The Competition between Hydrogen, Halogen, and Covalent Bonding in Atmospherically Relevant Ammonium Iodate Clusters

Iodine-containing clusters are expected to be central to new particle formation (NPF) events in polar and midlatitude coastal regions. Iodine oxoacids and iodine oxides are observed in newly formed clusters, and in more polluted midlatitude settings, theoretical studies suggest ammonia may increase growth rates. Structural information was obtained via infrared (IR) spectroscopy and quantum chemical calculations for a series of clusters containing ammonia, iodic acid, and iodine pentoxide. Structures for five of the smallest cationic clusters present in the mass spectrum were identified, and four of the structures were found to preferentially form halogen and/or covalent bonds over hydrogen bonds. Ammonia is important in proton transfer from iodic acid components and also provides a scaffold to template the formation of a halogen and covalent bonded backbone. The calculations executed for the two largest clusters studied suggested the formation of a covalent I₃$$ anion within the clusters.