Is Ortho-Terphenyl a Rigid Glass Former?

Ortho-terphenyl (OTP) has long been used as a model system
to study
the glass transition due to its apparent simplicity and a widespread
assumption that it is a rigid molecule. Here, we employ terahertz
time-domain spectroscopy and low-frequency Raman spectroscopy to investigate
the rigidity of OTP by direct observation of the low-frequency vibrational
dynamics. These terahertz phonons involve complex large-amplitude
atomic motions where intramolecular and intermolecular displacements
are often mixed. Comparison of experimental results with density functional
theory and ab initio molecular dynamics simulations
shows that the assumption of rigidity neglects important implications
for the glass transition and must be revisited. These results highlight
the significance of terahertz modes on elasticity, which will be even
more critical in more complex systems such as biomolecules.