Free Radical Chemistry Probed with Muon Spin Spectroscopy

本研究计划将粒子物理的高科技工具应用于基础化学的研究。最简单的化学物质是氢原子，它有一个电子绕着一个质子作为原子核运行。用一个正的介子取代质子会得到介子原子（Mu），它可以被认为是氢的一种轻同位素（质量0.11），类似于它较重的表兄弟氘(2)和氚(3)。和H一样，Mu可以和某些类型的分子反应生成自由基。因为介子来自自旋在一个方向上排列的束流线，所以有可能使用磁共振技术来跟踪在不同的化学环境中Mu发生的事情：它的反应速度有多快，它与分子的哪一部分反应，以及形成的mumuated自由基的性质。μ子自旋光谱学实验是在加拿大国家回旋加速器实验室（TRIUMF）进行的，这是世界上仅有的四个拥有此类实验设施的实验室之一。为什么要用这种奇特、昂贵的探针来研究“简单”的化学呢？这是因为介子自旋光谱学可以应用在更传统的方法不可能或给出混乱结果的情况下。前者的一个例子是核动力反应堆中使用的过热水中的辐射化学：我们已经研究了高达470°C和400 atm的水中的反应。AECL正在使用该结果来指导“第四代”超临界水冷反应堆的开发。

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.