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

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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.