Thursday, December 9, 2010

Simple valence bond model for a chemical reaction

Valence bond theory provides an intuitive picture of not just chemical bonding but bond breaking and making. For a reaction ab + c -> ac + b, one can write done the energy of the total system in terms of pairwise exchange J and coulomb integrals Q. This can be used to produce semi-empirical potential energy surfaces and/or diabatic states and coupling between them. This is at the heart of the treatment of coupled electron-proton transfer by Hammes-Schiffer and collaborators, discussed in previous posts. I struggled a bit to find the background of this. It goes back to London-Eyring-Polanyi-Saito (LEPS). A nice summary is the paragraph below taken from a paper by Kim, Truhlar, and Kreevoy. It provides a way to parametrise the Qs and Js in terms of empirical Morse potentials for the constituent molecules. At the transition state the gap to the next excited state is related to a singlet-triplet gap, a point emphasized by Shaik and collaborators.

More background is in material in the old text, Theoretical Chemistry by Glasstone (1944).

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