An experimental method for testing reactivity models: A high-pressure discharge-flow study of H plus alkene and haloalkene reactions

Theories of barrier height control in radical-molecule reactions must be te sted against data spanning a wide range in reactivity, by a method for sepa rating multiple, correlated terms in the theories. Here we present an analy sis technique designed to reveal reactant properties controlling reactivity and rate constant measurements for an extensive series of reactions where that control is very much in doubt. The measurements were made with a new h igh-pressure flow experiment designed specifically to facilitate the study of multiple radicals. The derivative technique consists of graphically anal yzing partial derivatives of modeled barrier heights, using measured barrie rs and reactant properties. We use this technique to uncover the governing parameters for hydrogen atom abstraction reactions, which are dominated by an essentially ionic excited state of the reactants. More generally, multip le excited states contribute to barrier formation. with different states do minating for different classes of reactions. The new experimental apparatus is a significantly more flexible (and much smaller) version of our origina l high-pressure flow system. In this case, we use hydrogen atoms as the att acking radical, enabling a study of hydrogen atom addition to alkenes, wher e reactivity may he controlled by ionic states, singlet-triplet splittings, reaction enthalpy, or a combination of these factors. By using hydrogen at oms, we eliminate potentially confounding influences on the ground state, a nd by selecting a series of alkenes and haloalkenes to systematically vary ionization potential, singlet-triplet splittings, and Jr-electron density, we lay the foundation for an extensive study of barrier height control for this reaction class. The data presented here include the first temperature- dependent measurements for 9 of the 13 reactions studied.