Free Radical Chemistry Probed with Muon Spin Spectroscopy

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In this research program the high-tech tools of particle physics are applied to study fundamental chemistry. The simplest chemical species is the hydrogen atom, which has a single electron in orbit around a proton as nucleus. Replacing the proton with a positive muon results in the muonium atom (Mu), which can be thought of as a light isotope of hydrogen (mass 0.11) similar to its heavier cousins deuterium (2) and tritium (3). Just like H, Mu can react with certain types of molecules to form free radicals. Because the muons come from a beam line where their spins are aligned in one direction, it is possible to use magnetic resonance techniques to follow what happens to Mu in its different chemical environments: how fast it reacts, which part of a molecule it reacts with, and the properties of the muoniated free radical formed. The muon spin spectroscopy experiments are performed at TRIUMF, Canada's national cyclotron laboratory, one of only four sites in the world where there are facilities for such experiments. Why use such an exotic, expensive probe to study "simple" chemistry? It is because muon spin spectroscopy can be applied in situations where more conventional methods are impossible or give confusing results. An example of the former is radiation chemistry in the superheated water used in nuclear power reactors: we have studied reactions in water as high as 470°C and 400 atm. pressure and the results are being used by AECL to guide the development of the "Generation-IV" supercritical-water-cooled reactor. **In a new project we are studying the chemistry of guest molecules inside gas hydrates similar to the infamous "ice crystals" that were involved in the Deepwater Horizon oil rig disaster in the Gulf of Mexico. Flames and explosions propagate through free radical reactions, yet almost nothing is known about the diffusion and interactions of atoms and radicals through the cavities of gas hydrate crystal structures. We have demonstrated the ability to detect Mu in hydrates of saturated organic molecules, and muoniated radicals in hydrates of similar unsaturated molecules. We have clear evidence that radicals behave differently in hydrates than in pure liquids at the same temperature. Further studies will involve lower temperatures, to investigate the change of molecular motion and reactivity as a function of temperature. The results will tell us how reactive free radicals behave in hydrate crystals and the conditions under which explosions occur. Our results on diffusion will also aid other researchers who are investigating the potential use of gas hydrates for hydrogen storage and carbon dioxide sequestration. **Another project concerns the free radical reactivity of novel organosilicon, organogermanium, and organophosphorus compounds. Synthetic chemists from around the world collaborate with us by sending their new materials because we can provide unique information on how their molecules react with free radicals. These materials are often produced in the search for new catalysts for industrial processes, such as radical-induced polymerization. However, there is no simple means of producing "normal" H atoms without producing many other highly reactive, competing free radicals. Our work provides a direct route to the free radical reactivity of the new candidate catalysts.