Chains of Hydrogen-Bonded Molecules: Structural Data and Localized Modes

Abstract

The acetanilide crystal (C₆H₅NHCOCH₃ or ACN) contains parallel one-dimensional networks of hydrogen-bonded....H -N-C-O.... amide groups, like polypeptides⁽¹⁾. ACN displays some anomalous infrared and Raman modes, the origin of which is still a subject of controversy.

Tentative explanations of these extra-intensities observed at low temperature have been given in terms of Davydov-like soliton theory⁽²⁾, vibron soliton⁽³⁾, “polaronic” localized mode^{(4) (5)}, nonlinearly coupled oscillators⁽⁶⁾. All these theories involve the self-trapping of the vibrational energy of the amide - 1 mode and postulate the existence of nonlinear coherent excitations at a microscopic scale. These quasi-solitons, assumed to have a very low damping rate, would be able to transfer energy without loss. The stake of these theories is, in a further step, to provide a plausible physical mechanism for the high efficiency of energy transport in biological molecules. This is the main reason why solving the acetanilide problem is so an attractive challenge.

Alternative interpretations of the anomalous modes in ACN involve Fermi resonance⁽⁷⁾, and two slightly non degenerate configurations of the amide proton⁽⁸⁾.