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

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.