Vibration-spectral studies of carboxylate ions. Part II. Substituted benzoate ions

Abstract

The frequency of the carboxylate stretching vibration near 1550 cm.^–1 in the benzoate ion ranges in the derivatives from 1500 cm.^–1(in the dianion of 4-hydroxybenzoic acid) to 1650 cm.^–1(in the zwitterion of picolinic acid) being strongly influenced by the polar effects of substituents. This vibration is often mixed, sometimes strongly with one or both aromatic skeletal stretching vibration(s) of very similar frequency (8b and 8a). The frequency of the carboxylate stretching vibration near 1400 cm.^–1 is not effected by polar effects and is almost constant except for small ortho effects. Appreciable mixing of this vibration with an aromatic skeletal stretching motion (19b) is rare. The observed behaviour of the carboxylate stretching frequencies is in accord with the classical structure of the carboxylate grouping, the ranging from 1500 to 1650 cm.^–1 being assigned to CO stretching and that near 1400 cm.^–1 to mainly C–O^– stretching.