Elucidation of Molecular-level Mechanisms of Oxygen Atom Reactions with Chlorine Ion in NaCl Solutions using Molecular Dynamics Simulations Combined with Density Functional Theory

Molecular dynamics simulations combined with density functional theory were performed to investigate the reaction mechanisms for oxygen atom and chlorine ion in NaCl solutions. Considering the ground and first excited states of the oxygen atom, four bulk systems, O(P-3)-NaCl-63H(2)O, O(D-1)-NaCl-63H(2)O, O(P-3)-4NaCl-63H(2)O, and O(D-1)-4NaCl-63H(2)O were studied. The main reaction pathway depended on the concentration of the aqueous electrolyte solution. For a dilute solution with a 1 : 64 ratio of NaCl to H2O, the singlet oxygen atom first attached a water molecule to produce a neutral OH radical, and then the OH radical and chlorine ion approach each other to form an intermediate structure [Cl horizontal ellipsis OH](-). Synchronously, the intermediate transferred charge to the solvated OH radical by accessing the hydrogen bond network. Finally, neutral HOCl and OH- ions were formed. For a dense solution with a 4 : 64 ratio of NaCl to H2O, a singlet oxygen atom was directly combined with Cl- to produce ClO- via an oxygen transfer reaction. The subsequent ab-initio energy computations indicated the most probable mechanism, where the O atom directly combines with Cl- ion to OCl-. This study provides insights into the fundamental mechanisms of the behavior of dissolved oxygen radicals in aqueous electrolyte solutions.