ULTRAVIOLET, INFRARED, AND RAMAN SPECTRA OF PROTONATED CARBOXYLIC ACIDS

Ultraviolet, infrared, and Raman spectra were measured for acetic acid and propionic acid in sulfuric acid solutions of various acidities. Infrared spectra of acetic acid and benzoic acid were measured in deuterosulfuric acid and aqueous deuterated sodium hydroxide solutions. From the detailed analyses of the ultraviolet absorption change the pKBH+ values of acetic and propionic acids were found to be −6.12 and −6.33, respectively. In the infrared and Raman spectra of the protonated acids, the C=O stretching band at about 1700 cm−1 disappeared and a new strong infrared absorption band appeared at about 1600 cm−1. The frequency of the latter band, which can be assigned to antisymmetric C—O stretching band, is very similar to that of the carboxylate group. The acidity dependence of these infrared spectra was parallel with that of the ultraviolet absorption. These facts confirm that the protonated carboxyl group has a skeleton [—C(OH)2]+ similar to the carboxylate group [—CO2]−.