Hypoiodous acid as guest molecule in protonated water clusters: A combinedFT-ICR/DFT study of I(H2O)(n)(+)

Cationic water clusters containing iodine, of the composition I(H2O)(n)(+), n = 0-25, are generated in a laser vaporization source and investigated by FT-ICR mass spectrometry. An investigation of blackbody radiation-induced fragmentation of size-selected clusters I(H2O)(n)(+), n = 3-15, under colli sion-free conditions revealed an overall linear increase of the unimolecula r rate constant with cluster size, similar to what has been observed previo usly for other hydrated ions. Above a certain critical size, I(H2O)(n)(+), n greater than or similar to 13, reacts with HCl by formation of the interh alide ICI and a protonated water cluster, which is the reverse of a known s olution-phase reaction. Accompanying density functional calculations illust rate the conceptual differences between cationic and anionic iodine-water c lusters I(H2O)(n)(+/-). While I-(H2O)(n) is genuinely a hydrated iodide ion , the cationic closed-shell species I(H2O)(n)(+) may be best viewed as a pr otonated water cluster, in which one water molecule is replaced by hypoiodo us acid. In the strongly acidic environment, HOI is protonated because of i ts high proton affinity. However, similar to the well-known H3O+/H5O2+ cont roversy in protonated water clusters, a smooth transition between H2IO+ and H4IO2+ as core ions is observed for different cluster sizes.