Hydrogen Bonds Induce Double-Well Spectroscopic Signatures in α‑Glycine

Hydrogen bonds in molecular crystals are often modeled
as double-well
potentials, yet direct evidence linking this potential form to vibrational
spectroscopic features remains elusive. In this study, we investigate
α-glycine, a hydrogen-bonded crystal that exhibits pronounced
Raman anomalies without undergoing a structural phase transition.
Through temperature- and polarization-dependent Raman spectroscopy,
supported by isotope substitution and first-principles calculations,
we identify two peaks whose behavior violates conventional Raman selection
rules. These peaks merge and narrow anomalously with temperature,
an effect that cannot be explained by harmonic models or thermal broadening.
Simulated spectra based on a weakly evolving asymmetric double-well
potential reproduce this merging, indicating that both peaks originate
from one double-well potential. Our results establish α-glycine
as a model system directly linking microscopic hydrogen-bond potentials
to vibrational spectroscopic features.