Thermochemistry of the Hypobromous and Hypochlorous Acids, HOBr and HOCl

Merits and Limitations of Radical vs. Nonradical Pathways in Persulfate-Based Advanced Oxidation Processes

Urbanization and industrialization have exerted significant adverse effects on water quality, resulting in a growing need for reliable and eco-friendly treatment technologies. Persulfate (PS)-based advanced oxidation processes (AOPs) are emerging as viable technologies to treat challenging industrial wastewaters or remediate groundwater impacted by hazardous wastes. While the generated reactive species can degrade a variety of priority organic contaminants through radical and nonradical pathways, there is a lack of systematic and in-depth comparison of these pathways for practical implementation in different treatment scenarios. Our comparative analysis of reaction rate constants for radical vs. nonradical species indicates that radical-based AOPs may achieve high removal efficiency of organic contaminants with relatively short contact time. Nonradical AOPs feature advantages with minimal water matrix interference for complex wastewater treatments. Nonradical species (e.g., singlet oxygen, high-valent metals, and surface activated PS) preferentially react with contaminants bearing electron-donating groups, allowing enhancement of degradation efficiency of known target contaminants. For byproduct formation, analytical limitations and computational chemistry applications are also considered. Finally, we propose a holistically estimated electrical energy per order of reaction (EE/O) parameter and show significantly higher energy requirements for the nonradical pathways. Overall, these critical comparisons help prioritize basic research on PS-based AOPs and inform the merits and limitations of system-specific applications.

Basis Set Effects in the Description of the Cl-O Bond in ClO and XClO/ClOX Isomers (X = H, O, and Cl) Using DFT and CCSD(T) Methods

The performance of a group of density functional methods of progressive complexity for the description of the ClO bond in a series of chlorine oxides was investigated. The simplest ClO radical species and the two isomeric structures XClO/ClOX for each X = H, Cl, and O were studied using the PW91, TPSS, B3LYP, PBE0, M06, M06-2X, BMK, and B2PLYP functionals. Geometry optimizations and reaction enthalpies and enthalpies of formation for each species were calculated using Pople basis sets and the (aug)-cc-pVnZ Dunning sets, with n = D, T, Q, 5, and 6. For the calculation of enthalpies of formation, atomization and isodesmic reactions were employed. Both the precision of the methods with respect to the increase of the basis sets, as well as their accuracy, were gauged by comparing the results with the more accurate CCSD(T) calculations, performed using the same basis sets as for the DFT methods. The results obtained employing composite chemical methods (G4, CBS-QB3, and W1BD) were also used for the comparisons, as well as the experimental results when they are available. The results obtained show that error compensation is the key for successful description of molecular properties (geometries and energies) by carefully selecting the method and basis sets. In general, expansion of the one-electron basis set to the limit of completeness does not improve results at the DFT level, but just the opposite. The enthalpies of formation calculated at the CCSD(T)/aug-cc-pV6Z for the species considered are generally in agreement with experimental determinations and the most accurate theoretical values. Different sources of error in the calculations are discussed in detail.

X-ray Diffraction and Density Functional Theory Provide Insight into Vanadate Binding to Homohexameric Bromoperoxidase II and the Mechanism of Bromide Oxidation

X-ray diffraction of native bromoperoxidase II (EC 1.11.1.18) from the brown alga Ascophyllum nodosum reveals at a resolution of 2.26 Å details of orthovanadate binding and homohexameric protein organization. Three dimers interwoven in contact regions and tightened by hydrogen-bond-clamped guanidinium stacks along with regularly aligned water molecules form the basic structure of the enyzme. Intra- and intermolecular disulfide bridges further stabilize the enzyme preventing altogether the protein from denaturing up to a temperature of 90 °C, as evident from dynamic light scattering and the on-gel ortho-dianisidine assay. Every monomer binds one equivalent of orthovanadate in a cavity formed from side chains of three histidines, two arginines, one lysine, serine, and tryptophan. Protein binding occurs primarily through hydrogen bridges and superimposed by Coulomb attraction according to thermochemical model on density functional level of theory (B3LYP/6-311++G**). The strongest attractor is the arginine side chain mimic N-methylguanidinium, enhancing in positive cooperative manner hydrogen bridges toward weaker acceptors, such as residues from lysine and serine. Activating hydrogen peroxide occurs in the thermochemical model by side-on binding in orthovanadium peroxoic acid, oxidizing bromide with virtually no activation energy to hydrogen bonded hypobromous acid.

