(HOCl) complexes (X = Cl, Br, I) in the Gas Phase with Cryogenic Ion Vibrational Spectroscopy: Hydrogen vs. Halogen Bonding*

The heterogenous reactions between hypochlorous acid (HOCl) and halides (X⁻= Cl⁻, Br⁻, I⁻) occurring at the air-water interface of sea spray aerosol (SSA) are thought to play a key role in the regulation of tropospheric ozone. While little is known about the molecular level mechanisms of such reactions, it has been suggested that they may involve the formation of X⁻(HOCl) ion-molecule complexes. Furthermore, these complexes are the basis for routine field observation of HOCl in the atmosphere using iodide and bromide adduct chemical ionization mass spectrometry (CIMS). In the presence of a halide, HOCl can participate in two well-known intermolecular interactions: ionic hydrogen (IHB) and halogen (IXB) bonding, and thus the study of X⁻(HOCl) in the gas phase presents a rare opportunity for a direct comparison between these two bonding motifs in a single, isolated system. Here, we generate X⁻(HOCl), X=Cl, Br and I ions by ligand exchange reactions with X⁻(H2O)n clusters in a 300 K ion guide, and report isomer-selective vibrational spectra obtained by IR photodissociation of weakly bound X⁻·HOCl⁻·H₂ complexes formed at ~20K. Although Cl⁻(HOCl) and Br⁻(HOCl) exclusively adopt IHB structures, both IHB and IXB isomers of I⁻(HOCl) are efficiently formed in a ratio that is highly dependent on the kinematics of the ligand exchange reaction. A detailed comparison of the binding energies, energy landscapes, complex geometries, and vibrational level structures of the two motifs is then explored using electronic structure calculations and anharmonic treatment of the nuclear motions.