On the OH Stretching and the Low-Frequency Vibrations of Carboxylic Acid Cyclic Dimers

Infrared spectra for the hydroxyl stretching region of matrix-isolated CF3COOH, CF3COOD, and CH3COOH are reported, and the vapor-phase spectrum of CF3COOH in the region below 500 cm−1 is given. Characteristic line spacings that are found to reproduce the observed spectral features of the high-frequency region are also found to be associated with the low-frequency spectra. Fundamental frequencies for the cyclic dimer ring vibrations are suggested. It is concluded that the well-known band broadening in the hydroxyl stretching region is due to combination bands of the low-frequency modes with the OH fundamental vibration and with nearby binary excitations of the COH(D) bending and the C–O stretching vibrations. A model such as that proposed by Marechal and Witkowski, which considers interactions between the low-frequency and the high-frequency vibrations, seems appropriate in order to explain the intensities of the high combination bands that occur. Further emphasis for the unusual vibrational properties of the mixed COH bending and C–O stretching coordinates in these acids is provided by the spectrum of matrix-isolated monomeric CF3COOH(D). In this case the nominal COH bending vibration of monomeric CF3COOH is clearly doubled while the corresponding COD vibration appears as a single band shifted to lower frequency. The behavior is compared with that for matrix-isolated acetic acid monomers.