Hydrogen Atom Transfer Reaction Free Energy as a Predictor of Abiotic Nitroaromatic Reduction Rate Constants: A Comprehensive Analysis.

A linear free energy model is presented that predicts the second order rate constant for the abiotic reduction of nitroaromatic compounds (NACs). Previously presented models use the one electron reduction potential EH1 (ArNO 2 ) of the NAC reaction ArNO2+e-→ArNO2•- . If EH1 (ArNO 2 ) is not available, it has been proposed that EH1 (ArNO 2 ) be computed directly or estimated from the gas phase electron affinity (EA). The model proposed below uses the Gibbs free energy of the hydrogen atom transfer (HAT) reaction ArNO2+H•→ArNOOH• as the parameter in the linear free energy model. Both models employ quantum chemical computations for the required thermodynamic energies. The available and proposed models are compared using experimentally determined second order rate constants from five investigations from the literature in which a variety of NACs were exposed to a variety of reductants. A comprehensive analysis utilizing all the NACs and reductants demonstrate that the HAT energy model and the experimental one electron reduction potential model have similar root mean square errors and residual error probability distributions. In contrast, the model using the computed electron affinity has a more variable residual error distribution with a significant number of outliers. The results suggest that a linear free energy model utilizing computed HAT reaction free energy produces a more reliable prediction of the NAC abiotic reduction second order rate constant than previously available methods. The advantages of the proposed HAT energy model and its mechanistic implications are discussed as well. This article is protected by copyright. All rights reserved.