FTIR investigation of polarizable hydrogen bonds in carboxylic acid–pyridine complexes in the mid- and far-IR region

Abstract

Twelve carboxylic acid–pyridine systems have been studied in chloroform solutions in the mid-IR and the far-IR regions as a function of the pK_a of the carboxylic acids at 25 °C and at –40 °C. If the acidity of the carboxylic acid is small a strongly asymmetrical double-minimum proton potential is present. With increasing acidity the hydrogen bonds become stronger, i.e. the bond length shortens and the proton potentials become more symmetrical in character. In these cases the barrier of the potentials becomes very low and the situation may be compared with a broad, flat, asymmetrical single minimum. Therefore, the continua in these complexes are very similar to those in systems with broad, flat, single-minimum proton potentials. With increasing polarity of the complexes the hydrogen bonds lengthen again and the double minimum becomes still more symmetrical. The two proton-limiting structures OH⋯N ⇌ O^–⋯H⁺N can be distinguished by different carbonyl bands; the band of the polar structure is found at relatively high wavenumbers indicating that the barrier is low. Almost symmetrical double-minimum proton potentials are now present. With the most acidic system at –40 °C, the polar structure has a larger weight.

In the far-IR spectra two hydrogen-bond vibrations are usually found which are not, however, caused by the two proton-limiting structures. One of these bands has more stretching and the other more librational character. As a rule, the second vibration, in which the whole carboxylic acid as well as the whole pyridine molecule perform a rotational motion against each other, is the less intense one. In the cases of the strongest hydrogen bonds at 25 °C, a slight broadening of the band with stretching character is observed.