Molecular filming: A director's grammar for visualizing chemistry in motion

Abstract

Abstract Time‐resolved x‐ray and electron scattering now follow the changing structures of molecules directly, on femtosecond timescales and sub‐ångström length scales. The community has long captured this achievement in one evocative phrase: the molecular movie. Yet that phrase names a product, the movie one obtains, rather than the practice that produces it. Drawing an explicit analogy to molecular editing in organic synthesis, where an action‐oriented reframing helped reorganize and reinvigorate a broad area of synthesis, I propose molecular filming as a complementary, action‐oriented paradigm in which the experimentalist is the director of an atomic‐scale movie. Building on more than two decades of my own group's molecular filming, spanning ultrafast electron diffraction, time‐resolved x‐ray liquidography, and time‐resolved serial femtosecond crystallography, I survey the field as a set of directorial choices: which trigger to call “Action!”, which imaging modality to use, which phase of matter to study, what temporal and structural information to capture, and how to reconstruct molecular structure from the resulting data. These choices encompass x‐ray and electron scattering, complementary structural probes, and increasingly automated, machine‐learning‐assisted analysis. I then make this logic explicit as a grammar of molecular filming that mirrors the cut‐and‐paste logic of molecular editing while describing motion rather than connectivity. Naming the practice, I argue, does for structural dynamics what naming molecular editing did for synthesis: it organizes a heterogeneous field, re‐values what a product‐focused view overlooks, and turns “we made a movie of X” into the sharper question “which scene can we not yet shoot?” I close by identifying the field's defining frontier: extending molecular filming from photoinduced unimolecular reactions to the thermal bimolecular reactions that dominate synthesis, and advancing from the structures of intermediates to those of transition states and conical intersections, and thus to the reaction coordinate itself.