Role of Gas-Phase Halogen Bonding in Ambient Chemical Ionization Mass Spectrometry Utilizing Iodine

Ambient ionization processes are becoming more widely used for the measurement of atmospherically relevant particles and gases. We report here ambient ionization mass spectra utilizing a commercial tincture of iodine and a piezoelectric discharge gun (PDG) to generate the ionizing reagents. Analytes include Cl-2, Br-2, HNO3, the C-1-C-9 series of saturated monocarboxylic acids, benzoic acid, 2,2-dimethyl-propanoic acid, 9-decenoic acid, and trichloroacetic acid. While Cl-2 and Br-2 form the [M + I](-) odide adducts, HNO3 and the organic acids show unexpected peaks corresponding to [2M - 2H + I](-). For HNO3, the new ion formed is interpreted as the [NO3-center dot center dot center dot IONO2] complex, where IONO2 is likely formed upon reaction of HOI with gaseous NO3-. Similarly, for the organic acids, the [2M - 2H + I](-) peaks are interpreted as [RC(O)O-center dot center dot center dot IOC(O)R] complexes formed by association of RC(O)O- with acyl hypoiodites [RC(O)OI]. It is proposed that the association of (1) Cl-2 and Br-2 with I-, (2) IONO2 with NO3- ions, and (3) RC(O)OI with carboxylate ions occurs via non-covalent halogen bonding. The results suggest the possibility that halogen bonding may play a role in chemical transformations in the atmosphere, particularly in particles where concentrations of iodinated species may be significant.