Temperature dependence of third order ion molecule reactions. The reaction H+3+2H2 = H+5+H2

Abstract

The rate constants k₁ for Reaction (1): H⁺₃+2H₂ = H⁺₅+H₂ were measured in the temperature range 100–300 °K. The temperature dependence of k₁ has the form k₁∝T⁻ⁿ, where n=2.3. Pierce and Porter have reported a much stronger negative temperature dependence with n=4.6. The difference arises from a determination of k₁ at 300 °K obtained by Arifov and used by Porter. The present k₁ (300 °K) =9×10⁻³⁰ (cm⁶ molecules⁻²⋅sec⁻¹). This is more than an order of magnitude larger than the Arifov value. The temperature dependence of third body dependent association reactions like (1) is examined on the basis of the energy transfer theory and the recently proposed trimolecular complex transition state theory by Meot‐Ner, Solomon, Field, and Gershinowitz. The temperature dependence of the rate constant for the reverse reaction (−1) is obtained from k₁ and the previously determined temperature dependence of the equilibria (1). k₋₁ gives a good straight line Arrhenius plot leading to k₋₁ =8.7×10⁻⁶ exp(−8.4/RT) cm³ molecules⁻¹⋅sec⁻¹. The activation energy is in kcal/mole. The preexponential factor is much larger than the rate constant for Langevin collisions. This is typical for pyrolysis of ions involving second order activation.