Studies of polyhalide ions in aqueous and non-aqueous solution by electrospray mass spectrometry

Abstract

Solutions of I₂ + X⁻ (X = Cl, Br, I. NCS) in both water and acetonitrile have been studied by electrospray mass spectrometry in order to identify the anions present. For I₂/I⁻, there is no evidence of the I₅⁻ anion proposed earlier to explain deviations from ideality in the formation of I₃⁻ in aqueous solution; rather, the equilibrium I₃⁻ + I⁻ ⇌ I₄²⁻ is shown to be compatible with the mass spectrometric results, and with the earlier literature. With Cl⁻, Br⁻ and NCS⁻, both classes of solution contain members of the series X₃⁻ ⇌ X₂I⁻ ⇌ XI₂⁻ ⇌ I₃⁻; this is ascribed to rapid redistribution processes rather than to the redox and/or photochemical reactions suggested by earlier workers. There is little qualitative difference between aqueous and acetonitrile solutions, except that aqueous I₂/Cl⁻ systems contain the ICl₄⁻ anion, whose decomposition in the gas phase produces the previously unknown ICl₃˙⁻ radical anion. No I₂X⁻ adducts were identified when X = OH⁻, NCO⁻, N₃⁻ or NO₂⁻.