Hydration of Negative Ions in the Gas Phase

The attachment of water molecules to the ions F⁻, Cl⁻, Br⁻, I⁻, O₂⁻, and NO₂⁻ was observed in the gas phase at water pressures from 0.1 to 1.5 torr and 292°K with an alpha‐particle ion source attached to a mass spectrometer. The negative ions were produced by adding a suitable compound RX to an 8‐torr oxygen carrier gas. The reaction involved was: RX+O₂⁻=X⁻+R+O₂. Equilibrium constants for a number of clustering reactions $(n − 1, n)$⁠: X⁻(H₂O)_n−1+H₂O=X⁻(H₂O)_n were determined. The hydration of the ions falls off with increase of ionic size. Comparison of the free energies $ΔG°2,3$ and $ΔG°3,4$ of the halogen and alkali ions with the Pauling radii shows that negative ions solvate more strongly than positive ions. Comparison with the Latimer, Pitzer, and Slanski radii shows that these radii predict stronger (2,3) and (3, 4) hydration for positive ions. The radii based on x‐ray measurements predict equal (2, 3) and (3, 4) hydration energies for positive and negative ions. The present data suggest that the single ion hydration energies derived by Latimer et al. and Verwey are either incorrect or due to stronger solvation by negative ions not in the inner hydration shell but in the subsequent hydration interactions.