Collaborative Research: Probing Undiscovered Reaction Pathways in the Decomposition of Highly Energized Molecules: Isomerization, Roaming, and Proton Coupled Electron Transfer

在化学系化学结构、动力学和机理(CSDM-A)计划的支持下，马凯特大学的Scott Reid教授和圣路易斯华盛顿大学的Richard Loomis教授将分别探索高激发分子的竞争双分子反应路径。带电的反应物分子可以通过多种机制松弛，包括(I)直接双分子反应，(Ii)异构化(分子结构和连接性的变化)，(Iii)漫游(导致意外的二次产物的远程分子间相互作用)，以及质子耦合电子转移(PCET)反应，这些反应发生在一个电子或质子从一个激发的反应物分子到另一个反应物分子的初始转移之后。对漫游、异构化和PCET过程的了解仍处于初级阶段。里德和卢米斯假设，与直接双分子反应竞争的路径是许多基本过程的核心，他们正在努力对决定其效率的因素以及这些路径如何决定产品的性质达成统一的理解。因此，里德和卢米斯教授领导的研究团队正在使用频率和时间分辨实验的强大组合，以及理论，来解开这些过程的动力学。实验将在真空、溶剂和固体基质中进行，产物的能量和产率是反应分子中能量沉积的函数。通过这种方式，研究团队将描述这些不同的途径及其效率如何因当地环境和激励而改变。该研究项目的合作性质为研究生和本科生提供了一系列重要技能领域的培训，为他们在科学领域的职业生涯做好准备。该项目还侧重于通过马奎特和华盛顿大学的一系列补充倡议，扩大科学、技术、工程和数学(STEM)中代表性不足群体的参与。该计划的一个显著组成部分是在研究生教育之初为高危学生开发高度实用的课程。这些课程建立在增强STEM多样性的原则上，通过向有前途的科学家提供他们在早期阶段取得成功所需的工具，来增强STEM的多样性。这个由马奎特大学和圣路易斯华盛顿大学的Scott Reid和Richard Loomis教授领导的合作研究项目的目标是表征与异构化、漫游和地面、激发和离子自由基表面的PCET反应相关的共同特征。所研究的体系分为两类：(I)哈龙的反应动力学，包括二溴乙烷、二氯乙烷和氟烷的异构体和它们的部分氚类似物，以及(Ii)氨与卤苯的电离络合物的反应。这些目标系统，即哈龙，对环境很重要，预计将展示上述所有反应途径，但又足够小，可以通过高水平的理论方法加以处理。两个实验室的互补和重叠的技能组合和技术使实验能够以高灵敏度、能量分辨率和时间分辨率进行。具体来说，将进行基于频率分辨荧光光谱、频率和时间分辨离子飞行时间速度图成像实验、超快瞬时吸收光谱和红外激发实验。选择这些反应系统的部分原因是能够以高灵敏度探测母分子或络合物和所有产物通道的性质。选定的系统也正在使用计算方法进行详细研究，实验结果为该理论的持续发展提供了严格的测试和里程碑。这项研究工作中的重要挑战包括反应物的特定状态制备和产品的状态分辨检测，这些挑战需要通过两个研究小组的共同努力来克服。学生培训机会和对扩大STEM教育和研究参与的重视进一步扩大了该项目的影响。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。

With support from the Chemical Structure, Dynamics, and Mechanisms-A (CSDM-A) Program in the Division of Chemistry, Professors Scott Reid at Marquette University and Richard Loomis at Washington University in St. Louis, respectively, will explore competing bimolecular reaction pathways of highly-excited molecules. Energized reactant molecules can relax via multiple mechanisms, including (i) direct bimolecular reactions, (ii) isomerization (changes in molecular structure and connectivity), (iii) roaming (long-range intermolecular interactions that lead to unexpected, secondary products), and proton-coupled electron transfer (PCET) reactions that occur following the initial transfer of an electron or proton from an excited reactant molecule to the other reactant molecule. The understanding of roaming, isomerization, and PCET processes are still at an elementary stage. Reid and Loomis hypothesize the pathways that compete with direct bimolecular reactions are central to many fundamental processes, and they are striving to develop a unified understanding of the factors that dictate their efficiencies and how these pathways dictate the properties of the products. Thus, the research teams led by Professors Reid and Loomis are using a powerful combination of frequency- and time-resolved experiments, together with theory, to unravel the dynamics of these processes. The experiments will be performed in vacuum, in solvents, and in solid matrices, and the energetics and yields of the products are characterized as a function of how much energy is deposited into the reacting molecules. In this manner, the research teams will characterize how these different pathways and their efficiencies are altered by local environment and excitation. The collaborative nature of the research project offers graduate and undergraduate students training in an array of important skill areas, preparing them for careers in science. The project also has a focus on broadening the participation of underrepresented groups in science, technology, engineering, and mathematics (STEM) through a number of complementary initiatives at Marquette and Washington University. A notable component of this program is the development of highly practical courses for at-risk students at the onset of their graduate education. The courses build on a principle of enhancing diversity in STEM, especially in academia, by providing promising scientists with the tools they need to succeed at an early stage.The goal of this collaborative research project led by Professors Scott Reid and Richard Loomis at Marquette University and Washington University in St. Louis, respectively, is the characterization of common features associated with isomerization, roaming, and PCET reactions on ground, excited, and ion radical surfaces. The systems being explored fall into two categories: (i) reaction dynamics of halons including the isomers of di-bromoethane, di-chloroethane, and halothane and their partially deuterated analogs, and (ii) reactions of ionized complexes of ammonia with halobenzenes. These target systems, the halons, are environmentally important, are expected to demonstrate the full range of reaction pathways listed above, and yet are small enough to be tractable via high-level theoretical methods. The complementary and overlapping skill sets and techniques in the two laboratories enable experiments to be undertaken with high sensitivity, energy resolution, and temporal resolution. Specifically, frequency-resolved fluorescence-based spectroscopy, frequency- and time-resolved ion time-of-flight velocity mapped imaging experiments, ultrafast transient absorption spectroscopy, and infrared excitation experiments will be pursued. These reaction systems were chosen, in part, because of the ability to probe the properties of the parent molecules or complexes and all of the product channels with high sensitivity. The selected systems are also being investigated in detail using computational methods, with the experimental results providing stringent tests and milestones for ongoing development of the theory. Important challenges in this research effort include state-specific preparation of the reactants and state-resolved detection of the products, challenges that are to be overcome through the combined effort of the two research groups. Student training opportunities and an emphasis on broadening participation in STEM education and research further broaden the impacts of the project.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.