Reactivity of Hydrogen Peroxide with Br and I Atoms

The reaction mechanisms of Br and I atoms with H₂O₂ have been investigated using DFT and high-level ab initio calculations. The H-abstraction and OH-abstraction channels were highlighted. The geometries of the stationary points were optimized at the B3LYP/aug-cc-pVTZ level of theory, and the energetics were recalculated with the coupled cluster theory. Spin-orbit coupling for each halogenated species was also explicitly computed by employing the MRCI level of theory. Thermochemistry for HOBr and HOI has been revised and updated standard enthalpies of formation at 298 K for HOBr and HOI are the following: Δ_fH°_298K(HOBr) = (-66.2 ± 4.6) kJ mol^-1 and Δ_fH°_298K(HOI) = (-66.8 ± 4.7) kJ mol^-1. The rate constants have been estimated using transition state theory (TST), canonical variational transition state theory (CVT), and CVT with small curvature tunneling (CVT/SCT) over a wide temperature range (250-2500 K). For the direct abstraction mechanism, the overall rate constant at 300 K was predicted to be 2.58 × 10^-16 and 7.42 × 10^-25 cm³ molecule^-1s^-1 for the Br + H₂O₂ and I + H₂O₂ reactions, respectively. The modified Arrhenius parameters have been estimated for the overall reactions: k_Br+H2O2(T) = 4.80 × 10^-26 T^4.31 exp(-5.51 (kJ mol^-1)/RT) and k_I+H2O2(T) = 3.41 × 10^-23 T^3.29 exp(-56.32 (kJ mol^-1)/RT).

Benchmark thermochemistry of chloramines, bromamines, and bromochloramines: halogen oxidants stabilized by electron correlation

The free energy of the formation of NH₂Br at 298 K can be estimated by taking into account the total atomization energy of NH₂Br and the atomic and molecular contributions to the enthalpy and the entropy of formation of NH₂Br at 0 K and 298 K.

Reactivity of BrCl, Br₂, BrOCl, Br₂O, and HOBr toward dimethenamid in solutions of bromide + aqueous free chlorine

HOBr, formed via oxidation of bromide by free available chlorine (FAC), is frequently assumed to be the sole species responsible for generating brominated disinfection byproducts (DBPs). Our studies reveal that BrCl, Br(2), BrOCl, and Br(2)O can also serve as brominating agents of the herbicide dimethenamid in solutions of bromide to which FAC was added. Conditions affecting bromine speciation (pH, total free bromine concentration ([HOBr](T)), [Cl(-)], and [FAC](o)) were systematically varied, and rates of dimethenamid bromination were measured. Reaction orders in [HOBr](T) ranged from 1.09 (±0.17) to 1.67 (±0.16), reaching a maximum near the pK(a) of HOBr. This complex dependence on [HOBr](T) implicates Br(2)O as an active brominating agent. That bromination rates increased with increasing [Cl(-)], [FAC](o) (at constant [HOBr](T)), and excess bromide (where [Br(-)](o)>[FAC](o)) implicate BrCl, BrOCl, and Br(2), respectively, as brominating agents. As equilibrium constants for the formation of Br(2)O and BrOCl (aq) have not been previously reported, we have calculated these values (and their gas-phase analogues) using benchmark-quality quantum chemical methods [CCSD(T) up to CCSDTQ calculations plus solvation effects]. The results allow us to compute bromine speciation and hence second-order rate constants. Intrinsic brominating reactivity increased in the order: HOBr ≪ Br(2)O < BrOCl ≈ Br(2) < BrCl. Our results indicate that species other than HOBr can influence bromination rates under conditions typical of drinking water and wastewater chlorination.

Antioxidant activity of unexplored indole derivatives: synthesis and screening

The present study envisaged the development of novel antioxidant candidates using the indole scaffold. Several tryptophan and tryptamine derivatives were synthesized, in particular prenylated indole compounds, and their scavenging activity for reactive oxygen species (ROS) and reactive nitrogen species (RNS) was investigated. The library substitution pattern included several alkyl chains at positions N-1, C-2 of the indole nucleus, including prenyl and isopentyl chain, as well as different groups at the side chain (C-3) that allowed the investigation of a possible radical stabilization. The results obtained showed that tryptophan (8), tryptamine (9), N-phthaloyl tryptamine (5) and N-prenyl tryptophan (13) were the most active against peroxyl radical (ROO(•)) with activities higher than Trolox, which was used as control. The scavenging of hypochlorous acid (HOCl) was also evaluated and tryptophan (8) and tryptamine (9) showed IC(50) of 3.50 ± 0.4 and 6.00 ± 0.60 μM, respectively. Significant activity was also found for the N-prenyl tryptophan (13) with an IC(50) of 4.13 ± 0.17 μM and C-2 prenylated derivative (14), with 4.56 ± 0.48 μM. The studies were extended to RNS and best results were obtained against peroxynitrite anion (ONOO(-)) in the presence of NaHCO(3). N-alkylated tryptophan (18) showed a high activity with an IC(50) of 14.0 ± 6.8 μM. The results show that the tested compounds are effective scavengers of ROS and RNS, and suggest that the radical stabilization is strongly dependent on the type of substituents on the indolic moiety and on their relative positions. In addition, the radical dissipation inside the indolic system is mandatory for the observed antioxidant activity.

