The current chemical synthesis problems of interest involve hundreds of atoms, with the current objective to obtain synthetic analogues of biological systems or nanometer scale active chemistry such as molecular motors. The nuclear fluctuations leading to the chemistry involve highly nonlinear couplings of all these degrees of freedom. For a system of N atoms, there are on the order of 3N independent degrees of freedom, or dimensions, to chemical problems. This classic thought experiment also touches upon one of the most significant intellectual questions in chemistry. It is the manipulation of barrier heights that gives chemists effectively exponential control over chemical outcomes. (4) The barrier height exponentially defines the statistical probability of a given pathway.
That is, chemistry by its very nature is a “race against time”. (3) It is the control over the barrier height that allows one form of chemistry, out of the multitude of possible rearrangements of the atoms, to occur faster than competing processes. This conceptualization of the very moment of chemistry is an important pedagogical tool as it directs attention to the relative arrangement of atoms defining the barrier to the reaction of interest.
However, the entire field of chemistry is unified through a thought experiment in which we conceptualize how molecules interconvert between different structures during passage through the transition state region. It is well-appreciated that each subdiscipline in chemistry has unique intellectual targets. (1, 2) Chemistry after all involves structural dynamics by definition, and to observe the interconversion of molecules from one structure to another would capture the very essence of chemistry. One of the dream experiments in chemistry is to directly observe atomic motions in real time during chemical reactions. We now have a new way to reformulate reaction mechanisms using an experimentally determined dynamic mode basis that in combination with recent theoretical advances has the potential to lead to a new conceptual basis for chemistry that forms a natural link between structure and dynamics. It is this reduction in dimensionality that makes chemical reaction mechanisms transferrable to seemingly arbitrarily complex (large N) systems, up to molecules as large as biological macromolecules ( N > 1000 atoms). Effectively, we can now directly observe the far-from-equilibrium atomic motions involved in barrier crossing and categorize chemistry in terms of a power spectrum of a few dominant reaction modes. The focus is on atomically resolved chemical reaction dynamics with pertinent references to work in other areas and forms of spectroscopy that provide additional information. The use of femtosecond Rydberg spectroscopy as a novel means to use internal electron scattering within the molecular reference frame to obtain similar information on reaction dynamics is also discussed. The review chronicles the first use of electron structural probes to study reactive intermediates, to the development of high bunch charge electron pulses with sufficient combined spatial-temporal resolution and intensity to literally light up atomic motions, as well as the means to characterize the electron pulses in terms of temporal brightness and image reconstruction. This objective has been met with electrons as the imaging source. This challenge has been cast as an imaging problem in which the technical issues reduce to achieving not only sufficient simultaneous space-time resolution but also brightness for sufficient image contrast to capture the atomic motions. The depiction of the nuclear configurations in space-time to understand barrier crossing events has served as a unifying intellectual theme connecting the different disciplines of chemistry. One of the grand challenges in chemistry has been to directly observe atomic motions during chemical processes.