'Anomalous' excitation in hydrogen-bonded molecular crystals - a Raman scattering study of specifically deuterated acetanilide (C6D5-CONH-CD3)

The focus of experimental and theoretical works about crystalline Acetanilide has been the “anomalous” temperature-dependent ir absorption and Raman peaks at about 1650 cm−1 and the multiband structure in the N-H stretch region. A lively discussion about the assignment of these “anomalous” bands has arisen and is still in progress. The present Raman experiments should be placed in this context as an attempt to identify the molecular degrees of freedom which originate the “anomalous” bands. In this aim Raman experiments have been performed on specifically deuterated Acetanilide [C6D5-CONH-CD3] single crystal in the low-frequency (phonon) and C=O stretching regions. On cooling a distinct band at about 1495 cm−1 increases in intensity. We assign this peak to the equivalent of the 1650 cm−1 band in Acetanilide. The temperature dependence of this Raman line was studied. The results are discussed in the light of the models proposed to explain the anomalous behaviour of the 1650 cm−1 Raman line in Acetanilide.