Prof. Scott Kable, University of Sydney, Australia

Reactions that don’t follow the transition state path

AACP Seminar Series

Every chemical reaction has its own unique characteristics â€" the identity of the chemical products is an obvious fingerprint, but the rate of the reaction and the energy deposited in the products is also characteristic of the mechanism. The notion of a “transition state (TS)”, proposed in the 1930’s, is a very powerful concept in helping us to understand and predict reaction mechanisms. The TS geometry represents the chemical (nuclear + electronic) structure at the maximum of the lowest energy pathway between reactant and product. Transition state theory (TST) proposes that all reaction trajectories pass through or near this configuration and that the energy and entropy of the TS, compared to the reactants, determines the rate of reaction. The potential energy surface (PES) after the TS determines how the energy is deposited into the products.

In this seminar I shall explain how detailed analysis of the energy disposed in the products reveals information about the reaction mechanism. I will illustrate two unusual mechanisms discovered recently. “Roaming”, first reported in 2004 in the photodissociation of H2CO [1], seems not to have a transition state â€" at least not one that TST can accommodate. It involves a long range excursion of a H-atom in the van der Waals region of the HCO fragment, leading to self-abstraction of the other H-atom. Roaming has also been well-characterised in CH3CHO, where the methyl group “roams” about the HCO core, abstracting H to produce CH4 + CO. [2] In the intervening 6 years, at least a dozen other systems have shown roaming mechanisms, however the theoretical calculation of the kinetics of roaming remains a challenge.

The other unusual mechanism involves H/D exchange in a unimolecular reaction. Photo-dissociation of CD3CHO or CH3CDO produces about 10% H/D exchange. Conventional TST underestimates the branching ratio of the exchange mechanism by more than two orders of magnitude. We propose a very unusual, high dimensional TS (ie not a true first order TS) to be responsible and that the very high entropy of this structure more than compensates for its higher energy. All ketones, aldehydes and alkenes should have a structure similar to the one proposed for acetaldehyde and this may lead to unimolecular H/D exchange in many more chemical systems.

[1] Signatures of H2CO photodissociation from two electronic states, H.M. Yin, S. H. Kable, X. Zhang and J.M. Bowman, Science, 311, 1443 (2006).
[2] Roaming dynamics: the dominant pathway to molecular products in acetaldehyde photodissociation, B.R. Heazlewood, M.J.T. Jordan, S.H. Kable, T.M. Selby, D.L. Osborn, B.C. Shepler, B.J. Braams, J.M. Bowman, Proc. Nat. Acad. Sci., USA, 105, 12719 (2008)

For more information contact Prof. Christine Payne (404-385-3125).