IR spectra of carboxylic acids in the gas phase: A quantitative reinvestigation

FTIR spectra of various isotopic forms of formic and acetic acids in the gas phase have been recorded in the range 500–4000 cm−1. The way for separating spectra due to monomers and to H-bonded cyclic dimers is described. A careful measurement of pressure and a precise control of temperature make it possible to measure intensities related to one molecule for all bands appearing in this region. It allows to measure the effects of H bonds on intensities of all bands. The comparison of intensities of νs (O–H↘⋅⋅⋅O) and νC=O bands between H-bonded and D-bonded dimers is particularly interesting as it confirms the existence of an anomalous isotope effect, which we propose to attribute, after analysis, to a nonadiabatic transfer of intensities between electronic and protonic transitions favored by the particular ring structure of these cyclic dimers (pseudo-Jahn–Teller effect). It might explain why simple double-well potentials have up to now failed to describe experimental results concerning transfers of protons through H bonds and it stresses the role that one may attribute to ring structures in describing dynamical properties of H bonds. The particular bandshape of νs is analyzed using a peeling-off procedure which allows, in a low resolution approximation, to eliminate features due to Fermi resonances. It allows to measure the magnitude of factor group splittings of νs modes, which had been scarcely performed before. Peeled-off spectra then appear as classical spectra of a rapid motion (νs ) modulated by low-frequency intermonomer modes of H bonds. Qualitative attributions of their submaxima is given in terms of a transition in νs accompanied by transitions of intermonomer modes. Stretching as well as bending intermonomer modes are shown to have an active part in the modulation of νs.