Collaborative Proposal: 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a （CSDM-A）项目的支持下，马凯特大学的Scott Reid教授和华盛顿大学的Richard Loomis教授将分别探索高激发分子的竞争性双分子反应途径。被激发的反应物分子可以通过多种机制放松，包括(i)直接的双分子反应，（ii）异构化（分子结构和连性的变化），（iii）漫游（导致意外的二次产物的远距离分子间相互作用），以及质子耦合电子转移（PCET）反应，这种反应发生在电子或质子从一个被激发的反应物分子转移到另一个反应物分子之后。对漫游、异构化和PCET过程的理解仍处于初级阶段。里德和卢米斯假设，与直接双分子反应竞争的途径是许多基本过程的核心，他们正在努力发展对决定其效率的因素的统一理解，以及这些途径如何决定产物的性质。因此，由里德教授和卢米斯教授领导的研究小组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，产物的能量和产率的特征是反应分子中沉积了多少能量的函数。通过这种方式，研究小组将描述这些不同的途径及其效率是如何被局部环境和激励改变的。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，为他们从事科学事业做好准备。该项目还侧重于通过马奎特大学和华盛顿大学的一些互补举措，扩大代表性不足的群体在科学、技术、工程和数学（STEM）领域的参与。该计划的一个显著组成部分是在研究生教育开始时为有风险的学生开发高度实用的课程。这些课程建立在增强STEM多样性的原则基础上，特别是在学术界，为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。这项合作研究项目的目标是由马奎特大学和华盛顿大学的斯科特·里德教授和理查德·卢米斯教授领导。Louis，分别描述了在地面、激发态和离子自由基表面上与异构化、漫游和PCET反应相关的共同特征。正在研究的体系分为两类：1)哈龙（包括二溴乙烷、二氯乙烷和卤代烷的异构体及其部分氘化类似物）的反应动力学；2)氨与卤代苯的电离配合物的反应动力学。这些目标体系，即哈龙，对环境具有重要意义，有望证明上述反应途径的全部范围，但又足够小，可以用高级理论方法处理。两个实验室的互补和重叠的技能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，频率分辨荧光光谱、频率和时间分辨离子飞行时间速度映射成像实验、超快瞬态吸收光谱和红外激发实验正在进行中。之所以选择这些反应系统，部分原因是它们能够以高灵敏度探测母体分子（或复合物）的性质以及所有产物通道（分子和原子）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特定状态的反应物制备和状态解决的产物检测，通过两个研究小组的共同努力，这些挑战正在被克服。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。

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, 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-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: 1) reaction dynamics of halons including the isomers of di-bromoethane, di-chloroethane, and halothane and their partially deuterated analogs, and 2) 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 to 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 are being 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 (molecular and atomic) 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 being overcome through the combined effort of the two resewarch 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.