Free Radical Chemistry Probed with Muon Spin Spectroscopy

在这个研究项目中，粒子物理的高科技工具被应用到基础化学的研究中。最简单的化学物种是氢原子，它只有一个电子在轨道上，围绕一个质子作为原子核。用正的Muon取代质子产生了Muonium原子(Mu)，它可以被认为是氢的一种轻同位素(质量为0.11)，类似于它较重的近亲氢(2)和氚(3)。就像H一样，Mu可以与某些类型的分子反应形成自由基。由于µ子来自一条光束线，它们的自旋在一个方向上对齐，因此有可能使用磁共振技术来跟踪Mu在不同化学环境中发生的事情：它的反应速度有多快，它与分子的哪个部分反应，以及产生的诱变自由基的性质。Muon自旋光谱实验是在加拿大国家回旋实验室TRIUMF进行的，这是世界上仅有的四个拥有此类实验设施的地点之一。为什么要用如此奇特、昂贵的探测器来研究“简单”的化学呢？这是因为Muon自旋光谱可以应用于更传统的方法不可能或给出令人困惑的结果的情况。前者的一个例子是核反应堆中使用的过热水中的辐射化学：我们研究了在高达470°C和400大气压的水中的反应。AECL正在使用压力和结果来指导“第四代”超临界水冷却反应堆的发展。在一个新的项目中，我们正在研究天然气水合物中客体分子的化学，类似于墨西哥湾深水地平线石油钻井平台灾难中臭名昭著的“冰晶”。火焰和爆炸通过自由基反应传播，但对原子和自由基通过气体水合物晶体结构的空穴扩散和相互作用几乎一无所知。我们已经证明了在饱和有机分子的水合物中检测Mu的能力，以及在类似的不饱和分子的水合物中检测Mu的能力。我们有明确的证据表明，在相同的温度下，自由基在水合物中的行为与在纯液体中的不同。进一步的研究将涉及较低的温度，以研究分子运动和反应性随温度的变化。结果将告诉我们活性自由基在水合物晶体中的行为以及发生爆炸的条件。我们在扩散方面的结果也将有助于其他正在研究天然气水合物用于氢气储存和二氧化碳封存的潜在用途的研究人员。另一个项目涉及新型有机硅、有机锗和有机磷化合物的自由基反应性。来自世界各地的合成化学家通过发送他们的新材料与我们合作，因为我们可以提供关于它们的分子如何与自由基反应的独特信息。这些材料通常是为了寻找用于工业过程的新催化剂而生产的，例如自由基引发聚合。然而，没有一种简单的方法可以在不产生许多其他高度活性的、相互竞争的自由基的情况下产生“正常的”氢原子。我们的工作为研究新的候选催化剂的自由基反应活性提供了一条直接途径。

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.