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

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Abstract
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.