(80b) Tuning Reaction Pathways for First-Principles Catalyst Design

We describe an approach where we optimize rate-limiting features of reaction coordinates by altering geometric properties of model catalyst fragments.  By varying such geometric properties and observing how features of reaction pathways are altered, we are able to identify structural features to target in database searches of characterized molecular scaffolds or to use to build new structures. We employ the reaction pathway of carbon dioxide hydration as our model, since its reaction pathway is characterized by a limited set of features when occurring at a single-metal-site catalyst.  Namely, these steps include formation of a reactive hydroxyl intermediate, oxidative attack on carbon dioxide, and release of the formed carbonic acid product.  We carry out density functional theory calculations augmented with a self-consistently calculated Hubbard U term (DFT+U) to explore the effect of variation of ligand identity (sp² or sp³ nitrogen motifs inspired by the natural enzyme carbonic anhydrase) for both Zn(II) and Co(II) metal centers. The comparison of binding, activation, and product release over a large range of interaction distances and angles suggests that four-coordinate, short Zn(II)-nitrogen(sp³) bond distances favor rapid turnover for carbon dioxide hydration. This design strategy is confirmed first by computationally characterizing the reactivity of a known mimic over a range of metal-nitrogen bond lengths.  A search of existing catalysts in a chemical database reveals structures that match the target geometry from model calculations, and subsequent calculations have identified these structures as potentially effective for carbon dioxide hydration. Time permitting, we will discuss how this approach may be used to seek out both novel catalyst scaffolds and in more complex reaction pathways.