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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 Cl2, Br2, HNO3, the C1-C9 series of saturated monocarboxylic acids, benzoic acid, 2,2-dimethylpropanoic acid, 9-decenoic acid, and trichloroacetic acid. While Cl2 and Br2 form the [M + I]- iodide 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-···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-···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) Cl2 and Br2 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.

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