## Doctoral Dissertations

Fall 1989

Dissertation

Chemistry

#### Degree Name

Doctor of Philosophy

Howard R Mayne

#### Abstract

This dissertation examines the effect of reagent rotation in elementary gas phase reactions. Historically, the effect of rotational excitation of the reagents of a chemical reaction on the reactive cross section has been poorly understood. One of the major reasons for this was the lack of a simple model in which the dynamics could be visualized. In this work, such a model is developed and utilized in order to elucidate trends in reactivity observed upon rotational excitation. Within the context of this model, exact quantum mechanical scattering calculations are performed for a simple atom-diatom system. These exact quantum mechanical probabilities of reaction as a function of rotational quantum number, P$\sp{\rm R}$(j), exhibit the characteristic "dip and climb" behavior observed in many classical trajectory calculations. These exact P$\sp{\rm R}$(j) are then compared to P$\sp{\rm R}$(j) obtained via several approximate quantum mechanical methods, for example, the Centrifugal Sudden (CS) and the Infinite Order Sudden (IOS). We find that, in general, the CS method does a good job reproducing exact P$\sp{\rm R}$(j). In contrast, the IOS method only reproduces the correct qualitative trends when the collision is "sudden" like.

The applicability of classical mechanics as it relates to rotational excitation is also investigated. Classical P$\sp{\rm R}$(j) obtained using the model are compared to the exact quantum mechanical P$\sp{\rm R}$(j). The viability of several classical mechanical approximate scattering techniques is also investigated. The classical CS approximation reproduces qualitative trends observed in the exact classical P$\sp{\rm R}$(j), while the classical IOS only reproduces the correct qualitative trends under "sudden" conditions.

Having established the accuracy of classical mechanics in dealing with rotational excitation we then utilized it to fully define the phenomena responsible for the trends observed in the classical P$\sp{\rm R}$(j). Lastly, full 3D classical trajectories are carried out for the F + H$\sb2$(0,j) and OH(0,j) + H$\sb2$(0,j$\sp\prime$) reactions. The model qualitatively reproduces the trends observed in the classical reaction cross section as a function of j, S$\sp{\rm R}$(j), for both reactions.

COinS