Two-dimensional hindered internal rotations in activated complexes of the form XH2

Abstract

For reactions of the form X+H2→XH+H, where X represents, H, O, F, or Cl and H represents light hydrogen or deuterium, the usual degenerate, harmonic, bending vibrations of activated complex theory have been replaced by a two-dimensional internal rotation hindered by a sinusoidal potential function. This replacement has the following effects: (a) The zero point energy of the complex is lowered, reducing the Arrhenius activation energy and increasing the reaction rate constant by 45% or less close to room temperature. (b) At very high temperatures, the Arrhenius plot is less strongly curved and the rate constant becomes less than that for the harmonic oscillator case. Provided the best available parameters and tunneling factors are used, the results are in agreement with experiment. At temperatures above about 1000 K, the results of the hindered rotor model approach those of a modified version of simple collision theory. For some reactions, the heat capacity of activation, a measure of Arrhenius plot curvature, exhibits two maxima as a function of temperature; the higher temperature maximum is caused by the hindered internal rotation, the low temperature one by tunneling.