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

在这个由化学结构、动力学和机理-化学系的项目资助的项目中，马萨诸塞大学阿默斯特分校的Ricardo Metz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具工业用途的化学品。尽管天然气主要是甲烷(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，然后用来生产塑料，如聚乙烯和聚丙烯，或其他更高价值的化学品。在工业上，生产乙烯和丙烯需要大量的能源，每生产一磅乙烯就会产生超过一磅的二氧化碳。梅茨小组使用专门的仪器，在高真空下的分子流与激光相互作用，生产乙烯或丙烯等产品。研究生和本科生都参与了这项研究项目。除了获得和传播分子系统化学的知识外，学生们还接受了先进激光技术和真空技术以及用于帮助解释实验结果的计算化学的培训。梅茨教授还通过Eureka帮助介绍八年级女孩从事STEM领域的研究！在激光烧蚀/流动反应器源中产生金属离子和金属团簇离子与乙烷和丙烷的络合物，通过超音速膨胀冷却，在质谱仪中选择质量，并使用振动和电子光谱表征它们的结构和成键。与金属的相互作用削弱了乙烷或丙烷配体中的碳-氢(C-H键)；这种减弱是随后键的催化激活的关键。这些实验测量了金属的C-H键减弱和极化的程度，因为它们导致了相应的C-H拉伸振动的较低频率。这些实验还揭示了在可能涉及几个异构体的情况下反应中间体和产物的一致性，例如竞争性C-H键和C-C键插入。正在对几种金属和具有不同金属原子数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、硫化物和卡宾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，并探测了激发电子态之间的耦合。金属卤化物的光解离也被用来启动金属阳离子和烷烃之间的双分子反应，使该团队能够研究具有受控撞击参数和取向的反应动力学，作为平移能量的函数。这项工作的更广泛的影响包括改进催化剂的潜在好处，以及在质谱学和激光光谱学方面培训本科生和研究生，以及更广泛的技能，如解决问题、实验设计、数据获取和分析以及结果的交流。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.