Triggering, Controlling and Imaging Chemical Reactions at the Single-Molecule Level by Electron Beam

我们怎么知道分子以一种方式而不是另一种方式反应？在一个给定的实验中，我们研究了以不同状态存在并具有不同动能的大分子群（数十亿或更多）以混沌方式相互碰撞的反应。即使在理想的情况下，在实验室实验中，通过集合平均分析技术（如光谱学或衍射）观察到的反应，也只能支持而不是证实所提出的机制，因为这些宏观测量不能排除可能存在另一种原子机制，导致相同的宏观观察。在实践中，关于分子间反应机制的明确信息只能通过在单分子水平上对反应物随时间转化为产物的直接观察来提供。在这种情况下，扫描探针显微镜（SPM）方法最近对化学反应的中间产物和产物的原子结构都有重要的研究。然而，由于AFM/STM的扫描性质以及分子必须被刺激（如热）“激活”的事实，SPM严重缺乏时间分辨率，在此期间分子保持不被观察到，从而引入了对物种集合的平均信息的需要（尽管比批量测量要小得多）。透射电子显微镜（TEM）为分子间反应提供了独特的机会，与SPM和气相分子光谱非常不同，但又高度互补。利用化学透射电镜的三个原理：(1)在纳米试管中对单个分子进行物理包裹和限制；（ii）从入射电子束到原子的直接动量传递；（iii）在单分子水平上直接拍摄化学键解离和形成的停止帧，这个EPSRC项目将解决同时触发和成像反应途径的挑战——从反应物到中间体到产物。分子被限制在二维空间，例如在纳米管中，而第三维空间则可以自由进行化学反应，电子束可以操纵这些分子，并在它们相互反应时成像。通过这种方式，化学家把单个分子放在“手术台”上，准备以原子级别的精度进行解剖和研究。在这个项目中开发的化学透射电镜的原理和方法有潜力成为分子反应的成像和分析工具，补充和支持当前的光谱、衍射和SPM方法。ChemTEM将在单分子水平上对直接空间的反应路径进行成像，并将使重要化学过程的反应机制得以阐明，如C-C键的形成和解离、脱氢和缩聚反应，从而改进高价值材料的制备合成和替代催化剂的设计。此外，ChemTEM在发现全新类型的化学反应方面具有巨大的潜力，这些化学反应不仅可以改变我们研究分子的方式，还可以启动合成化学研究的新浪潮，目前合成化学依赖于相对较少的反应类型。

How do we know that molecules react in one way rather than another? In a given experiment, we study the reactions of large ensembles of molecules (billions of billions or more) that exist in different states and possess different kinetic energies, colliding with each other in a chaotic manner. Even in an ideal case, a reaction observed in a laboratory experiment by ensemble-averaging analytical techniques, such as spectroscopy or diffraction, can only support rather than confirm a proposed mechanism, as these macroscopic measurements are unable to rule out that an alternative atomistic mechanism may also exist that results in the same macroscale observation. In practice, definitive information about the mechanisms of intermolecular reactions can be provided only by a direct observation at the single-molecule level of the reactants transforming into products over time. In this context, scanning probe microscopy (SPM) methods have recently shed important light on the atomic structures of both the intermediates and products of chemical reactions; however, SPM critically lacks time resolution due to the scanning nature of AFM/STM and the fact that the molecules must be 'activated' by a stimulus, such as heat, during which the molecules remain unobserved, thus introducing the need for averaging information over an ensemble of species (albeit much smaller than in the bulk measurement). Transmission electron microscopy (TEM) offers unique opportunities for intermolecular reactions, very different, yet highly complementary, to SPM and gas-phase molecular spectroscopy. Using the three principles of ChemTEM: (i) physical entrapment and confinement of individual molecules in nano test tubes; (ii) direct momentum transfer from the incident electron beam to atoms; (iii) stop-frame filming of chemical bond dissociation and formation in direct space at the single-molecule level, this EPSRC project will address the challenge of simultaneous triggering and imaging of reaction pathways - from reactants via intermediates to products. The molecules constrained in two dimensions, for example in a nanotube, while having the third dimension free for chemistry, will be manipulated by the electron beam and imaged as they react with each other. In this way, the chemist has the individual molecules on an 'operating table' as it were, ready to be dissected and studied with atomic-level precision. The principles and methodology of ChemTEM developed in this project have the potential to become an imaging and analytical tool for molecular reactions, complementing and bolstering current spectroscopy, diffraction and SPM methods. ChemTEM will image reaction pathways in direct space at the single-molecule level and will enable the elucidation of reaction mechanisms of important chemical processes, such as C-C bond formation and dissociation, dehydrogenation and polycondensation reactions, leading to the improved preparative synthesis of high-value materials and the design of alternative catalysts. In addition, ChemTEM has great potential for the discovery of entirely new types of chemical reactions that can transform not only the way we study molecules but also launch a new wave of research in synthetic chemistry, which currently relies on a relatively small number of reaction types.