HIO_x–IONO₂ Dynamics at the Air–Water Interface: Revealing the Existence of a Halogen Bond at the Atmospheric Aerosol Surface

Iodine is enriched in marine aerosols, particularly in coastal mid-latitude atmospheric environments, where it initiates the formation of new aerosol particles with iodic acid (HIO₃) composition. However, particle formation in polluted and semipolluted locations is inhibited when the iodine monoxide radical (IO) is intercepted by NO₂ to form the iodine nitrate (IONO₂). The primary fate of IONO₂ is believed to be, besides photolysis, uptake by aerosol surfaces, leading to particulate iodine activation. Herein we have performed Born–Oppenheimer molecular dynamics (BOMD) simulations and gas-phase quantum chemical calculations to study the iodine acids-iodine nitrate [HIO_x (x = 2 and 3)–IONO₂] dynamics at the air–water interface modeled by a water droplet of 191 water molecules. The results indicate that IONO₂ does not react directly with these iodine acids, but forms an unusual kind of interaction with them within a few picoseconds, which is characterized as halogen bonding. The halogen bond-driven HIO₃–IONO₂ complex at the air–water interface undergoes deprotonation and exists as IO₃^––IONO₂ anion, whereas the HIO₂–IONO₂ complex does not exhibit any proton loss to the interfacial water molecules. The gas-phase quantum chemical calculations suggest that the HIO₃–IONO₂ and HIO₂–IONO₂ complexes have appreciable stabilization energies, which are significantly enhanced upon deprotonation of iodine acids, indicating that these halogen bonds are fairly stable. These IONO₂-induced halogen bonds explain the rapid loss of IONO₂ to background aerosol. Moreover, they appear to work against iodide formation. Thus, they may play an important role in enhancing the amount of atmospherically nonrecyclable iodine (iodate) in marine aerosol.