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.