FTIR spectra of various isotopic forms of formic and acetic acids in the gas phase have been recorded in the range 500ndash;4000 cmminus;1. The way for separating spectra due to monomers and to Hhyphen;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 ngr;s(Ondash;Hdrarr;sdot;sdot;sdot;O) and ngr;C=Obands between Hhyphen;bonded and Dhyphen;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 (pseudohyphen;Jahnndash;Teller effect). It might explain why simple doublehyphen;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 ngr;sis analyzed using a peelinghyphen;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 ngr;smodes, which had been scarcely performed before. Peeledhyphen;off spectra then appear as classical spectra of a rapid motion (ngr;s) modulated by lowhyphen;frequency intermonomer modes of H bonds. Qualitative attributions of their submaxima is given in terms of a transition in ngr;saccompanied by transitions of intermonomer modes. Stretching as well as bending intermonomer modes are shown to have an active part in the modulation of ngr;s.
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