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.