Interrogation of the Intermolecular Forces That Drive Bulk Properties of Molecular Crystals with Terahertz Spectroscopy and Density Functional Theory

Identifying and characterizing intermolecular forces in the condensed phase is crucial for understanding both micro- and macroscopic properties of solids; ranging from solid-state reactivity to thermal expansion. Insight into these interactions enables a holistic comprehension of bulk properties, and thus understanding them has direct implications for supramolecular design. However, even modest changes to intermolecular interactions can create unpredictable changes to solid-state structures and dynamics. For example, copper-(II) acetylacetonate (Cu-(C₅H₇O₂)₂) and copper-(II) hexafluoroacetylacetonate (Cu-(C₅HF₆O₂)₂) exhibit similar molecular conformations, yet differences between the methyl and trifluoromethyl groups produce distinct sets of intermolecular forces in the condensed phase. Ultimately, these differences produce unique molecular arrangements in the solid state, with corresponding differences in material properties between the two crystals. In this work, terahertz spectroscopy is used to measure low-frequency vibrational dynamics, which, by extension, provide detailed insight into the underlying intermolecular forces that exist in each system. The experimental data is coupled to theoretical quantum mechanical simulations to precisely quantify the interplay between various energetic effects, and these results highlight the delicate balance that is struck between electronic and dispersive interactions that underpin the structural and related differences between the two systems.