Interactions of Cl− and OH Radical in Aqueous Solution

Ionic-Equilibrium-Based Mechanism of •OH Conversion to Dichloride Radical Anion in Aqueous Acidic Solutions by Kinetic and Theoretical Studies

A new mechanism for the dichloride radical anion (Cl₂^•-) formation in diluted acidic chloride solutions is proposed on the grounds of pulse radiolysis measurements of the optical absorption growth at 340 nm and the density functional theory and Hartree-Fock computations. We show that the rate of ^•OH conversion into Cl₂^•- is determined by the equilibrium concentration of the ionic pair H₃O⁺·Cl^-. According to the proposed mechanism, the diffusional encounter of ^•OH and H₃O⁺·Cl^- is followed by fast concerted charge/proton transfer ( k(25 °C) = 6.2 × 10¹² s^-1) to yield Cl^•, which then reacts with Cl^- to produce Cl₂^•-. The mechanism has been confirmed by the observed first-order growth of the Cl₂^•- absorption and a direct proportionality of the rate constant to the activities of H₃O⁺ and Cl^- ions. The salt effect on the rate of Cl₂^•- formation is due to the ionic strength effect on the equilibrium H₃O⁺ + Cl^- ⇄ H₃O⁺·Cl^-.

Mechanistic Insight into the Reactivity of Chlorine-Derived Radicals in the Aqueous-Phase UV-Chlorine Advanced Oxidation Process: Quantum Mechanical Calculations

The combined ultraviolet (UV) and free chlorine (UV-chlorine) advanced oxidation process that produces highly reactive hydroxyl radicals (HO^•) and chlorine radicals (Cl^•) is an attractive alternative to UV alone or chlorination for disinfection because of the destruction of a wide variety of organic compounds. However, concerns about the potential formation of chlorinated transformation products require an understanding of the radical-induced elementary reaction mechanisms and their reaction-rate constants. While many studies have revealed the reactivity of oxygenated radicals, the reaction mechanisms of chlorine-derived radicals have not been elucidated due to the data scarcity and discrepancies among experimental observations. We found a linear free-energy relationship quantum mechanically calculated free energies of reaction and the literature-reported experimentally measured reaction rate constants, k_exp, for 22 chlorine-derived inorganic radical reactions in the UV-chlorine process. This relationship highlights the discrepancy among literature-reported rate constants and aids in the determination of the rate constant using quantum mechanical calculations. We also found linear correlations between the theoretically calculated free energies of activation and k_exp for 31 reactions of Cl^• with organic compounds. The correlation suggests that H-abstraction and Cl-adduct formation are the major reaction mechanisms. This is the first comprehensive study on chlorine-derived radical reactions, and it provides mechanistic insight into the reaction mechanisms for the development of an elementary reaction-based kinetic model.

Correcting density-driven errors in projection-based embedding

Projection-based embedding provides a simple and numerically robust framework for multiscale wavefunction-in-density-functional-theory (WF-in-DFT) calculations. The approach works well when the approximate DFT is sufficiently accurate to describe the energetics of the low-level subsystem and the coupling between subsystems. It is also necessary that the low-level DFT produces a qualitatively reasonable description of the total density, and in this work, we study model systems where delocalization error prevents this from being the case. We find substantial errors in embedding calculations on open-shell doublet systems in which self-interaction errors cause spurious delocalization of the singly occupied orbital. We propose a solution to this error by evaluating the DFT energy using a more accurate self-consistent density, such as that of Hartree-Fock (HF) theory. These so-called WF-in-(HF-DFT) calculations show excellent convergence towards full-system wavefunction calculations.

Reducing density-driven error without exact exchange

PBE calculations, performed non-self-consistently on densities evaluated with Rung 3.5 density functionals, give improved performance for hydrogen transfer reaction barriers.

Effect of magnesium cation on the interfacial properties of aqueous salt solutions

Sodium chloride solutions have been used extensively as a model of seawater in both theoretical and experimental studies of the chemistry of sea salt aerosol. Many groups have found that chloride anions are present at the air-solution interface. This observation has been important for the development of a mechanism for the heterogeneous production of molecular chlorine from chloride in sea salt aerosol. However, while sodium chloride is a major constituent of seawater, it is by no means the only salt present. Seawater contains one Mg(2+) for every eight Na(+). Mg(2+) is naturally occurring in ocean waters from mineral deposits in the Earth's crust and biological sources. Mg(2+) forms a hexahydrate structure, rather than contact ion pairs with chloride anion, and this impacts the ordering of water in solution. In this study, we use molecular dynamics simulations, ab initio calculations, and vibrational sum frequency generation (SFG) spectroscopy to explore the effect of the Mg(2+) cation and its tightly bound solvation shell on the surface propensity of chloride, ion-ion interactions, and water structure of the air-solution interface of concentrated chloride salt solutions. In addition, we provide molecular level details that may be relevant to the heterogeneous reactions of chloride in deliquesced sea salt aerosols. In particular, we show that the presence of the divalent Mg(2+) cation does not modify the surface propensity of chloride compared to Na(+) and hence, its availability to interfacial reaction, although some differences in the behavior of chloride may occur due to specific ion interactions. In this work, we also discuss the SFG free OH band at the surface of salt solutions and conclude that it is often not straightforward to interpret.