Billiard-Ball Collisions in the Reactions of Hot Hydrogen Atoms. Isotope Effect on the Hydrogen Displacement in Methane

Abstract

The nature of collisions leading to reactions of hydrogen atoms of high kinetic energy (in the form of recoil tritium) is investigated. Two extreme models, both originally due to Libby, are examined. One assumes very weak coupling by valence bonds in the molecule attacked, so that the reaction can be represented by a quasi‐elastic atom‐atom collision (billiard‐ball model) and the other assumes strong valence bond coupling (epithermal model). Calculations on the billiard‐ball (hard sphere) model of the reaction T^*+CH₄→H+CH₃T leads to an estimate of its relative cross section as a function of energy. An isotope effect for reaction with CH₄ and with CD₄ is calculated and measured experimentally. Consideration of both the calculations and the experimental results indicate that the billiard‐ball model is unimportant in the reactions of gas phase hot hydrogen (as recoil tritium), and that reactions occur nearly exclusively at less than 10–20 ev of kinetic energy, by mechanisms involving strong bond coupling. The possible importance of the billiard‐ball model in the reactions of other hot atoms is briefly discussed.