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

Abstract

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(ArNO2) of the NAC reaction ArNO2+e−→ArNO2•−. If EH1(ArNO2) is not available, it has been proposed that EH1(ArNO2) be computed directly or estimated from the gas‐phase electron affinity (EA). The model proposed 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 5 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 EA 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. Environ Toxicol Chem 2020;39:1678–1684. © 2020 SETAC

Modeling the Reduction Kinetics of Munition Compounds by Humic Acids

“…The HAT energy, which corresponds to the transfer of a single hydrogen atom (i.e., 1e – + 1H + ), is described in eq for NAC/MC reduction where MCH • is the radical that results from the hydrogen atom (H • ) transfer to an MC molecule. While the aqueous-phase one-electron reduction potential ( E H 1 ) has been utilized previously as the NAC/MC energy descriptor in reduction kinetics LFERs, ,,,,− our more recent studies have shown that calculated HAT energies work as well, , and in some cases better, than the calculated E H 1 values.…”

“…where MCH • is the radical that results from the hydrogen atom (H • ) transfer to an MC molecule. While the aqueousphase one-electron reduction potential (E H 1 ) has been utilized previously as the NAC/MC energy descriptor in reduction kinetics LFERs, 3,11,14,15,30−32 our more recent studies have shown that calculated HAT energies work as well, 13,33 and in some cases better, 20 than the calculated E H 1 values. The computed standard state free energies of formation in eq 7 were obtained using the DFT functional M06-2X and 6-311++G(2d,2p) basis set.…”

“…The computed standard state free energies of formation in eq were obtained using the DFT functional M06-2X and 6-311++G­(2d,2p) basis set. , This functional and basis set were chosen because they had been shown to be successful in calculating NAC reduction energies in previous studies. ,, …”

“…34,35 This functional and basis set were chosen because they had been shown to be successful in calculating NAC reduction energies in previous studies. 20,32,33 Since the structure of the QFGs is not known, a direct calculation of the HAT oxidation energies using quantumchemical calculations is not possible. We developed a relationship between the HAT oxidation energy (transfer of 1H • ) and the reduction potential (transfer of 2H • , or 2e − and 2H + ), which were selected to fit the experimental reduction profile, as discussed earlier.…”

Electron Transfer Energy and Hydrogen Atom Transfer Energy-Based Linear Free Energy Relationships for Predicting the Rate Constants of Munition Constituent Reduction by Hydroquinones

“…Given that ET and HAT energies accurately describe the reduction rate constants of mono-nitro NACs − and that they can be computed from molecular structures without experimental data, the suitability of both energies as thermodynamic descriptors for predicting MC reduction rate constants was evaluated. Surprisingly, ET and HAT energies and experimental second-order rate constants of MCs are scarce or nonexistent in the literature, particularly for NTO and DNP.…”