Spiers Memorial Lecture. State-resolved studies of unimolecular reactions

Abstract

During the last decade, the development of molecular beam and laser technologies has permitted a look into the transition-state region at the dynamics of chemical bond formation and breaking. State-selective preparation of reactants and state-resolved detection of products permits an exacting quantitative test of dynamical theories that are based on accurate ab initio potentials. The control of reaction rate and energy release to products by vibrations at the transition state is illustrated for ketene fragmentation over a simple barrier to yield triplet methylene and carbon monoxide. The barrier to exchange of carbon atoms in ketene has a shallow well at its top; this causes quantum dynamical resonances for motion along the reaction coordinate which are seen as peaks in the isomerization rate constant as a function of energy. For dissociation to singlet methylene and carbon monoxide, there is no barrier to the reverse reaction. The reaction rate is controlled by a variational transition state which tightens as energy increases. The transition state is defined on a separate vibrationally adiabatic potential-energy surface for each product vibrational state. The dynamics of energy flow between transition state and separated fragments are in the strongly coupled limit near threshold (0–200 cm^–1). For higher energies the methylene rotations are significantly colder than predicted statistically and thus the dynamics must be partially adiabatic.