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

Abstract

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 (HIOx(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 non-recyclable iodine (iodate) in marine aerosol.

Experimental evidence that halogen bonding catalyzes the heterogeneous chlorination of alkenes in submicron liquid droplets

“…Finally, it is likely, and has indeed been shown theoretically that weak interactions, such as halogen and hydrogen bonding at aerosol surfaces can play a substantial role in determining heterogeneous reaction rates and mechanisms. [8][9][10][11] The chlorination reaction studied here is perhaps unique in that it is relatively slow reaction compared to others (e.g. ozonolysis and free radical reactions), thereby allowing the coupling and competition of the elementary steps shown in Fig.…”

“…The reaction rate of a gas phase molecule with an aerosol or droplet surface is governed by a set of kinetic steps that include adsorption/desorption, solvation/de-solvation, and diffusion. [1][2][3][4][5] The relative rates and coupling of these steps can oen yield new pathways or catalytic mechanisms, [6][7][8][9][10][11][12][13] which are largely absent for a reaction occurring solely within a single phase. Given the central role that aerosols play in air pollution, 14,15 climate [16][17][18] and human health, 19 developing better insight, using laboratory measurements, 20 into the molecular factors that enhance, suppress or even catalyze multiphase chemical reactions is an important challenge.…”

“…27 Theoretical work has also shown the importance of relatively weak interactions, such as hydrogen bonding, in governing reaction mechanisms at the surface of atmospheric droplets. [8][9][10] A recent study 11 revealed the importance of halogen bonded complexes in mediating iodine reactions at droplet surfaces. In most cases, a halogen bond is weaker than a hydrogen-bond with typical bond lengths $2.8Å, which form between an electrophilic molecule containing a halogen atom X (X ¼ I, Br, Cl and F), and a nucleophilic molecule containing atoms, such as N, O, S etc.…”

Insights into the Chemistry of Iodine New Particle Formation: The Role of Iodine Oxides and the Source of Iodic Acid

“…Higher level of theory calculations (CCSD(T)/aug-cc-pVTZ + LANL2DZ//M06-2X/aug-cc-pVDZ + LANL2DZ) confirmed that the reaction R3.1 , facilitated by the formation of a IO 3 –IONO 2 halogen-bonded adduct, is exothermic and barrierless. 33 This is not too surprising, considering that halogen bonding has been found to play an important role in the iodine CIMS when used for detecting HNO 3 . 34 Here, we have revisited our previous calculations 22 and extended them to reactions R6.2 , R3.2 , R3.3 , and R7 using the larger aug-cc-pVQZ basis set (see Methods).…”

“…Our ab initio calculations indicated that different reactions between I x O y with x = 2 and NO 3 – , Br – , CH 3 COO – , and H 3 O + are exothermic and can potentially generate some of the ions and cluster ions that have been attributed to HIO 3 , in particular IO 3 – and HIO 3 ·NO 3 – in the nitrate anion CIMS. 22 , 33 For example, …”