Characterizing Bond Activation in Alkanes and Alkyl Halides by Metal and Metal Cluster Cations using Spectroscopy and Photofragment Imaging

在这个由化学部门化学结构、动力学和机理a项目资助的项目中，马萨诸塞大学阿姆赫斯特分校的里卡多·梅斯教授和他的学生正在研究如何将烷烃和烷基卤化物转化为更有用的工业化学品。虽然天然气的主要成分是甲烷（CH4），但它也含有高达6%的乙烷和4%的丙烷。乙烷和丙烷非常有用，因为它们可以转化为乙烯和丙烯，然后用于生产聚乙烯和聚丙烯等塑料，或其他更高价值的化学品。在工业上，乙烯和丙烯的生产需要大量的能源，每生产一磅乙烯，就会产生一磅多的二氧化碳。梅茨团队使用专门的仪器，在高真空下的分子流与激光相互作用，产生乙烯或丙烯等产品。研究生和本科生都参与了这个研究项目。除了获得和传播分子系统的化学知识外，学生们还接受了先进激光技术和真空技术的培训，以及用于帮助解释实验结果的计算化学。梅斯教授还通过“尤里卡！程序。在激光烧蚀/流动反应器源中制备金属离子和金属簇离子与乙烷和丙烷的配合物，通过超音速膨胀冷却，在质谱仪中选择质量，并使用振动和电子能谱对其结构和键合进行表征。与金属的相互作用削弱了乙烷或丙烷配体中的碳氢键；这种弱化是键随后催化活化的关键。实验测量了碳氢键被金属削弱和极化的程度，因为它们导致相应的碳氢键拉伸振动频率较低。这些实验还揭示了在可能涉及多个异构体的情况下，如竞争性碳氢键和碳氢键插入时，反应中间体和产物的身份。目前正在对几种金属和具有不同数量金属原子的团簇进行这些研究，以确定导致依赖于尺寸的反应性的电子和结构因素。金属卤化物、硫化物和二氧化碳阳离子光解的互补速度图成像研究测量了动能释放和碎片各向异性，确定了准确的键强度，探测了激发态之间的耦合。金属卤化物的光解也被用于启动金属阳离子和烷烃之间的双分子反应，使团队能够在控制冲击参数和方向的情况下研究反应动力学，作为平动能量的函数。这项工作的广泛影响包括改进催化剂的潜在好处，以及培养本科生和研究生在质谱和激光光谱方面的技能，以及更广泛的技能，如解决问题、实验设计、数据采集和分析以及结果交流。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

In this project, funded by the Chemical Structure, Dynamics, and Mechanisms-A program of the Chemistry Division, Professor Ricardo Metz and his students at the University of Massachusetts Amherst are studying how alkanes and alkyl halides can be converted to more industrially useful chemicals. Although natural gas is mostly methane (CH4), it also has up to 6% ethane and 4% propane. Ethane and propane are extremely useful, as they can be converted into ethylene and propylene, which are then used to produce plastics like polyethylene and polypropylene, or other higher value chemicals. Industrially, the production of ethylene and propylene require a great deal of energy and they produce over a pound of carbon dioxide for every pound of ethylene produced. The Metz group uses specialized instruments in which streams of molecules under high vacuum interact with laser light to produce products like ethylene or propylene. Both graduate and undergraduate students are involved in this research project. In addition to gaining and disseminating knowledge about the chemistry of molecular systems, the students are receiving training in advanced laser techniques and vacuum technologies, as well as computational chemistry, which is used to help interpret the experimental results. Professor Metz is also helping to introduce eighth-grade girls to research in STEM fields through the Eureka! program.Complexes of metal ions and metal cluster ions with ethane and propane are produced in a laser ablation/flow reactor source, cooled via supersonic expansion, mass selected in a mass spectrometer and their structure and bonding is characterized using vibrational and electronic spectroscopy. Interaction with the metal weakens the carbon-hydrogen (C-H bonds) in the ethane or propane ligand; this weakening is key to the subsequent catalytic activation of the bond. The experiments measure the extent of C-H bond weakening and polarization by the metal, as they lead to lower frequencies for the corresponding C-H stretch vibrations. These experiments also reveal the identity of reaction intermediates and products in cases where several isomers could be involved, as in competitive C-H and C-C bond insertion. These studies are being carried out for several metals and for clusters with differing numbers of metal atoms to determine the electronic and structural factors that lead to size-dependent reactivity. Complementary velocity map imaging studies of metal halide, sulfide and carbene cation photodissociation measure kinetic energy release and fragment anisotropy, determining accurate bond strengths and probing the couplings between excited electronic states. Photodissociation of a metal halide are also being used to initiate a bimolecular reaction between a metal cation and an alkane, allowing the team to study reaction dynamics with controlled impact parameter and orientation, as a function of translational energy. The broader impacts of this work include potential benefits from improved catalysts as well as training undergraduate and graduate students in mass spectrometry and laser spectroscopy and in broader skills such as problem solving, experimental design, data acquisition and analysis and communication of their results.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.