Molecular Interaction and Orientation of HOCl on Aqueous and Ice Surfaces

Abstract

The interaction and orientation of hypochlorous acid (HOCl) on the ice surface has been of great interest as it has important implications to ozone depletion. As HOCl interacts with the ice surface, previous classical molecular dynamics simulations suggest its OH moiety orients to the outside of the ice surface, whereas the quantum calculations performed at 0 K indicate its Cl atom is exposed. To resolve this contradiction, herein, Born–Oppenheimer molecular dynamics simulations are adopted, and the results suggest that at ambient temperature, the interaction between HOCl with interfacial water is dominated by the robust H-bond of (HOCl)­H–O­(H2O). As a result, the HOCl mainly acts as the proton donor to the water surface, which thus can participate in proton transfer reactions via the promotion of interfacial water. Moreover, the Cl atom of HOCl is found to be exposed to the outside of the water surface. Therefore, during the heterogeneous reactions of HOCl on the water surface, the Cl atom becomes the reactive site and is easily attacked by other species.

Molecular Insights into the Spontaneous Generation of Cl2O in the Reaction of ClONO2 and HOCl at the Air–Water Interface

“…The solvation and adsorption of these two molecules have been calculated previously. It was observed that ClONO 2 and HOCl have lower free energy at the air–water interface than in the gas phase and bulk liquid water, indicating that these two molecules have an affinity for the air–water interface. , Here, AIMD simulations coupled with the metadynamics method were performed to calculate the free-energy change as a function of the distance between ClONO 2 and HOCl at the air–water interface and in bulk liquid water, as shown in Figure a. The minima of the free-energy profiles for the molecules at the air–water interface and for the molecules in the bulk liquid water are located at d = 5.12 and 5.06 Å, respectively, reflecting that ClONO 2 and HOCl prefer to form complexes both at the air–water interface and in the bulk liquid water.…”

Can Ozone Dissociate at the Surface of Water (Water Droplet and Ice) without Light?

“…For the water droplet, we have run the simulation at 250 and 300 K, whereas, for the ice surface, simulation was run at 190 and 220 K. The simulation was performed using an integration step of 0.5 fs for a total time of 1 ps for the ice surface and 1.5 ps for water droplets. Further, we have used 191 water molecules to model the water droplet, and 270 water molecule to model the ice surface − (for detail of modeling the water droplet and ice surface, see section 1 of the Supporting Information).…”

A Molecular Dynamic Study of the Effects of Surface Partitioning on the OH Radical Interactions with Solutes in Multicomponent Aqueous Aerosols

“…MD has been extensively used to investigate the orientation of molecules near the air–water interface for specific solutes at varying concentrations. ,,− It is generally observed that alkyl groups will orient toward the surface due to their nonpolar hydrophobic nature, , with varying angles relative to the water surface. ,, For example, surface-adsorbed small brominated halomethanes are preferentially oriented with the carbon atom adsorbed on the water surface, , while halocarbons with longer nonpolar hydrocarbon chains are found to be have a parallel alignment at the water surface . Similarly, acetonitrile is found to lie nearly flat at the water surface. , Ab initio quantum-mechanical molecular dynamic simulations (QM/MM) have been performed to specifically investigate the effect of solute confinements in aerosols. , The photosensitizer, imidazole-2-carboxaldehyde, is found to orient at the water surface and to have different absorption cross sections and spin orbit constants in the bulk or within the interface.…”