Low-frequency single-crystal Raman, far-infrared, and inelastic neutron scattering studies of acetanillde at low temperature

Single-crystal Raman, far-infrared (far-IR), and inelastic neutron scattering (INS) spectra of acetanilide (ACN) in the low-frequency region (20-200 cm^-1) were obtained as a function of temperature. At 20 K, a total of 29 low-frequency Raman-active vibrational modes were resolved and assigned to a unique symmetry species. For comparison, a total of 23 far-IR bands were observed in the 20-200-cm^-1 region at 20 K. Factor group analysis of ACN predicts 24 Raman-active and 15 IR-active phonon bands. The greater-than-expected number of observed bands in the low-frequency region was assigned to the presence of low-frequency internal modes that exhibit anomalous frequency-shift and line-narrowing behavior upon cooling. The most striking change occurs in the B_1g (xy) polarization where a single band at 126 cm^-1 at 305 K splits into three distinct, well-resolved bands at 133,142, and 152 cm^-1 at 20 K. INS spectra of ACN and its deuterated isotopomers demonstrate clearly that the 142-cm^-1 band corresponds to a methyl torsion. In addition, low-frequency bands at 104 and 189 cm^-1 were assigned to internal modes. In contrast to the phonon modes, the low-frequency internal modes all exhibit greater line broadening with temperature.