Dynamics at Unimolecular Transition States
Ketene undergoes three unimolecular reactions with thresholds near 29,000 cm−1, dissociation to either the singlet or triplet state of methylene and carbon monoxide and isomerization through exchange of its methylene and carbonyl carbons atoms. Energy-resolved rate constant measurements near the threshold for triplet methylene formation reveal a stepwise increase in rate at the energy corresponding to each quantized energy level for vibration orthogonal to the reaction coordinate in the region of the transition state. Thus the dynamics in the immediate region of the transition state are approximately adiabatic. The isomerization reaction has a high barrier with a broad shallow well at the top surrounding the oxirene configuration. This well gives rise to quantum resonances for motion along the reaction coordinate and a reaction rate which exhibits narrow peaks as the total energy increases above the reaction threshold. These results provide a clear and detailed confirmation of the fundamental hypothesis of the statistical transition state theory (RRKM) that reaction rates are controlled by the number of energetically accessible vibrational levels at the transition state and a striking demonstration of the importance of the detailed shape of the potential energy surface in the region of the transition state.
KeywordsTransition State Potential Energy Surface Vibrational Level Total Angular Momentum Reaction Coordinate
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