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

Abstract

The rates of formation of hydrates of H₃O⁺ in the reactions $${\mathrm{H}}_3{\mathrm{O}}^{+}({\mathrm{H}}_2\mathrm{O}{)}_{\text{n−1}}+{\mathrm{H}}_2\mathrm{O}\mathrm{+M\leftrightarrows }{\mathrm{H}}_3{\mathrm{O}}^{+}({\mathrm{H}}_2\mathrm{O}{)}_n+\mathrm{M}\text{, n\hspace{0.17em}=\hspace{0.17em}1, 2, 3,}$$ 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 ${\mathrm{E\hspace{0.17em}/\hspace{0.17em}P}}_0$ in the range 4–30 V cm⁻¹·torr⁻¹. Values for $k_1{\mathrm{,\; k}}_2,\mathrm{and}{k}_3$ 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 $\mathrm{E\hspace{0.17em}/\hspace{0.17em}P}$ 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.