Velocity resolved kinetics of enantioselective olefin epoxidation on chiral surfaces

• Applicant: Privatdozent Dr. Tim Schäfer
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 457424069

In the here proposed project we intend to study the dynamics and kinetics of enantioselective olefin epoxidation on chiral surfaces by combining molecular beam surface scattering experiments with velocity map imaging techniques. The epoxidation of olefins on metal surfaces is a technologically important class of chemical reactions, and so far corresponding reactions have been extensively investigated in high detail under ultra-high vacuum conditions. However, none of these studies focuses on the reaction’s enantioselectivity when the reaction occurs on chiral surfaces, as chirality detection is demanding for surface desorbing reaction products at low concentration in the gas phase. We plan to identify the reaction products’ chirality by measuring the multi-photon photoelectron circular dichroism (MP-PECD) after pulsed laser 2+1 resonant enhanced multi-photon ionization (REMPI) of the epoxides at 400 nm. Experiments will be performed similar to pump-probe laser experiments. A short molecular beam pulse of the prochiral olefin initiates the reaction by adsorption at the surface (pump) and reaction products will be probed after desorption by laser ionization. By time delaying molecular beam pulse and laser ionization, reaction kinetics can be inferred on a microsecond timescale. We record either photoelectrons for chirality detection or photoions for product assignment by time-of-flight mass spectrometry (ToF-MS). The same detector provides velocity map images of reaction products, which are used to record dynamics of site specific reaction channels. We will investigate the reactions of the prochiral olefins styrene, 3,3-dimethyl-1-butene, and trans-methyl-styrene on chiral coinage metals. On achiral coinage metal surfaces all these olefins partially oxidize to racemic mixtures of corresponding chiral epoxides when reacting with oxygen. We will introduce enantioselectivity to the reaction by performing the reaction on chiral surfaces. For this, we will follow two approaches: at first, we will perform epoxidation experiments on naturally chiral surfaces, which have intrinsically chiral atomic structures. In a second approach, we plan to investigate the epoxidation on chirally modified surfaces. We will produce such surfaces by the adsorption of functionalized chiral organic molecules. The experiments will produce valuable data for the microscopic understanding of heterogeneously catalyzed enantioselective reactions by addressing among others the following questions: how does the epoxidation reaction rate of chosen olefin molecules depend on the combination of chiral reaction product and surface chirality? Are there systematic trends when changing the prochiral olefin reactants? Can we induce enantioselectivity when adsorbing chiral modifier molecules at the surface?

• DFG Programme: Research Grants