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

摘要

The rate constantsk1for Reaction (1): H+3+2H2= H+5+H2were measured in the temperature range 100ndash;300thinsp;deg;K. The temperature dependence ofk1has the formk1prop;Tminus;n, wheren=2.3. Pierce and Porter have reported a much stronger negative temperature dependence withn=4.6. The difference arises from a determination ofk1at 300thinsp;deg;K obtained by Arifov and used by Porter. The presentk1(300thinsp;deg;K) =9times;10minus;30(cm6thinsp;moleculesminus;2sdot;secminus;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 Meothyphen;Ner, Solomon, Field, and Gershinowitz. The temperature dependence of the rate constant for the reverse reaction (minus;1) is obtained fromk1and the previously determined temperature dependence of the equilibria (1).kminus;1gives a good straight line Arrhenius plot leading tokminus;1=8.7times;10minus;6exp(minus;8.4/RT) cm3thinsp;moleculesminus;1sdot;secminus;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.