High-pressure Raman study of vibrational spectra in crystalline acetanilide

We have studied the effect of pressure on the low-frequency lattice modes and the amide-I (N-C?O stretching) vibrational modes in crystalline acetanilide (${\mathrm{C}}_6$ ${\mathrm{H}}_5$ ${\mathrm{NHCOCH}}_3$) in the temperature range 80–300 K by means of Raman spectroscopy. The Raman intensity of the 1650-${\mathrm{cm}}^{\mathrm{-}1}$ band, which appears upon cooling, is enhanced by applying pressure. The energy difference between the amide-I phonon (${\mathit{A}}_{\mathit{g}}$ mode) and the 1650-${\mathrm{cm}}^{\mathrm{-}1}$ bands does not change appreciably under pressure up to at least 4 GPa. These results are analyzed in terms of the self-trapped model in which a single lattice mode couples with the amide-I excitation by taking into account the effect of pressure on the low-frequency lattice modes and on the dipole-dipole interactions associated with the amide-I vibration. A band is observed at 30 ${\mathrm{cm}}^{\mathrm{-}1}$ below the amide-I phonon band at low temperatures with a pressure above ∼2 GPa.