Mapping the polymorphic transformation gateway vibration in crystalline 1,2,4,5-tetrabromobenzene

Area of Science:

• Solid-state chemistry
• Materials science
• Physical chemistry

Background:

• Thermosalient materials exhibit a macroscopic change in response to temperature variations.
• 1,2,4,5-tetrabromobenzene (TBB) is a known thermosalient compound.
• Understanding the molecular mechanisms of thermosalience is crucial for designing novel functional materials.

Purpose of the Study:

• To investigate the molecular mechanism behind the thermosalient behavior of 1,2,4,5-tetrabromobenzene (TBB).
• To elucidate the role of specific vibrational modes in inducing the polymorphic phase transition.
• To computationally determine the energy barrier and stabilization of the high-temperature polymorph.

Main Methods:

• Low-frequency Raman spectroscopy (10-250 cm⁻¹) was employed to study TBB polymorphs.
• Solid-state density functional theory (DFT) simulations were utilized for computational analysis.
• Laser power was carefully controlled to manage polymorph populations and obtain high-quality spectra.

Main Results:

• Distinct Raman vibrational spectra were obtained for both the beta (low-temperature) and gamma (high-temperature) TBB polymorphs.
• A low-frequency rotational mode at 15.5 cm⁻¹ in beta-TBB was identified as the key initiator of the phase transition.
• DFT calculations revealed a 2.40 kJ mol⁻¹ energy barrier separating the polymorphs, with gamma-TBB showing enhanced cohesion.

Conclusions:

• The polymorphic phase transition in TBB is initiated by a specific low-frequency rotational vibration.
• This vibrational mode acts as a gateway for the transition and stores mechanical strain.
• The gamma-TBB polymorph is stabilized by increased cohesion energy, providing insight into thermosalient mechanisms.