based on their EA values. We report here a new set of LFERs relating EA and k for NACs. Reduction of substituted nitrobenzenes mediated by quinones, natural organic matter, Fe(II) complexes, and the (CH₃)₂COH radical was examined using EA values calculated from quantum mechanics. For monosubstituted nitrobenzenes without ortho substituents, strong linear correlations were found between EA and log k, leading to accurate estimates of k. Deviations between measured and estimated k values for most ortho-substituted and/or polysubstituted compounds were somewhat higher, but were accurate to within approximately an order of magnitude using the same LFER for all compounds. We report estimates of 169 reduction rate constants for 23 compounds in nine reducing systems for which no measured values are available.