Crystallization Caught in the Act with Terahertz Spectroscopy: Non‐Classical Pathway for l‐(+)‐Tartaric Acid

Crystal formation is a highly debated problem. This report shows that the crystallization of l‐(+)‐tartaric acid from water follows a non‐classical path involving intermediate hydrated states. Analytical ultracentrifugation indicates solution clusters of the initial stages aggregate to form an early intermediate. Terahertz spectroscopy performed during water evaporation highlights a transient increase in the absorption during nucleation; this indicates the recurrence of water molecules that are expelled from the intermediate phase. Besides, a transient resonance at 750 GHz, which can be assigned to a natural vibration of large hydrated aggregates, vanishes after the final crystal has formed. Furthermore, THz data reveal the vibration of nanosized clusters in the dilute solution indicated by analytical ultracentrifugation. Infrared spectroscopy and wide‐angle X‐ray scattering highlight that the intermediate is not a crystalline hydrate. These results demonstrate that nanoscopic intermediate units assemble to form the first solvent‐free crystalline nuclei upon dehydration. Caught in the act: By employing THz waves, it is shown that the crystallization of L‐(+)‐tartaric acid from water follows a non‐classical path involving intermediate hydrated states. The figure shows crystallization of l‐(+)‐tartaric acid and the balloons on the top structural schemes of the molecular arrangement in the different stages.