Molecular mechanism of NDMA formation from N,N-dimethylsulfamide during ozonation: Quantum chemical insights into a bromide-catalyzed pathway

During ozonation of drinking water, the fungicide metabolite NN-dimethyIsulfamide (DMS) can be transformed into a highly toxic product, N-nitrosodimethylamine (NDMA). We used quantum chemical computations and stopped-flow experiments to evaluate a chemical mechanism proposed previously to describe this transformation. Stopped-flow experiments indicate a pK(a) = 10.4 for DMS. Experiments show that hypobromous acid (HOBr), generated by ozone oxidation of naturally occurring bromide, brominates the deprotonated DMS- anion with a near-diffusion controlled rate constant (7.1 +/- 0.6 X 10(8) M-1 s(-1)), forming Br-DMS- anion. According to quantum chemical calculations, Br-DMS has a pK(a) similar to 9.0 and thus temains partially deprotonated at neutral pH. The anionic Br-DMS- bromamine can react with ozone with a high rate constant (105 2.5 M-1 s(-1)), forming the reaction intermediate (BrNO)(SO2)N(CH3)(2)(-). This intermediate resembles a loosely bound complex between an electrophilic nitrosyl bromide (BrNO) molecule and an electron-rich dimethylaminosulfinate ((SO2)N(CH3)(2)(-)) fragment, based on inspection of computed natural charges and geometric parameters. This fragile complex undergoes immediate (10(10 +/- 2.5) s(-1)) reaction by two branches: an exothermic channel that produces NDMA, and an entropy-driven channel giving non-NDMA products. Computational results bring new insights into the electronic nature, chemical equilibria, and kinetics of the elementary reactions of this pathway, enabled by computed energies of structures that are not possible to access experimentally.

Quantum chemical studies of reactive species in water

Daniela Trogolo

In this thesis, quantum chemical methods have been applied to elucidate the thermodynamics and the kinetics of reactions involving reactive species in water. Due to their high reactivity in water, many transient species are difficult to study by experimental means only. Here, quantum chemical models are used to provide a deeper insight into the chemical nature and aqueous behaviors of such species. In the first chapter, I investigate the gas phase electronic structure and the thermodynamics of inorganic chloramines, bromamines, and bromochloramines, collectively termed halamines. The halamines are halogen oxidants that arise from reactions between ammonia and hypohalous acids during water disinfection processes, and these reactive species are implicated in the formation of disinfection byproducts that are harmful to human health. Despite their relevance in both drinking water chemistry and in biochemistry, the stabilities and speciation of these molecules are difficult to investigate by experimental means. To accurately predict the electronic structures and gas phase thermodynamic properties of halamines, I design a computational protocol, TA14, based on the high-quality Weizmann and Feller-Peterson-Dixon composite methods. TA14 combines a systematic sequence of wave function theory calculations, including the evaluations of dynamical and static electron correlation (CCSDTQ), core/valence electron correlation contributions, scalar and spin-orbit relativistic contributions, and VPT2 anharmonic vibrations. Using TA14, I successfully assess the gas phase total atomization energies, free enthalpies of formation, and Gibbs free energies of formation of halamines within uncertainty bounds of 1-3 kJ/mol. Analysis of the energy components contributing to total atomization energies of halamines reveals that N-Cl and N-Br bonds are held together mostly or entirely by electron correlation forces, with small or even negative Hartree Fock contributions. For example, the Hartree Fock component of the total atomization energy is negative for both NBr3 and NBr2Cl, implying that these molecules would be predicted as unstable without accounting for dynamical electron correlation. Reported thermochemical data enable the determination of equilibrium constants for reactions involving halamines, opening possibilities for more quantitative studies of the chemistry of these poorly understood compounds. In the second chapter, I evaluate the aqueous equilibria and speciation of halamines. I combine theoretical benchmark-quality gas phase Gibbs free energies of formation (chapter 2) with the computed Gibbs free energies of solvation, thereby obtaining aqueous phase Gibbs free energies of formation for halamines. The 'half-and-half' solvation approach, based on averaging the estimates of SMD implicit solvent model and the cluster-continuum solvent model, produces an average error of 3.3 kJ/mol in the free energies of solvation for a set of structurally related molecules containing H, N, O, and Cl. Taking into consideration the combined uncertainties of the computed gas free energy of formation values and the computed free energy of solvation values, we assign an uncertainty of 6-7 kJ/mol to the theoretical standard Gibbs free energies of formation in aqueous phase of halamines. Aqueous Gibbs free energies of formation values are key thermodynamic properties for investigating chemical processes involving halamines during drinking water treatment. The newly reported thermodynamic data can be used to determine the stabilities (reaction equilibria) of halamines in water. Based on our estimated uncertainties of 6-7 kJ/mol in the computed aqueous free energy of formation values of halamines, we expect roughly 1 order-of-magnitude uncertainty in the aqueous equilibrium constants for the reactions leading to the production of halamines in water. ...

