FTIR INVESTIGATION OF POLARIZABLE HYDROGEN-BONDS IN CARBOXYLIC ACID-PYRIDINE COMPLEXES IN THE MID-IR AND FAR-IR REGION

Twelve carboxylic acid-pyridine systems have been studied in chlorofor m solutions in the mid-IR and the far-IR regions as a function of the pK(a) of the carboxylic acids at 25 degrees C and at -40 degrees C. If the acidity of the carboxylic acid is small a strongly asymmetrical d ouble-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 case s the barrier of the potentials becomes very low and the situation may be compared with a broad, flat, asymmetrical single minimum. Therefor e, the continua in these complexes are very similar to those in system s with broad, flat single-minimum proton potentials. With increasing p olarity of the complexes the hydrogen bonds lengthen again and the dou ble minimum becomes still more symmetrical. The two proton-limiting st ructures OH ... N reversible arrow O-... H+N can be distinguished by d ifferent carbonyl bands; the band of the polar structure is found at r elatively high wavenumbers indicating that the barrier is low. Almost symmetrical double-minimum proton potentials are now present. With the most acidic system at -40 degrees C, the polar structure has a larger weight. In the far-IR spectra two hydrogen-bond vibrations are usuall y found which are not, however, caused by the two proton-limiting stru ctures. One of these bands has more stretching and the other more libr ational character. As a rule, the second vibration, in which the whole carboxylic acid as well as the whole pyridine molecule perform a rota tional motion against each other, is the less intense one. in the case s of the strongest hydrogen bonds at 25 degrees C, a slight broadening of the band with stretching character is observed.