INFRARED SPECTROSCOPY AND HYDROGEN BONDING — BAND-WIDTHS AND FREQUENCY SHIFTS

Abstract A review is given of the main experimental facts concerning the modification of vXH vibration bands by hydrogen bonding, and of the various explanations advanced in the literature for the low frequencies, and the broad and (sometimes) smooth contours observed for these bands under conditions of strong hydrogen bonding. It is considered that the breadth of the vXH bands is most satisfactorily accounted for in terms of (1) a quantum mechanical energy level scheme proposed by Stepanov involving a strong anharmonic coupling between the vXH and vXH ···· Y types of vibration, together with (2) Fermi resonance between the vXH vibration and overtones etc., involving lower frequency fundamentals. A modification of Stepanov's original scheme accounts for the persistence of broad vXH regions (sometimes consisting of many sub-bands) at low temperatures. It is pointed out that variations in the width of γXH absorption bands can also be understood in an analogous fashion in terms of an interaction of this vibration with low frequency torsional motions of the molecules. It seems not unlikely that many band-width phenomena of vibrational spectra in the liquid and solid states are caused by anharmonic interactions between high frequency vibrations and low frequency motions controlled by intermolecular forces.