Vibrational Structures in the He(I) Photoelectron Spectra of Carboxylic Acids

Abstract

Abstract The first and the second bands of the photoelectron spectra of acetic, propionic, and isobutyric acids were recorded, and the vibrational structures in the spectra were studied by comparing with the results of formic acid. The first band exhibits a νc=o, a δOH, and a ρCOOH vibrational progression, and this implies that an oxygen lone pair orbital, from which the first band of the spectra originates, contributes not only to a C=O bonding and the bonding between carbonyl oxygen and hydroxyl hydrogen, but to the settling of carboxyl group in the molecular plane. The second band exhibits a νc=o, a νc-o vibrational progression with very high frequency for the νc=o as observed in the case of formic acid. Thus the vibrational features of the second band are consistent with the ionization of an antisymmetric π orbital localized mainly on carbonyl and hydroxyl oxygen atoms. Since the 0-0 vibrational transitions for the second ionization are out of the Franck-Condon region, the second adiabatic ionization potential was estimated by comparing the vibrational fine structures of the spectra with that of formic acid. The second adiabatic ionization potential of acetic acid so determined is 11.63 eV.