Lone Pair Ionization Potentials of Carboxylic Acids Determined by He(I) Photoelectron Spectroscopy

Abstract

Abstract Photoelectron spectra of some simple carboxylic acids and halogen substituted acetic acids were observed. Each spectrum contains a band which is associated with the ionization from an oxygen lone pair (n0) orbital of carboxyl group. The band associated with the n0 orbital is clearly separated from other bands, and its shape and width are quite similar among the various carboxylic acids. In the spectra of halogen substituted acetic acids, n0 band appears around the region of the lone pair bands of halogen atoms. The ionization potentials for oxygen lone pair orbital of carboxylic acids are measured and compared with the results of some semi-empirical SCF MO calculations. The agreements are quite satisfactory when ionization potentials are obtained from the difference of total energies of neutral molecule and its corresponding cation calculated using CNDO/2 or INDO method. MINDO/2 method also gives excellent values when Koopmans’ theorem is applied.