A Unified Linear Free Energy Relationship for Abiotic Reduction Rate of Nitroaromatics and Hydroquinones Using Quantum Chemically Estimated Energies

Abstract

Determining the fate of nitroaromatic compounds (NACs) in the environment requires the use of predictive models for compounds and conditions for which experimental data are insufficient. Previous studies have developed linear free energy relationships (LFERs) that relate the thermodynamic energy of NAC reduction to its corresponding rate constant. We present a comprehensive LFER that incorporates both the reduction and oxidation half-reactions through quantum chemically calculated energies.

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.…”

“…Model predictions for these curves were based on the general rate law (eq )­ where k R is the second-order rate constant, [H 2 Q N, i ], [HQ A, i – ], and [MC] are the concentrations of the reduced neutral and anionic forms of the QFGs and the NAC/MC, respectively, and i is the index of the functional groups. The second-order rate constants are derived from a LFER proposed in a previous study that utilizes the energies of both NAC/MC reduction as well as hydroquinone oxidation where Δ r G HAT 0 (MC) is the HAT reduction energy for the NAC/MC, and Δ r G HAT 0 (H 2 Q N, i ) and Δ r G HAT 0 (HQ A, i – ) are the HAT oxidation energies for the ith functional group. The slope and intercept ( a = 0.43, b = 4.2) were taken from the original LFER study and were calculated from the regression of rate constants from seven NACs and three hydroquinones under two protonated states each (six reductants total) .…”

“…The second-order rate constants are derived from a LFER proposed in a previous study that utilizes the energies of both NAC/MC reduction as well as hydroquinone oxidation where Δ r G HAT 0 (MC) is the HAT reduction energy for the NAC/MC, and Δ r G HAT 0 (H 2 Q N, i ) and Δ r G HAT 0 (HQ A, i – ) are the HAT oxidation energies for the ith functional group. The slope and intercept ( a = 0.43, b = 4.2) were taken from the original LFER study and were calculated from the regression of rate constants from seven NACs and three hydroquinones under two protonated states each (six reductants total) . 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.…”

“…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. ,, …”

“…Such relationships already exist for simpler reductants such as hydroquinones, − iron complexes, , and iron oxides, − but the complex and the poorly defined structure of HAs has to date inhibited the application of generalized models for all but the simplest idealized systems. To overcome this obstacle, we built on the success of a previous model for hydroquinone reductants , and modeled HA as a collection of quinone-like functional groups (QFGs) of different reduction potentials. The hydroquinone model utilized quantum chemically calculated hydrogen atom transfer (HAT) energies of NAC/MC reduction and hydroquinone oxidation in a linear free energy relationship (LFER) to predict second-order rate constants for the overall reaction.…”