Exposing Key Vibrational Contributions to Properties of Organic Molecular Solids with High Signal, Low Frequency Neutron Spectroscopy and Ab Initio Simulations

Stability and response of supramolecular forms is important to many areas in materials science, and contributions from vibrations can be crucial. We have collected the first spectra of organic molecular crystals and polymorphic cocrystals using the next-generation, high-signal VISION spectrometer in the far-infrared (FIR) and mid-infrared (MIR) range. Unambiguously different spectral signatures were found for carbamazepine and two polymorphs of the carbamazepine-saccharin cocrystal, including numerous modes undetectable with optical methods. For FIR-range frequencies, in addition to correct calculation of peak positions, accurate line-shape features were reproducible by simulation using ab initio vibrational calculations. High confidence spectral assignments and thermochemical information for this region are thus expected, and a benchmark for vibrational ab initio calculations is provided, creating a significant addition to methods used in investigation and design of organic materials.