Effect of the resonance of the C–H and O–H bond stretching vibrations on the IR spectra of the hydrogen bond in formic and acetic acid

Abstract

It is shown that the resonance of the O–H and C–H bond stretching vibrations is responsible for a noticeable intensity redistribution effect in the IR spectra of associated formic acid molecules in the gaseous phase. This effect is manifested by a considerably high growth in intensity of the ν C–H band, which overlaps the ν O–H band contour in the spectra. A vibronic coupling of the Herzberg–Teller-type expressed by the second order term in the perturbation theory is the most probable source of these spectral effects. The presented mechanism explains the variation of the effect magnitude accompanying the phase transitions. The proposed model also facilitates the understanding of the H/D isotopic effects in the spectra as well as the essential difference in the corresponding spectral properties between the formic and the acetic acid.