EPFL2016

Formation and reactivity of inorganic and organic chloramines and bromamines during oxidative water treatment

Jeremy Samuel Arey, Michèle Bernadette Heeb, Daniela Trogolo, Urs von Gunten

The formation and further reactions of halamines during oxidative water treatment can be relevant for water quality. In this study, we investigated the formation and reactivity of several inorganic and organic halamines (monochloramine, N-chloromethylamine, N-chlorodimethylamine, monobromamine, dibromamine, N-bromomethylamine, N,N-dibromomethylamine, and N-bromodimethylamine) by kinetic experiments, transformation product analysis, and quantum chemical computations. Kinetic model simulations were conducted to evaluate the relevance of halamines for various water treatment scenarios. Halamines were quickly formed from the reaction of chlorine and bromine with ammonia or organic amines. Species-specific second-order rate constants for the reaction of chlorine and bromine with ammonia, methyl- and dimethylamine were in the order of 10(6)-10(8) M(-1)s(-1). The formed halamines were found to be reactive towards phenolic compounds, forming halogenated phenols via electrophilic aromatic substitution (phenol and resorcinol) or quinones via electron transfer (catechol and hydroquinone). At near neutral pH, apparent second-order rate constants for these reactions were in the order of 10(-4)-10(-1) M(-1)s(-1) for chloramines and 10(1)-10(2) M(-1)s(-1) for bromamines. Quantum chemical computations were used to determine previously unknown aqueous pK(a) values, gas phase bond dissociation energies (BDE) and partial atomic charges of the halamines, allowing a better understanding of their reactivities. Kinetic model simulations, based on the results of this study, showed that during zhlorination inorganic and organic chloramines are the main halamines formed. However, their further reactions with organic matter are outcompeted kinetically by chlorine. During ozonation, mainly inorganic bromamines are formed, since ozone quickly oxidizes organic amines. The further reactions of bromamine are typically outcompeted by ozone and thus generally of minor importance. The use of peracetic acid for saline ballast water treatment can result in the formation of substantial amounts of bromamines, which can react with dissolved organic matter and contribute to the formation of brominated products. (C) 2016 Elsevier Ltd. All rights reserved.

Elsevier2017

Gas-Phase Ozone Reactions with a Structurally Diverse Set of Molecules: Barrier Heights and Reaction Energies Evaluated by Coupled Cluster and Density Functional Theory Calculations

Jeremy Samuel Arey, Peter Rudolf Tentscher, Daniela Trogolo

Reactions with ozone transform organic and inorganic molecules in water treatment systems as well as in atmospheric chemistry, either in the aqueous phase, at gas/particle interfaces, or in the gas phase. Computed thermokinetic data can be used to estimate the reactivities of molecules toward ozone in cases where no experimental data are available. Although the gas-phase reactivity of olefins with ozone has been characterized extensively in the literature, this is not the case for the richer chemistry of ozone with polar molecules, which occurs in the aqueous phase or in microhydrated environments. Here, we selected a number of model reactions with small molecules (ethene, ethyne, hydrogen cyanide, hydrogen chloride, ammonia, bromide, and trimethylamine) to study the accuracy of different quantum chemical methods for describing the reactivities of these molecules with ozone. We calculated benchmark electronic energies of gas-phase reactions of these systems with single reference coupled cluster (CC) theory. These benchmark results for the binding energy in the van der Waals complex, the energy of the transition structure, and the reaction energy were estimated to be accurate within 1-2 kcal mol(-1). Singlet oxygen (O-1(2)) is a common product of ozone reactions. Coupled cluster calculations with up to perturbative quadruples (CCSDT(Q)) were needed to obtain reaction energies accurate within 1 kcal mol(-1) when this species was involved. In (micro)hydrated environments or at interfaces, coupled cluster methods are prohibitively expensive in most cases. We tested the suitability of some contemporary density functional theory (DFT) methods to reproduce the benchmark electronic energy differences. Range-separated functionals were found to be promising candidates to estimate forward barrier heights, with LC-omega PBE rivaling the accuracy of CCSD(T). For energies of reaction, however, DFT methods exhibited large systematic errors, depending on their fraction of orbital exchange. This was found to worsen when O-1(2) is a product, and no safe recommendation can be given for DFT reaction energies in such cases.

AMER CHEMICAL SOC2019