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

Solutions of I-2 + 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 I-2/I-, there is no evidence of the I-5(-) anion proposed earlier to explain deviations from ideality in the formation of I-3(-) in aqueous solution; rather, the equilibrium I-3(-) + I- reversible arrow I-4(2-) 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 X-3(-) reversible arrow X2I- reversible arrow XI2- reversible arrow I-3(-); 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 I-2/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-, N-3(-) or NO2-.