Experimental Validation of Hydrogen Atom Transfer Gibbs Free Energy as a Predictor of Nitroaromatic Reduction Rate Constants

Abstract

Nitroaromatic compounds (NACs) are a class of prevalent contaminants. Abiotic reduction is an important fate process that initiates NAC degradation in the environment. Many linear free energy relationship (LFER) models have been developed to predict NAC reduction rates. Almost all LFERs to date utilize experimental aqueous-phase one-electron reduction potential (E H 1) of NAC as a predictor, and thus, their utility is limited by the availability of E H 1 data. A promising new approach that utilizes computed hydrogen atom transfer (HAT) Gibbs free energy instead of E H 1 as a predictor was recently proposed. In this study, we evaluated the feasibility of HAT energy for predicting NAC reduction rate constants. Using dithionite-reduced quinones, we measured the second-order rate constants for the reduction of seven NACs by three hydroquinones of different protonation states. We computed the gas-phase energies for HAT and electron affinity (EA) of NACs and established HAT- and EA-based LFERs for six hydroquinone species. The results suggest that HAT energy is a reliable predictor of NAC reduction rate constants and is superior to EA. This is the first independent, experimental validation of HAT-based LFER, a new approach that enables rate prediction for a broad range of structurally diverse NACs based solely on molecular structures.

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

“…According to the mechanism proposed by Hartenbach et al (2008) and Hofstetter et al (2008), NAC reduction occurs through sequential electron and proton transfer in multiple steps, rather than in one single step as a hydrogen atom. This is consistent with the observations that reductants that can donate electrons but not hydrogen atoms, such as Fe(II) complexes (Schwarzenbach and Gschwend 1990; Naka et al 2006) and the fully deprotonated form of hydroquinones (Murillo‐Gelvez et al 2019), are highly reactive toward NACs. Given that Gibbs free energy is a state function, the HAT energy of an NAC is identical to the sum of the free energies associated with transferring an electron and a proton serially to the NAC.…”

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

“…Aqueous‐phase EA and HAT reaction energies were fit to experimental second‐order rate constants using Supplemental Data, Equations S8 and S10, respectively (Figure 1). From the previous batch reduction experiments (Murillo‐Gelvez et al 2019), rate constants were available for 2 protonated states for each of the 3 reduced hydroquinones: doubly and singly protonated lawsone (H 2 LAW − and HLAW 2– ), doubly and singly protonated AQS (H 2 AQS − and HAQS 2− ), and doubly and singly protonated AQDS (H 2 AQDS 2− and HAQDS 3− ). The H and H 2 in the nomenclature correspond to the number of protonated hydroxyl groups that are present.…”

“…Second‐order rate constants for the reduction of 7 NACs were measured under various reducing conditions (Murillo‐Gelvez et al 2019). The NACs employed were 2‐amino‐4‐nitroanisole, 4‐amino‐2‐nitroanisole, 2‐chloro‐4‐nitroaniline, 4‐nitroanisole, 4‐nitrotoluene, nitrobenzene, and N ‐methyl‐4‐nitroaniline.…”

“…aq = aqueous; AQS = anthraquinone-2-sulfonate; AQDS = anthraquinone-2,6-disulfonate; H 2 LAW − and HLAW 2-= doubly and singly protonated lawsone; H 2 AQS − and HAQS 2− = doubly and singly protonated AQS; H 2 AQDS 2− and HAQDS 3− = doubly and singly protonated AQDS; RMSE = root mean square error; R = gas constant; T = temperature. protonated AQS and AQDS hydroquinones across both reactions, which was likely due to their lower relative concentrations at the pH conditions tested, as well as the NAC 4amino-2-nitroanisole behaving as an outlier in the original experiment (Murillo-Gelvez et al 2019).…”

“…This observation suggests that the mechanism for the reduction as well as the rate‐limiting step in the reaction between NACs and these different hydroquinones are the same. This relationship is most prominent in a study by Murillo‐Gelvez et al (2019), where a total of 6 hydroquinone species (3 hydroquinones [Supplemental Data, Figure S1], each with 2 protonation states) were fit to LFERs with a single common slope and 6 separate intercepts unique to the different hydroquinone species (Supplemental Data, Figure S2). If these intercepts correspond to the intrinsic reactivity of the hydroquinone species toward NACs, we hypothesize, a thermodynamic property of hydroquinones that represents their reactivity as a reductant may be incorporated into existing LFERs to produce a single comprehensive LFER that encompasses both NACs and hydroquinones.…”