Spectroscopic evidence for Davydov-like solitons in acetanilide

Detailed measurements of infrared absorption and Raman scattering on crystalline acetanilide [${\left(\mathrm{C}{H}_3\mathrm{CONH}{C}_6{H}_5\right)}_x$] at low temperature show a new band close to the conventional amide I band. Equilibrium properties and spectroscopic data rule out explanations based on a conventional assignment, crystal defects, Fermi resonance, and upon frozen kinetics between two different subsystems. Thus we cannot account for this band using the concepts of conventional molecular spectroscopy, but a soliton model, similar to that proposed by Davydov for $\alpha$-helix in protein, is in satisfactory agreement with the experimental data.