Activation Energies in Computational Chemistry - A case study

Book chapter, 2011

A straight-forward way to learn how different complementary properties of a catalyst control a resulting activation energy is proposed within the frame work of Density Functional Theory. It is argued that in special cases the activation energy can be approximated from the crossing of the two vibrational modes’ harmonic oscillator potentials corresponding to the reactant and the product, respectively. The procedure is argued to be applicable in cases were traditional transition state search algorithms such as the synchronous transit or the nudged elastic band methods are of limited use. The constraints of the present approach include accessibility of reactant and product structures as well as availability of normal modes pointing towards the transition state. The usefulness of the proposed procedure is demonstrated for the O-O bond formation step in the water oxidation reaction (OER). A comparative study of the activation energy for said reaction is understaken, employing (i) a molecular manganese dimer, (ii) an embedded manganese dimer, and (iii) an embedded cobalt dimer. In case of the two latter, an MgOx(OH)y is used as support. It is shown how the activation barrier for said reaction step is influenced by mainly two factors, (a) the flexibility of a catalyst and (b) the equilibrium O—O distance of the dioxo- species. It is demonstrated how in case of a flexible molecular catalyst, the influence of the O—O distance is negliable, while it is decisive to the activation energy in case of a more stiff embedded catalyst.