Simulations at the air-water interface shows that IONO2 does not react directly with iodine acids, but forms a halogen bond, for the first time proposed to occur on an atmospheric aqueous surface.

The atmospheric chemistry of iodine is important because of its role in catalytic ozone loss and new particle formation, which have an influence in the energy balance of the atmosphere. Iodine nitrate (IONO2) is one of the main reservoir species in the atmospheric iodine cycle. It is formed via the reaction of iodine oxide and nitrogen dioxide. The primary fate of IONO2 is believed to be, besides photolysis, uptake by aerosol surfaces, leading to particulate iodine activation.

In this study, we have conducted simulations on the interaction of IONO2 with the iodine acids (HIO3 and HIO2) that compose atmospheric ultrafine iodine particles. 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. 

Manoj Kumar, Tarek Trabelsi, Juan Carlos Gómez Martín, Alfonso Saiz-Lopez and Joseph S. Francisco. HIOx-IONO2 dynamics at the air-water interface: Revealing the existence of a halogen bond at the atmospheric aerosol surface. Journal of the American Chemical Society. DOI: 10.1021/jacs.0c05232