Towards molecular movies: exploring reaction dynamics using electron diffraction

我们对周围世界的许多知识和理解都来自于对分子结构的考虑。但科学家如何知道在分子或原子水平上正在发生的事情呢？衍射技术可以直接给我们提供信息，如分子采用的几何结构，该几何结构是否随物质的物理状态而变化，以及当两个或更多分子反应时产生什么产物。在20世纪，至少有22项诺贝尔奖获奖，这些奖项是基于使用X射线和电子衍射进行的结构研究，导致了DNA的双螺旋结构和血红蛋白在生命周期中的作用等重要发现。在21世纪，新的目标是了解化学反应的动力学。这不仅要求我们观察反应发生前后的结构，还要求我们更深入地了解反应是如何以及为什么以特定的方式进行的，并最终利用这些信息来控制反应。利用泵浦-探测实验来研究化学、生物和材料科学中的超快事件已经开始彻底改变我们对化学反应的理解。这类实验使用强激光为分子提供能量(泵浦)，改变其基本结构，然后进行观察(探测)。到目前为止，重点一直是在泵浦和探测阶段使用激光，或者最近使用X射线衍射来探测结构。衍射法直接产生分子的瞬时结构，这比从光谱学中推断结构信息要好得多。我的研究更进一步，使用电子衍射作为探针来研究在不同时间尺度上发生变化的化学物种的结构。电子特别适合于研究气相中的结构，在气相中，由于缺乏邻近分子的影响(固态技术的一个问题)，可以研究模型系统。电子是有效的分子结构探测器，具有高散射截面和低比例的非弹性散射(几乎不包含结构信息)。因为电子是带电的，所以它们相互排斥。当需要非常短的电子脉冲时，这会产生后果，而实验室中时间分辨率的理论极限是0.5皮秒。其他地方已经进行了实验，并被报道为飞秒电子衍射--这是一种误导，因为技术规定皮秒限制仍然存在。然而，使用能量非常高的电子有可能突破这一障碍。这样的电子通常用于加速器物理，在那里它们被加速，直到发出X射线。我最终将利用这些电子发出长度约为100飞秒的脉冲；当用于衍射实验时，这些电子将允许观察到化学键的形成和断裂。我将使用超快电子衍射方法的一个领域是研究氢键，这在化学和生物学中非常重要，在许多分子物种中很常见，如水、DNA和蛋白质。尽管多年来对氢键的形成和断裂的机制进行了研究，但仍然有许多问题没有得到解答。与氢键相关的过程被称为快速质子传输，被认为发生在许多生物系统中，在这些系统中，能量从一种形式转换为另一种形式。有人提出，在有一个以上氢键的系统中，快速的质子传输遵循固定的模式。我还将与合成化学家密切合作，以确保我正在研究对当今化学家真正重要的系统，使我的工作有别于其他目前正在实践超快电子衍射的人。

So much of our knowledge and understanding of the world around us comes from a consideration of the structures of molecules. But how do scientists know what is happening at a molecular or atomic level? Diffraction techniques can give us directly information such as the geometry that a molecule adopts, whether that geometry changes depending on the physical state of the substance, and what products are yielded when two or more molecules react. In the 20th century no fewer than 22 Nobel Prizes were awarded for work based around structural studies using X-ray and electron diffraction, leading to such important discoveries as the double-helix structure of DNA and the role of haemoglobin in the life cycle. In the 21st century the new goal is to understand the dynamics of chemical reactions. This requires us not just to observe structures before and after reactions have occurred, but also to gain a deeper knowledge of how and why reactions proceed in particular ways and, ultimately, to use this information to control reactions.The use of pump-probe experiments to study ultrafast events in chemistry, biology and materials science has already begun to revolutionise our understanding of chemical reactions. Such experiments use an intense laser beam to provide energy to molecules (the pumping), changing their fundamental structures, which are then observed (probed). Until now the emphasis has been on using lasers for both the pump and probe phases or, more recently, using X-ray diffraction to probe the structures. Diffraction methods yield transient structures of molecules directly, which is greatly preferable to inferring structural information from spectroscopy.My research takes this one step further and uses electron diffraction as a probe to study the structures of chemical species undergoing changes that occur on a variety of timescales. Electrons are particularly well suited to studying structures in the gas phase, where the lack of influence from neighbouring molecules (an issue with solid-state techniques) allows model systems to be studied. Electrons are efficient probes of molecular structure, with a high scattering cross section and a low proportion of inelastic scattering (which contains little or no structural information). Because electrons are charged they repel one another. This has consequences when very short pulses of electrons are required, and the theoretical limit of temporal resolution in a laboratory is 0.5 picoseconds. Experiments have been performed elsewhere and reported as femtosecond electron diffraction - this is misleading as the technology dictates that the picosecond limit remains. However, it is possible to break through this barrier using electrons with very high energies. Such electrons are routinely used in accelerator physics, where they are sped up until X-rays are emitted. I will ultimately harness these electrons to give pulses with a length of around 100 femtoseconds; when used in a diffraction experiment these electrons will allow the formation and breaking of chemical bonds to be observed.One area where I will use ultrafast electron-diffraction methods is in the study of hydrogen bonds, which are of utmost importance in chemistry and biology and are common in many molecular species such as water, DNA and proteins. Despite many years of work into the mechanisms of the formation and breaking of hydrogen bonds there are still many unanswered questions. A process related to hydrogen bonding, called fast proton transport, is believed to occur in many biological systems where energy is converted from one form to another. It has been proposed that, in systems with more than one hydrogen bond, fast proton transport follows set patterns. I will also work closely with synthetic chemists to ensure that I am studying the systems that really matter to chemists today, setting my work apart from others who are currently practicising ultrafast electron diffraction.