Velocity resolved kinetics of enantioselective olefin epoxidation on chiral surfaces

本课题拟结合分子束表面散射实验和速度图成像技术，研究手性表面对映选择性烯烃环氧化反应的动力学和动力学。金属表面烯烃的环氧化反应是一类技术上重要的化学反应，迄今为止，在超高真空条件下对相应的反应进行了广泛而详细的研究。然而，这些研究都没有关注当反应发生在手性表面时反应的对映选择性，因为手性检测要求在低浓度的气相下表面解吸反应产物。我们计划通过测量脉冲激光2+1共振增强多光子电离（REMPI）后400 nm环氧化物的多光子光电子圆二色性（MP-PECD）来鉴定反应产物的手性。实验将进行类似泵浦探测激光实验。前手性烯烃的短分子束脉冲通过表面（泵）吸附引发反应，反应产物在激光电离解吸后被探测。通过时间延迟分子束脉冲和激光电离，可以在微秒级时间尺度上推断反应动力学。我们记录光电子用于手性检测或光电子用于飞行时间质谱（ToF-MS）的产品分配。同样的探测器提供反应产物的速度图图像，用于记录现场特定反应通道的动力学。我们将研究前手性烯烃苯乙烯，3,3-二甲基-1-丁烯和反式甲基苯乙烯在手性铸币金属上的反应。在非手性铸币金属表面，当与氧反应时，所有这些烯烃部分氧化成相应的手性环氧化合物的外消旋混合物。我们将通过在手性表面上进行反应来引入对映选择性。为此，我们将采用两种方法：首先，我们将在具有固有手性原子结构的天然手性表面上进行环氧化实验。在第二种方法中，我们计划研究手性修饰表面的环氧化反应。我们将通过吸附功能化的手性有机分子来制造这样的表面。这些实验将通过解决以下问题，为多相催化对映选择反应的微观理解提供有价值的数据：所选烯烃分子的环氧化反应速率如何取决于手性反应产物和表面手性的组合？前手性烯烃反应物的变化是否有系统的趋势？当在表面吸附手性修饰剂分子时，我们能诱导对映体选择性吗？

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?