Photoelectron spectra of acetaldehyde and acetyl halides

The He I photoelectron spectra of acetaldehyde and acetyl fluoride, chloride and bromide have been obtained. The spectra are interpreted in terms of molecular orbital structures, by comparison with results for related molecules and by a consideration of the inductive and resonance effects. The results of CNDO/2 and INDO semi-empirical calculations are also reported. It is found that the oxygen “lone pair” is progressively stabilized as the electronegativity of the halogen increases. The vibrational structure of the first photoelectron band of acetaldehyde is compared with the observed structure of Rydberg transitions in the optical spectrum.