H-LOCALIZED MODE IN CHAINS OF HYDROGEN-BONDED AMIDE GROUPS

New infrared measurements of the anomalous amide modes in acetanilide and its derivatives are presented. Preliminary results of structural d ata obtained by neutron diffraction at low temperature are also descri bed. Besides the well-known anomalous amide-1 mode (1650 cm-1), it is shown that the NH out-of-plane bend (770 cm-1) and the ''H-bond strain '' (at about 105 cm-1) exhibit an anomalous increase of intensity prop ortional to the law exp(-T2/THETA2), suggesting that the amide proton bears a significant electronic distribution as formerly observed for H - localized modes. Structural data, moreover, show that the thermal el lips of the amide proton has an increasing anisotropy at 15 K. Conside ring these new results, the theoretical model of a self-trapped ''pola ronic'' state seems to be the most consistent with the whole set of ob served anomalies in this family of crystals.