Thermochemistry of new molecular species: SBr and HSBr

Total energies, optimized geometries, and vibrational frequencies of SBr and HSBr have been evaluated at the coupled cluster level of theory with the correlation consistent basis sets. Extrapolated to the complete basis set limit and with corrections for core-valence, scalar relativistic, and spin-orbit effects, atomization energies were computed and then combined with the experimental heats of formation of the atomic species to generate very accurate heats of formation for the species SBr and HSBr. For SBr, we predict 37.45 and 36.07 kcal/mol for DeltaHf(0 K) and DeltaHf (298.15 K), respectively, in very good agreement with the inferred experimental values of 37.98 and 36.15 kcal/mol. For HSBr, the estimate turns out to be 10.38 and 8.29 kcal/mol for DeltaHf (0 K) and DeltaHf (298.15 K), respectively. Using the more recent HBrO experimental heat of formation at 298.15 K of Lock et al., [J. Phys. Chem. 100, 7972 (1996)] the inferred experimental value for HSBr is predicted to be 8.15 kcal/mol, compared with 8.65 kcal/mol derived from the data of Ruscic and Berkowitz [J. Chem. Phys. 101, 7795 (1994)]. Considering the better agreement of the result with that predicted using the experimental value of DeltaHf(298.15 K) of Lock et al., the author also supports the suggestion made by Denis [J. Phys. Chem. A. 110, 5887 (2006)] that the result of Lock et al. should be preferred over the one of Ruscic and Berkowitz. For DeltaHf(0 K), the author found 10.38 and 10.56 kcal/mol, respectively, for the theoretical and inferred experimental estimates.

Reaction of hypochlorous acid with imidazole: Formation of 2-chloro- and 2-oxoimidazoles

Reaction of hypochlorous acid (HOCl) with imidazole (Im) taken as a model for the 5-membered ring of guanine, leading to the products 2-chloro- and 2-oxo-imidazoles was investigated at the B3LYP/6-31+G* and B3LYP/AUG-cc-pVDZ levels of density functional theory. For all cases, single point energy calculations were performed at the MP2/AUG-cc-pVDZ level of theory using the geometries optimised at the B3LYP/AUG-cc-pVDZ level. Intrinsic reaction coordinate calculations were performed to ensure genuineness of all the calculated transition states. Effect of aqueous media was investigated by solvating all the species involved in the reactions using the polarizable continuum model. It is found that 2-chloroimidazole (2-ClIm) can be formed following three different reaction schemes while 2-oxoimidazole (2-oxoIm) can be formed following two different reaction schemes. The calculated barrier energies show that formation of 2-oxoIm would be less favored than that of 2-ClIm, which explains the experimental observations on relative yields of 8-chlorodeoxyguanosine and 8-oxodeoxyguanosine.

Predicting New Molecular Species of Potential Interest to Atmospheric Chemistry: The Isomers HSBr and HBrS

Coupled cluster singles and doubles with perturbative contributions of connected triples CCSD(T) theory with a series of correlation consistent basis sets was used to predict the existence and characterize for the first time the structures, harmonic frequencies, and energetic quantities of the isomeric species HSBr and HBrS, as well as the transition state connecting them. These calculations consider extrapolation to the complete basis set (CBS) limit, corrections for scalar relativistic effects using the second-order Douglas-Kroll-Hess Hamiltonian, and also correlation of the bromine d electrons in addition to the 14 valence electrons. The species HSBr was found to be more stable than HBrS by 50.93 kcal/mol, with a high barrier height of 60.00 kcal/mol for the interconversion into HBrS. The smaller barrier of 7.90 kcal/mol (ZPE included) for the reverse process, however, should favor a rapid interconversion of HBrS into HSBr if HBrS can also be initially present in a potential synthetic route. If trapped in a matrix, their harmonic frequencies will allow for an unambiguous distinction between the two species. Scalar relativistic corrections and correlation of 24 electrons, although minor for the present purpose of a first time, but accurate, characterization of these species, are needed if chemical accuracy is also pursued. A test of the DFT/B3LYP approach in describing this type of system resulted in good energetic quantities, but geometric parameters and frequencies still lack spectroscopic accuracy. Whether HSBr can act as a temporary bromine reservoir and/or a source of reactive bromine and HS radicals requires further studies that are underway in our group.