Water Cluster Ions: Rates of Formation and Decomposition of Hydrates of the Hydronium Ion

The rates of formation of hydrates of H₃O⁺ in the reactions H₃^O+(H₂O)_n-1+H₂O+M⇆H₃O⁺(H₂O)_n+M, n = 1, 2, 3, were studied using drift tube techniques. Experiments were performed at 337°K with mixtures of 0.5% to 1.8% partial pressure of water in argon at total pressures from 0.6 to 3.0 torr. Rate constants for the forward reactions decreased slowly with increasing E / P₀ in the range 4-30 V cm^-1.torr^-1. Values for k₁, k₂, and k₃ at 337°K were 6 × 10^-28, 6 × 10^-28, and 2 × 10^-28 cm⁶/sec. Limits were estimated for the reverse reactions. Mobilities of the cluster sequence from H₃O⁺ to H₉O₄⁺ were almost independent of E / P in the range studied, with zero-field values of 3.0, 2.5, 2.2, and 2.0 cm²V^-1.sec^-1 in close agreement with the polarization-limit Langevin theory.