Ephrath Solel

The reaction pathway on the potential energy surface (i.e. the minimal energy pathway, MEP) determines the reaction’s reactivity. Most reaction analyses focus only on one dimension of the MEP, namely the relative energies of reactant, transition structure (TS) and product. However, the MEP has also other dimensions, or characteristics, such as slope and width. These are rarely considered, but can nevertheless influence reaction rates directly, especially when tunneling dominates, or indirectly through their effect on barrier heights, for thermal rates. While chemists are used to thinking in terms of manipulating barrier heights by (de)stabilization of the TS or product, ways to manipulate barrier widths or slopes remain ill understood. Using computational tools to analyze MEPs, we explore methods to manipulate barrier widths. In this presentation, I will show examples of effects that influence barrier widths, with a special emphasis on synchronicity. In many catalytic reactions, multiple bond-forming/breaking events occur during the same elementary step. The degree of reaction synchronicity (i.e. how simultaneously different events occur along the reaction coordinate), is directly related to the shape and width of the reaction barrier. We used DFT methods to investigate the guanidine-catalyzed ring-opening polymerization of γ-butyrolactone. Our results show that by introducing different substituents onto the catalyst, we can systematically control bond strengths and reaction synchronicity, and thus barrier width. Computed thermal and tunneling rates revealed an increase in tunnelling contribution with increasing reaction synchronicity. Our results suggest synchronicity as a new rational design principle for increasing tunneling rates, with the potential to guide the design of catalysts with significant tunneling rate accelerations.